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*Correspondin

216925.

E-mail addres

0277-3791/$ - see

doi:10.1016/j.qua

Quaternary Science Reviews 23 (2004) 1699–1722

Mineral magnetism and geomagnetic secular variation of marine andlacustrine sediments from central Italy: timing and nature of local and

regional Holocene environmental change

Timothy C. Rolpha,*, Luigi Vigliottib, Frank Oldfieldc

aGeology Division, School of Environmental and Life Sciences, University of Newcastle, University Drive, Callaghan, NSW 2308, AustraliabCNR Instituto di Geologia Marina, Consiglio Nazionale della Ricerche, Via P. Gobetti 101, Bologna 40129, Italy

cDepartment of Geography, University of Liverpool, Liverpool L69 3BX, UK

Received 20 January 2003; accepted 8 February 2004

Abstract

Sediment core palaeomagnetic and mineral magnetic records from two crater lakes in central Italy and from the western margin of

the Adriatic Sea have been used to evaluate local and regional responses to Holocene environmental change. In all cores, sediment

magnetism reflects the interplay between catchment material and the in situ production of bacterial magnetite (magnetotactic

bacteria). In the lakes, the earliest Holocene sediments record a waning catchment input that we attribute to rising lake levels and

increased tree cover in the catchment. From B9000 to 5000 yr BP, both lakes become anoxic, a consequence of water-mass

stratification driven by high lake levels. Bottom-water anoxia also developed in the Adriatic, with sapropel S1 produced between

B9000 and 7000 yr BP. Subsequently, the lake and Adriatic mineral magnetic records show evidence for increased catchment

delivery, consistent with pollen evidence for Bronze Age deforestation. In the lakes, this evidence is first recorded at B4300 yr BP

and a number of distinct clearance events are recorded. In comparison, at Adriatic site RF93-30, lithogenic input increases abruptly

at B3500 yr BP and is followed by a slowly changing record of waxing and waning sediment delivery.

Inter-site comparisons of palaeomagnetic data point to a possible link between the magnitude of the bacterial magnetite

component and the recorded magnetic inclination. The sites are at near identical latitudes and have similar sediment accumulation

rates but the Adriatic sites have a core-average magnetic inclination that is some 10� steeper than the lake average values. We

suggest that the large dipole moment of the magnetosome chains, which in life produce the passive alignment of the bacterium along

the local geomagnetic field line, produce a more faithful (albeit smoothed) record of the geomagnetic field.

r 2004 Elsevier Ltd. All rights reserved.

1. Introduction

Magnetic minerals are produced, transported, accu-mulated and modified within surface environments, andit is well established that the magnetic content ofsediments provides a sensitive medium for recordingenvironmental change. Sediments record this environ-mental change through temporal and spatial variationsin the mineralogy, concentration and grain size of themagnetic carriers. Such variations in sediment magneticproperties may also correspond to changes in thephysical, chemical, biological and isotopic characteris-tics of the sediments. In this paper, we discuss the

g author. Tel.: +61-249-215402; fax: +61-249-

s: Tim.Rolph@newcastle.edu.au (T.C. Rolph).

front matter r 2004 Elsevier Ltd. All rights reserved.

scirev.2004.02.005

palaeoenvironmental information derived from sedi-ment cores collected from two crater lakes near Rome:Lakes Albano and Nemi; and from sediments on thewestern margin of the Adriatic (Fig. 1). These data wereobtained within the framework of an EC Project(PALICLAS: PALaeoenvironmental analyses of ItalianCrater Lake and Adriatic Sediments): a multi-disciplin-ary investigation focused on the Lateglacial andHolocene climate of central Italy (Oldfield, 1996). Here,we discuss only the results obtained from the Holocenesediments, using the secular-variation (SV) curvesobtained from the same cores to establish the relativetiming of environmental change in the two areas. Acommon chronology is established through comparisonsof the core SV records with archaeomagnetic data andwith the UK Holocene SV curve. Finally, the combina-tion of mineral magnetism and palaeomagnetism has

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Fig. 1. Location map showing the sampling locations and the cores

(open symbols) discussed in the text.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–17221700

allowed us to evaluate the extent to which thepalaeomagnetic record is influenced by changes inmagnetic mineralogy driven by environmental change.

2. Study area and materials

2.1. The lakes

Lakes Albano and Nemi are located on the westernside of the Alban Hills, B15 km to the southeast ofRome (Fig. 1A). The lakes fill the most recent craters ofthe Alban Hills Volcano, a volcano of the RomanComagmatic Province of central Italy. Both lakes arehydrologically confined, with maximum depths of 175m(Albano) and 32m (Nemi). The Lake Albano crater isthought to have formed atB45 kyr (Villa et al., 1999) atthe onset of the most recent phase of activity (thehydromagmatic phase of De Rita et al., 1988). Recentuplift of the Alban Hills volcano has produced a periodof intense seismic activity (Amato et al., 1994) and itseems likely that similar activity has occurred through-out the lifetime of the lakes. Post-depositional dis-turbance of the sediment fill is therefore likely, andnumerous turbidites have been identified in the uppermetre of Nemi sediments (Masaferro et al., 1993).

Erosion within the crater controls sediment input tothe lakes, and the rate of supply will reflect such factorsas precipitation; the degree of soil formation; the natureand density of vegetation; and the available catchmentarea. Chemical and/or biogenic sediments will supple-ment this detrital input. Changes in the relativeimportance of the detrital and authigenic componentswill reflect changes in, for example, water temperature,evaporation, lake productivity, and detrital input (theinput of magmatic CO2 may be an additional factor,Martini et al., 1994). Any change in these variousparameters is likely to reflect a local response to globalclimate change, although modification of the localenvironment through human occupation will be a factorin the more recent past.Long cores were collected from two sites in Nemi and

six sites in Albano using a Kullenberg piston-coringsystem (Fig. 1B and C). Sites were chosen with referenceto seismic cross-sections obtained using a 3.5 kHzechosounder (Niessen et al., 1993). This paper focuseson two sites, P.NEMI94-1B, a 9.14m core collectedfrom 30m water depth in Lake Nemi, and P.ALB94-1E,a 13.8m core collected from a water depth of 70m inLake Albano. Short cores were also collected at each siteto provide minimally disturbed material from the upperB1m of sediment. Pollen data (Lowe et al., 1996)suggest that the Holocene/Pleistocene boundary islocated at a depth of B585 cm in P.ALB94-1E, withthe Younger Dryas stadial present between 585 and625 cm. In P.NEMI94-1B, the Holocene boundaryoccurs at the base of the core. The 4.1 kyr BP AvellinoTephra is found at 3.55m in P.ALB94-1E and at 4.50min P.NEMI94-1B.The lithology and chronology of the lake cores have

been described in detail elsewhere (Chondrogianni et al.,1996) and only a summary is given here. The Holocenesediments in both cores can be sub-divided into threeunits. In P.ALB94-1E, unit I (0–3.8m) is composed oforganic-rich silts, unit II (3.8–5.1m) is dominated bydark mud, but includes laminations of diatom-rich,organic-rich, carbonate-rich and clastic-rich layers,while unit III (5.1–5.8m) comprises a monotonous darkmud. A major erosional event, or hiatus, has beenidentified at the boundary between units II and III. InP.NEMI94-1B, unit I (0–5.2m) is composed of diatombeds and laminae intercalated with massive mud andindistinctly laminated mud. Unit II (5.2–8.4m) has bothmassive mud and laminated mud intercalated withdiatom beds and laminae, while unit III (8.4–9.1m)comprises massive mud intercalated with laminatedmud. The laminations are interpreted as annual varves,while the more massive mud intervals represent episodeswhere varve formation was interrupted by increaseddetrital input (Chondrogianni et al., 1996). Varvecounting of the recent sediments in Nemi attests tosedimentation rates that vary between 3.8 and 6mm/yr

ARTICLE IN PRESST.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–1722 1701

since AD 1830 (Masaferro et al., 1993). This comparesto average sedimentation rates (above the AvellinoTephra) of 1.1mm/yr for P.NEMI94-1B, and 0.9mm/yrfor P.ALB94-1E.Existing catchment soils are thin, poorly developed,

and rich in fragments of the bedrock, a feldspathoidalbasalt with an average chemistry of K-foidite to tephrite(Trigila et al., 1995). In order to establish a representa-tive catchment magnetic signature, 15 soil samples werecollected from locations distributed around the catch-ment. If core magnetic data deviates from this catch-ment signature then we need to consider factors such asauthigenic modification of the core magnetic mineral-ogy, or temporal variations in soil properties.

2.2. The Adriatic

The Adriatic Sea comprises a narrow, elongate,epicontinental shelf surrounding the 250m deep Meso-Adriatic Depression (MAD) (Fig. 1A). Sediment inputcurrently derives mainly from the Italian mainland.Rivers draining the Po Plain to the northwest supply themajority of this material, with drainage from theApennine chain supplying most of the remainder(Trincardi et al., 1996). The rate of sediment suppliedby these rivers will reflect climatic, and more recentlyhuman, impact on catchment erosion, although theongoing uplift of the Apennine chain could introduce anadditional, but likely minor, variable. Thus, thesediment record of the Adriatic will contain informationthat reflects a regional response to global climatechange, compared with the more localised responserecorded by the lake sediments.Seismic reflection data indicate the presence of four

Late Quaternary systems tracts (Trincardi et al., 1996).The youngest is a high-stand systems tract (HST) thatdeveloped after the early Holocene sea-level rise. ThisHST unit comprises a mud wedge, up to 30m thick inplaces, confined to the western margin of the Adriatic.The distribution of the HST unit reflects the role of thewestern margin as a depocentre, a role that arises fromthe westerly bias of sediment input, reinforced byoceanographic circulation that redistributes sedimentalong the western margin towards the southeast(Malanotte Rizzoli and Bergamasco, 1983). CoresRF93-30 and PAL94-9 (Fig. 1A) are located within thisunit. Core RF93-30, located at a water depth of 77m,consists of 6.27m of homogeneous grey mud. The baseof the HST is recognised at 5.9m by a dramatic decreasein foraminiferal abundance above this point (Asioli,1996). Calibrated AMS 14C results from the HST(Langone et al., 1996) give dates (in years BP) of650750, at 129 cm; 1210780, at 219 cm; 2720740, at365 cm; and 36607100, at 527 cm. Of these, the firstthree have been rejected because of inconsistency withthe pollen record and with the palaeomagnetic correla-

tion of the core (Oldfield et al., 2003). Below the HST isa condensed interval that has provided an AMS 14C dateof 6180 yr BP at 599 cm depth. Core PAL94-9, retrievedfrom a water depth of 104m, consists of 6.1m ofhomogeneous mud underlain by 60–70 cm of organic-rich silty mud with occasional laminated intervals. Thiscore encompasses the full Holocene, with an erosionalsurface at 650 cm produced during the Lateglacialmarine transgression (Trincardi et al., 1996). A seriesof calibrated AMS 14C results (in years BP; Langoneet al., 1996) provide dates of 1790780, at 122 cm;5570780, at 338 cm; 7210760, at 433.5 cm; 92207130,at 523.5 cm; and 17,1807130, at 675 cm.

3. Methods

All cores were passed though a magnetic susceptibilitycore scanner before splitting and sub-sampling. Corehalves were sampled by carefully pressing perspex cubes,of 6 cm3 volume, into the sediment. Samples fromP.NEMI94-1B and PAL94-9 were analysed at the CNRInstituto per la Geologia Marina magnetic laboratory inBologna, while cores P.ALB94-1E and RF93-30 wereanalysed at the University of Liverpool EnvironmentalMagnetism and Geomagnetic laboratories.Natural remanent magnetisation (NRM) was investi-

gated using a Molspin spinner magnetometer andstepwise alternating field (AF) demagnetisation. Weakersamples from the anoxic sections within the lake cores(see later discussion) were analysed using a high-temperature (FIT) SQUID system (Liverpool) or JelinekJR-4 spinner magnetometer (Bologna). Remanencedirections were determined using orthogonal projectionsand principal component analysis.Rock-magnetic techniques were applied in the follow-

ing sequence, although not all samples were subjected tothe full suite of measurements.

* Low frequency (0.465 kHz) magnetic susceptibility(k).

* Anhysteretic remanent magnetisation (ARM; AF of100mT with a DC biasing field of 0.95mT) followedby AF demagnetisation of the ARM.

* High-field isothermal remanent magnetisation (IRM;produced in a 1T field) with subsequent backfieldremagnetisation at 20, 40, 100 and 300mT.

Although insufficient to saturate any antiferromag-netic minerals in the samples, the high-field IRM ishereinafter referred to as saturation IRM (SIRM).These techniques can identify down-core variations in

the bulk magnetic properties of the sediments, which canarise from changes in grain size, chemistry andconcentration of the magnetic mineral content. Expres-sing the magnetic data as percentages, or as interpara-metric ratios, provides information on changing

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magnetic grain-size or changing chemistry. Ratios usedhere are kARM/k (kARM, the anhysteretic susceptibility,is calculated by dividing the ARM intensity by thebiasing field), SIRM/k and ARM/SIRM. ARM isdisproportionately responsive to the stable singledomain content (SSD;B0.1–0.03 mm in magnetite, Kinget al., 1982; Maher, 1988), while k is most sensitive tothe superparamagnetic (SP) content (o0.03 mm inmagnetite). A general fining of the magnetic grain-sizewill produce an increase in those ratios with ARM as thenumerator, provided the reduction in grain size does notshift the distribution dominantly into the SP region. Inthe latter case, an increase in the relative contribution ofSP grains will reduce those ratios that have k as thedenominator. Here, the ARM/SIRM quotient is useful,as it will only respond to changes in those grain sizesthat lie above the SP boundary. Opposite trends inkARM=k and ARM/SIRM may therefore indicate thatchanges in the ultrafine component have produced arelative change in the SP to SSD contributions. Changesin the antiferromagnetic content (haematite and/orgoethite) can be identified through a change in themagnetically ‘‘hard’’ component of SIRM (HIRM; i.e.coercive force region above 300mT). Expressing thishard component as a percentage of the SIRM(%HIRM) can identify variations in the relativecontribution of the canted-antiferromagnetic compo-nent.A number of sub-samples were selected for further

analysis using Curie (JS � T) curves, hysteresis curves(between 71T); and detailed IRM acquisition experi-ments (up to 2.5 T). These techniques provide additionalconstraints on the interpretation of down-core varia-tions in magnetic mineralogy.

4. Results and interpretation

4.1. Rock-magnetic properties of the lake sediments and

catchment soils

The stratigraphic variation of the bulk magneticmeasurements and ratios are shown in Figs. 2 and 3,together with the stratigraphic sub-division of each coreinto the three units described above in Section 2.1. Theshaded vertical bands in Fig. 3 indicate the range ofinterparametric values obtained from the catchment soilsamples. Regions of the core that contain onlyunmodified catchment magnetic material will produceinterparametric values that fall within the shaded bands.Regions of the core that produce interparametric valuesfalling outside of the shaded bands can be explained byan additional source of magnetic material, by changes inthe characteristics of the catchment input, or by both ofthese factors.

The gross trends in the magnetic concentration indiceshave a commonality between the two cores that suggestsa natural sub-division into four zones (Fig. 2). Themagnetic characteristics of each of the four zones are asfollows.Zone 4, which represents the oldest Holocene sedi-

ment, shows an upward-decreasing trend in all bulk-parameters that culminates in the extremely low valuesseen in zone 3. Zone 4 also shows highest values of theARM ratios, suggesting that this early Holocene sectionof the core contains the greatest proportion of SSDferrimagnets. For the most part, the magnetic ratios inthis zone also show the greatest deviation from the soilenvelope values. The zone 4–zone 3 boundary iscoincident with the boundary between lithological unitsIII and II.Zone 3 has the lowest SIRM, HIRM and ARM

values and mainly negative magnetic susceptibilityvalues. Negative values of kARM=k and SIRM/k havebeen re-set to zero. Catchment-derived material hasbeen identified within these core intervals (Chondro-gianni et al., 1996), and the lower limit for ARM/SIRMvalues in zone 3 is closer to the soil envelope values thanin all other zones. Therefore, the very low magneticvalues in this zone point to an increased dilution ofmagnetic material and/or to some in situ modification ofthe magnetic minerals. Core maximum values for%HIRM occur within this zone, confirming that themagnetic mineralogy here differs significantly from thatin the other zones. In P.ALB94-1E, the zone 3–zone 2boundary is coincident with the boundary betweenlithological units II and I, whereas in P.NEMI94-1B, thelithological boundary occurs some 30 cm above themagnetic boundary.The onset of zone 2 shows an abrupt increase in

magnetic concentration, reaching a maximum for eachcore in the lower part of this zone. Above this point,there are localised peaks in the magnetic concentrationindices superimposed on an upward-decreasing trend.The peaks in P.NEMI94-1B are more numerous, andgenerally have larger amplitudes. Peaks at 450 cm(Avellino Tephra) and 380 cm in P.NEMI94-1B can becorrelated with peaks at 355 and 320 cm in Albano,indicating that earlier peaks seen at 560 and 500 cm inP.NEMI94-1B are both absent from P.ALB94-1E. Theabsence of the two latter peaks is consistent with theobservation that erosion has removed an approximately2000-yr long section of P.ALB94-1E (Chondrogianniet al., 1996; Guilizzoni et al., 2002) at a depth thatcorresponds to our designation of the zone 3/zone 2boundary within the core. This erosion also explains thediscrepancy between the two cores in the relativeposition of the magnetic (zone 3–zone 2) and lithological(unit II–unit I) boundaries. All zone 2 magnetic ratiosfluctuate about mean values that are either greater than,or at the upper limit of, the catchment envelope values.

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Fig. 2. Stratigraphic variation of bulk magnetic parameters in the lake sediment cores suggests a four-zone sub-division that is common to both

lakes. The greatly reduced magnetic content of zone 3 is the result of ferrimagnetic dissolution by reductive diagenesis.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–1722 1703

This implies that the average magnetic grain-size isgenerally finer than the catchment material. Ratiosincorporating ARM show a pattern of behaviour that isthe inverse of the magnetic concentration indices; peaksin the latter correspond in the ARM ratios to minimathat are closer to the catchment envelopes.

Zone 1 has generally low bulk magnetic concentra-tion, particularly in P.ALB94-1E, although P.NEMI94-1B does show some minor peaks in magnetic concentra-tion in the topmost metre. Interparametric ratio valuesfor P.NEMI94-1B are little different from the meanvalues seen in zone 2, implying that zone 1 has simply

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Fig. 3. Stratigraphic variation of interparametric ratios identifies temporal changes in magnetic grain-size and/or chemistry within the lake sediment.

The vertical shaded regions represent the ranges of interparametric values exhibited by catchment soil samples. Lake sediment intervals that plot

within these shaded regions are dominated by a catchment magnetic component. Note the enhanced hard (H)IRM component within the zone of

ferrimagnetic dissolution.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–17221704

seen a decrease in magnetic concentration. The ratios inP.ALB 94-1E indicate quite different behaviour, the lowmagnetic concentration in zone 1 coinciding with bothan increase in relative ferrimagnetic grain-size and anincrease in the proportion of antiferromagnetic minerals.

Although both cores show this gross zonation, thereare differences between the two cores in the mean valueof each quotient, and their stratigraphic variation. Theorigin of these differences, and the interpretation of themagnetic zones, will be considered later.

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0.0

0.2

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1.2

Soil 1 Soil 2 Soil 3

-200 -150 -100 -50 00.0

0.2

0.4

0.6

0.8

1.0

1.2

193 cm 302 cm 530 cm

Temperature oC

κ/κ R

Tκ/

κ RT

Fig. 5. Normalised magnetic susceptibility of soil and P.ALB94-1E

samples during warming from �196�C. The Verwey transition is

present in the lake samples.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–1722 1705

Identification of the magnetic carriers in the lakesediments and soils facilitates a more comprehensiveinterpretation of the stratigraphic changes seen in themagnetic parameters. Samples from zone 3 have such alow magnetic content that they only provided reliableand accurate data from the IRM analyses. For the otherzones, samples from P.ALB94-1E were subjected to thefull suite of measurements listed earlier, whereasP.NEMI94-1B samples were subjected only to thedetailed IRM acquisition measurements. Consequently,and for the reasons of close proximity and similarity inlake and catchment characteristics, the inferences basedon analyses of P.ALB94-1E samples are assumed also toapply to P.NEMI94-1B.Stepwise IRM acquisition measurements (Fig. 4)

indicate that zones 1, 2 and 4 are dominated byferrimagnetic material, with saturation essentially com-plete by 300mT. The soil samples show similar IRMacquisition characteristics, but with a narrower range ofvariation; this is illustrated by plotting the full range ofvariation in soil IRM acquisition characteristics as ashaded envelope in Fig. 4. The peak IRM intensities ofzone 3 samples (A404, A501, N595 and N732 cm) areabout two orders-of-magnitude weaker than zone 2 andzone 4 samples, although the initial (below 50mT) IRMacquisition behaviour is similar to that seen in thestronger samples. A significant canted-antiferromag-netic component in the sample means that saturation isnot achieved at a maximum field of 2.5 T. The canted-antiferromagnetic component accounts for a similarfraction of the IRM as the ferrimagnetic component insamples A404, N595 and N732 cm, and is greater thanthe ferrimagnetic component in sample A501 cm.Fig. 5 shows the low-temperature magnetic suscept-

ibility measurements; samples A193 and A302 cm have

0

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1 10 100 1000Field (mT)

Nor

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ized

Mag

netis

atio

n

A101 cmA197 cmA289 cmA404 cmA501 cmA530 cmN 76 cmN304 cmN595 cmN732 cmN860 cmSoils (Range)

Zone 1Zone 2Zone 2Zone 3Zone 3Zone 4Zone 1Zone 2Zone 3Zone 3Zone 4

Fig. 4. Normalised IRM acquisition curves for selected samples from

P.ALB94-1E and P.NEMI94-1B (prefixed A and N, respectively). The

sample codes indicate the core depth from which samples were

obtained, while the zone number refers to the magnetic zones identified

in Figs. 2 and 3. The shaded region identifies the range of acquisition

curves obtained from catchment soil samples.

been corrected for the paramagnetic component. Theinterpretation of low-temperature magnetic susceptibil-ity data is equivocal (e.g. Radhakrishnamurty et al.,1981; Senanayake and McElhinny, 1981; Moskowitzet al., 1998) so we restrict discussion to the moreprominent features. All soil samples show a relativelyrapid increase in susceptibility at lower temperatures.This behaviour is consistent with a decrease in thecoercivity of SP grains as they exceed their blockingtemperatures. The lake samples show a similar initialincrease in susceptibility, but in these samples, theincrease passes into the Verwey transition, characterisedby a peak in susceptibility at B155�C that indicatesthe presence of unoxidised magnetite ( .Ozdemir et al.,1993). This implies an additional phase in the lakesamples that does not derive from the catchmentinput. It should be noted that the uncharacteristi-cally high temperature of the Verwey transition isan artefact of the experimental procedure. It wasdifficult to ensure the accurate positioning of thethermocouple within the small, dry sediment samplesand consequently thermal conduction along the thermo-couple leads resulted in sample temperature laggingB30–40�C behind the thermocouple at the transitiontemperature.

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0 100 200 300 400 500 600 7000.0

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0 100 200 300 400 500 600 7000.0

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0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700

A193 cm A530 cmA306 cm

Soil 1 Soil 2

Temperature oCTemperature oC Temperature oC

Soil 3

Fig. 6. Thermomagnetic curves for selected soil samples and P.ALB94-1E samples (prefixed A). For each sample, the magnetisation has been

normalised to the pre-heating value.

-300 -200 -100 0 100 200 300

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193 cm 306 cm 530 cm

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-1.0

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Soil 1 Soil 2 Soil 3

Nor

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ized

Mag

netis

atio

n

Fig. 7. Normalised hysteresis curves for selected soil samples (upper)

and P.ALB94-1E samples (lower). The more open loops of the lake

samples point to the presence of a harder ferrimagnetic component.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–17221706

Thermomagnetic analyses are shown in Fig. 6. Theheating curves of soils 1 and 3 are dominated by a Ti-poor titanomagnetite phase with a Curie point (TC) ofB430�C and both heating curves show a ‘tail’ thatdisappears at B580�C. On cooling, the titanomagnetiteTC has increased toB460�C and for soil 3 the ‘tail’ seenon the heating curve has been lost. Soil 2 has a Ti-poortitanomagnetite phase with TCB560�C that on coolingis lowered to B530�C. The titanomagnetite is probablyderived from the volcanic rocks of the catchment andthe Curie curves thus show little evidence of ferrimag-netic material of pedogenic origin. Lake samples A193and A306 cm (zone 2) both show a titanomagnetitephase with a Curie point that falls betweenB440�C and480�C, similar to the dominant phase in the soilsamples. Both heating curves also show a minormagnetite phase, which is consistent with the resultsfrom the low-temperature susceptibility measurements.Cooling curves give magnetite Curie points, but sampleA193 cm shows an increase in magnetisation aftercooling, while sample A306 cm shows a small decrease.This may simply indicate different organic contents (andthus redox conditions during heating) in the samples.The sample from zone 4, A530 cm, is much weaker,significantly reducing the signal-to-noise ratio. Onheating, this sample shows a single, magnetite Curiepoint (TCE580�C), with no evidence for a titanomag-netite component. This may be equivalent to the minormagnetite phase seen in the other two lake samples, andimplies that all lake samples contain a magnetite phasethat does not come from the catchment.Hysteresis loops shown in Fig. 7 have had the

paramagnetic component removed. The zone 2 samples

(A193 and A306 cm) are nearly identical and show littledifference in HC or MRS=MS values. The weaker zone 4sample (A530 cm) had negative slopes at high fields,

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0 2 4 6 8 10 120.0

0.1

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0.6

HCR / HC

MR

S / M

S SoilLake sedimentFIN

ER

COARSER

Fig. 8. Hysteresis parameters for the soil and lake sediment samples

plotted as a simplified Day plot. The magnetic mineralogy is multi-

component and therefore the trend displayed by the position of

individual points provides only an indication of changes in the

aggregate magnetic grain-size of the sample group.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–1722 1707

indicating the increasing influence of diamagneticminerals. Values of HC and MRS=MS were, nonetheless,similar to the other samples. In comparison, the soilsamples show significantly reduced MRS=MS and HC

values. The hysteresis and IRM backfield data havebeen combined in Fig. 8 in a simplified form of Day plot(Day et al., 1977). The Day plot can be used to infervariations in the magnetic grain-size of a suite ofsamples containing a single ferrimagnetic phase. TheP.ALB94-1E samples show evidence of (at least) twomagnetic phases and therefore the plot is used only toprovide an indication of changes in the aggregatemagnetic grain-size.

4.2. Rock-magnetic properties of the Adriatic cores

The behaviour of the bulk magnetic parameters isshown in Fig. 9, together with the core stratigraphydescribed in Section 2.2. A log scale has been used whenextreme values in a particular parameter are such that alinear scale is inadequate for displaying in detail the fullrange of variation. In RF93-30, the magnetic suscept-ibility and SIRM data have an essentially identicalpattern, showing three distinct peaks of variousmagnitudes and an underlying trend that shows agradual upward decrease. The HIRM data differ fromthe SIRM data at the very base of the core, where itdrops to low values, and between the peaks at 525 and295 cm, where it is convex to the right. In the kARM data,the narrow peaks at 525 and 295 cm occur withinbroader zones of increased kARM values. The peak at525 cm is produced by a tephra layer (Calanchi et al.,1996) and, by inference, the peaks at 295 and 118 cmmay possibly reflect currently unidentified tephra-richhorizons. This supposition is supported by the recogni-

tion of glass shards in the sediment at these depthintervals (Langone et al., 1996). Calanchi et al. (1998)noted that the Avellino Tephra is geochemically distinctfrom the tephra in RF93-30, assigning the latter to anunclassified status. However, it may be that the latter isthe Astroni tephra, recently identified by Siani et al.(2001) in a core collected from the southern Adriatic.This tephra has an age range (1s) of 4291–4146 cal. yrBP, which is similar to that of the Avellino Tephra.In core PAL94-9, bulk magnetic parameters, other

than HIRM, exhibit a quite consistent pattern ofvariation above 580 cm. There are broad maximacentred at B280, 190 and 50 cm, and a minimumcentred at 475 cm. The HIRM data show considerablescatter throughout much of this interval, which largelymasks a broad maximum centred at B190 cm. TheHIRM data also show a broad maximum betweenB440and 510 cm that corresponds to the minimum seen in theother bulk magnetic parameters. Below 5.9m, the lowestB80 cm of sediment (except the basal point) shows alarge increase in magnetic susceptibility that coincideswith a similar rise (proportionally) in HIRM; a smallerrise in SIRM; and a decrease in kARM. This zonecorresponds to a change in lithology from silty mudbelow, to homogeneous mud above, with the formerinterpreted as a unit formed during the Lateglacial–Early Holocene marine transgression (middle TST,Trincardi et al., 1996). The basal point occurs belowan erosional surface, within Lateglacial (low stand)sediments.Some 120 km separate the two Adriatic sites, and so

the bulk magnetic properties cannot provide the type ofsimple correlation that was achieved between the twolake cores. The 14C results for PAL94-9 (Langone et al.,1996) yield a date of 1790780 yr BP at 122 cm depth,and 5570780 yr BP at 338 cm. Assuming a constantaccumulation rate between 122 and 338 cm, the tephrafound at B525 cm in RF93-30 would be found atB220 cm in PAL94-9, an estimate supported by thepalaeomagnetic correlations discussed later. The lattercore has a double peak between 200 and 250 cm in themagnetic susceptibility and SIRM data, the deeper ofwhich corresponds to a small minimum in the kARMdata.Magnetic parameter ratios and %HIRM are pre-

sented in Fig. 10. For RF93-30, the ARM ratios showthat the broad zones of higher kARM values reflectincreases in the proportion of the fine-grained (SD toPSD) fraction, while the narrow peaks in the magneticconcentration indices correlate with increases in theproportion of the coarser-grained fraction. The varia-tion in %HIRM shows essentially the inverse of thepattern displayed by the kARM data. Therefore, theincrease in the fine, ferrimagnetic component thatdominates the kARM data effectively diminishes therelative proportion of the hard canted-antiferromagnetic

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Fig. 9. Stratigraphic variation of bulk magnetic parameters in the Adriatic cores. Note the three localised peaks in concentration in RF93-30,

the lowest of which has been identified as a tephra with a Somma-Vesuvius geochemical signature. Sapropel S1 is identified in PAL94-9 by a

zone of reduced magnetic content centred at 485 cm. Selected parameters are plotted on a log scale to accentuate the variability seen at lower

values.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–17221708

component. This suggests that the two componentshave different sources. In core PAL94-9, all ratiosdecrease significantly in the bottom metre of the core,related to the change in lithology, and there is a strongminimum centred at 480 cm. There are also lesssignificant minima centred at 100, 240 and 290 cm, but

of these, only the 240 cm feature occurs in all ratios. ThekARM=k data indicate that these minima are allassociated with a decrease in the proportion of ultrafineferrimagnets. The minimum centred at 480 cm correlateswith Adriatic sapropel layer S1 (Alvisi and Vigliotti,1996), within which sulphate reduction and dissolution

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Fig. 10. Stratigraphic variation of interparametric ratios clearly identifies the sapropel interval in PAL94-9 as having an increased magnetic grain-

size and an increased proportion of antiferromagnetic minerals. In RF93-30 the peaks in magnetic concentration are identified as intervals of

increased magnetic grain-size, while the fluctuation of %HIRM reflects a changing lithogenic contribution. The importance of magnetotactic bacteria

is reflected in ARM-based ratios that are consistent with values obtained from intact magnetosome chains.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–1722 1709

of ferrimagnetic grains has occurred. The preferentialdissolution of the finer fraction has resulted in amajor decrease in the ratios, as well as in the bulkmagnetic parameters. The small minima at 240 cmmay represent the tephra seen in RF93-30, as outlinedabove. A notable feature of the magnetic ratios

in PAL94-9 is that they display mean values thatare two to three times greater than the equivalent ratioin RF93-30, implying a finer mean ferrimagnetic grain-size for the former. Conversely, the HIRM values inPAL94-9 are typically smaller than the values seen inRF93-30.

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There was limited material available from the Adriaticcores and samples were exchanged between laboratoriesfor a number of tests. Consequently, samples could notbe sub-sampled for hysteresis or thermomagneticexperiments. The samples selected for IRM acquisitionexperiments represent the extremes in magnetic beha-viour within each core. Fig. 11 presents the data in theform of normalised IRM acquisition curves. In RF93-30, the sample from 116 cm is a possible tephra-richlayer; the sample from 153 cm has a low ARM valuecombined with a high percentage of HIRM; while thesample from 583 cm has a large ARM but a lowpercentage of HIRM. In PAL94-9, the sample from231 cm is from a zone of reduced interparametric ratios,perhaps representing a tephra-rich horizon; the samplefrom 291 cm occurs within a region displaying thelargest kARM values for the core; the sample from483 cm occurs within the zone of reduced ferrimagneticconcentration of sapropel layer S1; while the samplefrom 625 cm is from the region of increasing magneticsusceptibility and decreasing kARM values towards thebase of the core. The samples R153, R583, P291, P483and P625 cm show similar behaviour up to 70mT, butwhile samples R583 and P291 cm reach maximum IRMvalues by 400mT, the samples R153, P483 and P625 cmdo not achieve saturation at the maximum availablefields. This behaviour indicates a relatively largercanted-antiferromagnetic component in the latter group,with sample R483 cm, from sapropel S1, havingthe largest such component (in relative terms). Incomparison, the samples R116 and P231 cm showthe most rapid increases in IRM at lower fields,indicating that the tephra-rich layers have a coarserferrimagnetic grain-size than is typical for these Adriaticsediments.

1 10 100 1000

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Mag

netis

atio

n

Field (mT)

R116 R153 R583 P231 P291 P483 P625

Fig. 11. Normalised IRM acquisition curves for selected samples from

Adriatic cores RF93-30 and PAL94-9 (prefixed R and P, respectively).

The samples R116 and P231 cm are from core intervals categorised as

possible tephra-rich horizons.

5. Palaeomagnetic measurements

5.1. Alternating field demagnetisation

Characteristic remanence directions were establishedby orthogonal vector analysis of remanence measure-ments acquired during stepwise AF demagnetisation.Initial tests on pilot samples were used to establish thecoercivity range of the characteristic remanence. Thepilot samples were taken from a number of differentdepths to take account of the established down-corevariations in magnetic mineralogy. The bulk of thesamples were then processed using four demagnetisationsteps chosen within the established coercivity interval.Median destructive fields generally fall between 15 and35mT, with samples from RF93-30 showing a narrowerrange of B25–35mT. The more strongly magnetisedsamples maintained palaeomagnetic stability up to80mT, but the weaker samples showed reduced stabilityas their remanence approached the magnetometer noiselevel after fewer demagnetisation steps. The lowferrimagnetic content of zone-3 lake samples resultedin the majority failing to yield stable directions.Examples of orthogonal vector plots are shown inFig. 12.

5.2. Secular-variation records

The palaeomagnetic data are displayed in Fig. 13. Theopen symbols are the raw data while the solid linerepresents a best fit using a Savitsky–Golay (Savitskyand Golay, 1964) smoothing filter, which fits a low-order polynomial to a fixed number of data pointswithin a moving window. This smoothing procedureprovides better recovery of true peak amplitude andwidth relative to a simple moving average. Core breaksare indicated by horizontal broken lines and the localgeocentric axial dipole (GAD) inclination of 61� ismarked with a vertical, broken line. The mean NRMinclination of each site is shown as a shaded zone, with awidth equal to the inclination standard deviation.Scattered data from the regions of low ferrimagneticcontent in the lake cores have not been included, whilethe P.ALB94-1E record has been extended to include thedepth interval that contains a Younger Dryas pollensignal (Lowe et al., 1996). This extension to the recordwill allow comparison with the lowest metre of corePAL94-9.Complete declination records were achieved by the

rotation of individual core sections to align declinationtrends across core breaks. Before this manipulation, weremoved any large declination changes occurring withina few centimetres of the core breaks. This subjectiveprocedure may fail to distinguish between declinationchanges that reflect true geomagnetic change anddeclination change produced by subtle rotations that

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W

N, UP

N

E, UP

N

E, UP

N

E, UP

WN W N

P.ALB94-1E139 cm

P.ALB94-1E346 cm

P.NEMI94-1B250.5 cm

P.NEMI94-1B262.5 cm

E, UPE, UP N, UP N, UPPAL94-9263 cm

RF93-30615 cm

RF93-30411 cm

RF93-3048.5cm

Max. Field 50 mT

Max. Field 110 mT

Max. Field 40 mT

Max. Field 40 mT

Max. Field 70 mT

Max. Field 70 mT

Max. Field 80 mT

Max. Field 40 mT

NRM=3.5 mAm-1 NRM=26.8 mAm-1 NRM=48.8 mAm-1 NRM=41.0 mAm-1

NRM=5.6 mAm-1 NRM=12.1 mAm-1 NRM=16.8 mAm-1NRM=9.7 mAm-1

Fig. 12. Normalised orthogonal (Zijderveld) projections of NRM vectors during AF demagnetisation. Sample declinations are with respect to the

core section fiducial mark, rather than geographic coordinates. Open (closed) symbols represent the horizontal (vertical) projection of the NRM

vector. The initial NRM intensity and the maximum demagnetisation field are given for each sample.

-30 0 30 60 90-30 0 30 60 900

100

200

300

400

500

600

-30 0 30 60 90 -30 0 30 60 90

DECLINATION

Dep

th (

cm)

Pal94-9RF93-30P.ALB94-1EP.NEMI94-1B

20 40 60 8020 40 60 8020 40 60 80 20 40 60 80

INCLINATION

Pal94-9RF93-30P.ALB94-1EP.NEMI94-1B

Fig. 13. SV curves for the lake and marine cores. Horizontal broken lines indicate core breaks and vertical broken lines define the GAD inclination

for each site. The vertical shaded bands define the core mean (71s) inclination values. The solid line fitted to each data set was obtained using a

Savitsky–Golay smoothing filter.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–1722 1711

can occur during, or after, coring. For example, justbelow 200 cm in the P.NEMI94-1B record, a westerlyswing that occurs in the upper 20 cm of Section 2 is notconsistent with the behaviour seen in the base of Section1. In this particular case the swing has been attributed tocore twisting and the points have not been used in thesmoothing operation. Construction of core inclinationrecords involved only the removal of abrupt inclinationswings localised at core breaks. As with the declination

records, this subjective process may fail to distinguishbetween rapid geomagnetic change and sediment dis-ruption.The upper section of both the P.ALB94-1E and

P.NEMI94-1B SV curves were produced by combiningdata from the long cores with data from a short corecollected at the same station. The process is illustratedfor the Nemi curve in Fig. 14, where the depth scale ofthe long core (P.NEMI94-1B) has been shifted to

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20 40 60 80-30 0 30 600 25 50 75 1000

100

0 10 20 30NRM (10-3 Am-1) DECLINATION INCLINATIONk (10-5 SI)

Dep

th (

cm)

Fig. 14. Illustration of the final correlation achieved between the

upper part of the Lake Nemi long core (P.NEMI 94-1B) with an

adjacent short core (NEMI 2). The correlation process uses the rock-

magnetic data, supported by the palaeomagnetic data. Open (closed)

symbols represent data from NEMI 2 (P.NEMI 94-1B).

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–17221712

provide the best visual correlation of the palaeomagneticand mineral magnetic data. In the final composite SVrecord, the short-core (NEMI 2) depth scale has beenstretched (from 100 to 137 cm) to fit the long-core depthscale. The declination record obtained from Section 1 ofthe Albano long-core (P.ALB94-1E) showed evidence ofsediment deformation, changing smoothly by more than90� in the top 68 cm of the section and then suffering anabrupt B40� jump. This behaviour suggests coretwisting and the Section 1 data was discarded in favourof the short-core record. However, there is still a notableincrease in palaeomagnetic scatter in the compositeupper section of the Albano record. This may reflectsediment disturbance during coring, or during the sub-sampling procedure, and therefore correlations are morespeculative for this core interval. The upper 29 cm ofRF93-30 were used for 210Pb dating (Oldfield et al.,2003), and material was not available for magneticanalyses. The remainder of the upper section of RF93-30 shows considerable scatter in declination data, andthe inclination values are rather high for this latitude.Again, any palaeomagnetic correlations within thisinterval are more speculative.

5.3. Core correlations and chronologies

The establishment of time relationships between thefour sites requires intercorrelations, and ultimatelychronologies to be developed that are independent ofthe palaeoclimatic records of the cores. Our approachhere is to utilise the position of the 4.1 kyr BP AvellinoTephra (lake cores), calibrated AMS14C dates (Adriaticcores, Langone et al., 1996), and correlations based onthe SV records. The calibrated 14C dates are shown intheir stratigraphic locations in Fig. 15 (we use here only

the two accepted AMS14C dates for RF93-30, Oldfieldet al., 2003). Further age control points are provided bycomparison of our SV curves with the UK Holocene SVmaster curve (Turner and Thompson, 1982). Archae-omagnetic data for France (Daly and LeGoff, 1996;Gallet et al., 2002) and palaeomagnetic results fromhistoric Mt. Etna lavas (Rolph and Shaw, 1987) havebeen used to further refine the core chronologies for thelast 3000 years (Fig. 16). The UK SV master curve,archaeomagnetic and Mt. Etna data have been trans-formed to the mean geographic location of the core sitesusing the method of Noel and Batt (1990). Thistransformation compensates for the influence of sitelatitude on SV amplitudes (Itota et al., 1997).Inclination shallowing can be a problem for sedimen-

tary palaeomagnetic records (e.g. Verosub, 1977), so ourcorrelation process involves matching patterns of varia-tion, rather than magnitudes. Nevertheless, the inclina-tion records should provide a more robust correlationsince inclination will be less affected than declination bydifferential rotation between sections, or by twisting ofthe sediment during the coring phase. The inclination-based correlation was then evaluated by transferring thecorrelation tie lines to the declination profiles. Minoradjustments were then made using declination featuresthat could be clearly traced between the cores.Correlations between sites, and with the UK SV

master curve and archaeomagnetic data are shown inFigs. 15 and 16; the characters used to label SV featureson the UK SV and archaeomagnetic curves are takenfrom Turner and Thompson (1982). To improve clarity,the raw data points have been excluded from the SVcurves. The inclination data provide the best correlationwith the reference curve, in particular the data fromP.NEMI94-1B and RF93-30, both of which display UKSV master curve features a to Z (and possibly y). Thebasal sediment (below 590 cm) of RF93-30 represents acondensed sequence (Trincardi et al., 1996), whichmakes correlation of this core interval more equivocal.The most recent sediment appears to be missing fromcore PAL94-9, but reasonable correlations are possiblewith a number of inclination features from b to i andpossibly also m and n: However, between features i and mcorresponding to the interval that contains the sapropel,the inclination record of PAL94-9 correlates poorly withthe other inclination records. The identification ofdeclination features is typically less certain, with theexception of features a, d, e and f. However, within thesapropel interval, it is the declination, rather than theinclination record, which correlates more convincinglywith the UK master curve. The lower part of theP.ALB94-1E record contains the Younger Dryas stadialbetween 590 and 630 cm, and we have attempted tocorrelate this lower interval with both the PAL94-9 andthe UK records. The original chronology for the UKrecord gives a date of 10 kyr for the base. However, our

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ζ

-30 0 30 600

100

200

300

400

500

600

-30 0 30 60 -30 0 30 60-30 0 30 60 -30 0 30 600

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

40 50 60 700

100

200

300

400

500

600

40 50 60 70 40 50 60 70 40 50 60 70 40 50 60 700

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

DECLINATION

INCLINATION

Pal94-9RF93-30P.ALB94-1EP.NEMI94-1B UK

9450

7210 ± 60

5570 ± 80

1790 ± 80

AvellinoTephra

11165

a b

de'

e

f

gg'

h'

h

i

9450

11165

7210 ± 60

1790 ± 80

AvellinoTephra

YD

α

γδ

ε

η

κ

λ

ι

θ

µν

β

Dep

th (

cm)

Dep

th (

cm)

Yea

rs B

PY

ears

BP

YD

9220 ± 130

9220 ± 130

5570 ± 80

sapropel

6180 ± 80

sapropel

3660 ± 100

3660 ± 100

6180 ± 80

ε′

c

Pal94-9RF93-30P.ALB94-1EP.NEMI94-1B UK

?

Fig. 15. Comparison of the smoothed lake and marine SV records with the published UKHolocene master curve (Turner and Thompson, 1981). The

UK data have been transformed to the site latitude of the sediment cores. The sub-horizontal lines indicate proposed correlations of individual

features. AMS 14C dates are indicated for the two Adriatic cores and the position of the Avellino Tephra is identified for the lake cores. The two 14C

dates on the UK curve have been recalculated from the original data (Turner and Thompson, 1981) using Calib4.3 (Stuiver and Reimer, 1993).

Shaded regions indicate the Younger Dryas interval in the Albano core and the sapropel interval in core PAL94-9.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–1722 1713

correlations with the P.ALB94-1E and PAL94-9 recordsimply an older base for the UK record. To investigatethese older correlations, we attempted a recalibration(using Calib4.3; Stuiver and Reimer, 1993) of theuncalibrated radiocarbon dates used to establish thechronology of the UK SV master curve (Turner and

Thompson, 1981). In the original study, Turner andThompson (1981) assumed that calibration changedsmoothly between 6500 and 10,000 yr BP, with zerocorrection at the latter date. After recalibration, thecalendar ages of inclination features m and n on the UKSV master curve are significantly older (increasing from

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-30 0 30 600

100

200

300

400

500

600

-30 0 30 60 -30 0 30 60-30 0 30 60

40 50 60 700

100

200

300

400

500

600

40 50 60 70 40 50 60 70 40 50 60 70

DECLINATION

INCLINATION

PAL94-9RF93-30P.ALB94-1EP.NEMI94-1B

5570 ± 80

1790 ± 80

AvellinoTephra

5570 ± 80

1790 ± 80

AvellinoTephra

-30 0 30 600

1000

2000

3000

4000

5000

40 50 60 700

1000

2000

3000

4000

5000

Dep

th (

cm)

Dep

th (

cm)

Yea

rs B

PY

ears

BP

β

α

γ

δ

abc

d

e

f

ε'

Archaeomagnetic Data

PAL94-9RF93-30P.ALB94-1EP.NEMI94-1B Archaeomagnetic Data

Fig. 16. Comparison of the smoothed lake and marine SV records with archaeomagnetic data for France (solid line, Daly and LeGoff, 1996; broken

line, Gallet et al., 2002) and Sicily (solid symbols, Rolph and Shaw, 1987). The archaeomagnetic data have been transformed to the site latitude of the

sediment cores. The lines indicate proposed correlations of individual features.

T.C. Rolph et al. / Quaternary Science Reviews 23 (2004) 1699–17221714

8800 to 9450 yr BP for m and from 9700 to 11,165 yr BPfor n). If we accept these revised dates (marked on theUK SV master curve in Fig. 15), the UK record extendsto B11,500 yr BP, coincident with age estimates of theYounger Dryas–Holocene transition (Gulliksen et al.,1998).For PAL94-9, there are notable discrepancies between

the calibrated 14C dates and the age of the relevant

sediment interval as estimated from the palaeomagneticcorrelation. The most notable inconsistencies occur forthe calibrated dates of 1790780 and 92207130 yr BP.While an acceptance of the recalibrated ages for the UKSV master curve will reduce the significance of the latterdiscrepancy, we also need to consider the possibility thatthe NRM acquisition process results in the SV recordsof our sediment cores lagging behind the age of the

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sediment. The question of delayed remanence acquisi-tion is explored further below.

5.4. Resolution and precision of the secular-variation

record

There are significant differences between the averageinclination values of the four sites (see Fig. 13).P.NEMI94-1B and P.ALB94-1E give average values of49.4�76.9� and 50.8�79.7�, respectively, while RF93-30 and PAL94-9 give average values of 58.6�79.5� and60.9�77.3�. These differences are notable because thereis only a 0.5� variation in latitude between the sites.P.NEMI94-1B and RF93-30 encompass near identicaltime intervals, and have near identical accumulationrates, so it does not appear that the inclinationdifferences can be attributed to geomagnetic behaviour,or to the time window available for post-depositional re-orientation (Verosub, 1977). Comparisons with theGAD inclination (61�, see Fig. 13) suggest thatinclination shallowing has occurred in the lake sedi-ments, but not in the Adriatic cores (in particularPAL94-9). We also note that the Adriatic cores displaythe largest ARM-based ratios, suggesting a greater roleplayed by bacterial magnetite in the magnetisation ofthese cores. A link between magnetic grain-size andinclination shallowing is consistent with the concept of amore efficient acquisition of post-depositional rema-nence (PDRM) by small ferrimagnets, which are betterable to rotate within water-filled pore space before de-watering and sediment compaction (cf. Verosub, 1977).However, a recent reassessment of sediment magnetisa-tion (Katari et al., 2000) indicates that such a realign-ment mechanism is unlikely because flocculation andaggregation will incorporate detrital magnetite withinlarger aggregates. This does not specifically address thecase of magnetosomes, which are formed authigenicallyby bacteria inhabiting micro-aerophilic to anaerobicconditions (cf. Bazylinski and Moskowitz, 1997).Depending on the location of the oxycline, thesemagnetosomes may form above or below the sedi-ment–water interface. In the latter case, they formwithin the sediment pore-space, rather than becomingincorporated in the sediment after death as a componentof detrital input. Magnetosomes display a large dipolemoment, sufficient to produce their passive orientationalong geomagnetic field lines. The ARM/SIRM valuesobserved in PAL94-9, and within some regions of RF93-30, are typical of values associated with intact magneto-somes (these intact chains of bacterial magnetiteparticles yield ARM/SIRM values between 0.15 and0.25, Moskowitz et al., 1993). Individual bacterialmagnetite particles vary in size between 30 and120 nm, and many types of bacteria produce magnetiteparticles that have aspect ratios of 2:1 or smaller (cf.Yamazaki and Kawahata, 1998). These characteristics

should limit mechanical re-orientation during sedimentcompaction, provided the bacterial magnetite particlesdo not become attached, post-mortem, to the sedimentmatrix (Deamer and Kodama, 1990; Lu et al., 1990;Katari et al., 2000). Some support for this suggestioncomes from a mathematical model of compactionrelated inclination shallowing for a bimodal distributionof magnetite grains (model 1c of Arason and Levi,1990). This model produces an inverse relationshipbetween inclination shallowing and the ratio of equidi-mensional grains to acicular grains. Magnetosomeparticle morphology approximates the former, suggest-ing that sediments with a greater relative proportion ofmagnetosome particles will provide a more faithfulrecord of magnetic inclination. However, if the magne-tosomes are produced within the sediment, then theremanence contributed by the magnetosomes will post-date the host sediment. Moreover, it is clear that such aPDRM acquisition process will also tend to smooth thegeomagnetic signal, thereby reducing the amplitude ofSV features (Lund and Keigwin, 1994). The amount ofgeomagnetic smoothing and the time lag of themagnetosome remanence will be dependent on thesediment accumulation rate and the size of the sedimentdepth interval within which magnetotactic bacteria areactive. Core PAL94-9, which has the greatest proportionof magnetosomes, also shows the smallest SV amplitude,supporting this hypothesis.

6. Discussion

6.1. Origin of the magnetic minerals and controls on their

stratigraphic variation

At the four sites, the mineral magnetic results haveidentified significant stratigraphic variations in thequantities, proportions and average grain sizes of theconstituent magnetic minerals. In core RF93-30, oppo-site patterns in the variation of kARM and HIRMsuggest that an increase in the ultrafine ferrimagneticfraction (kARM component) is accompanied by adecrease in the canted-antiferromagnetic fraction.TEM images show that magnetosome particles arepresent throughout RF93-30 (Gibbs, 2000), and weattribute the opposite trends in kARM and HIRM tochanges in the magnetosome and lithogenic magneticcomponents, respectively (Oldfield et al., 2003). Asnoted earlier, ARM/SIRM ratios between 0.15 and 0.25are characteristic of intact magnetosomes (Moskowitzet al., 1993) and in RF93-30 the lower limit of thisARM/SIRM range is exceeded within the zones ofhighest kARM values. We do not have TEM images forcore PAL94-9, but in the upper 450 cm the ARM/SIRMratio consistently exceeds 0.22, except within the intervalcontaining the possible tephra (200–250 cm). These high

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ARM/SIRM values are consistent with magnetosomesdominating the magnetic content of this second Adriaticcore. The SV correlations show that site PAL94-9 hasapproximately half the sediment accumulation rate ofthe RF93-30 site. The difference in ARM-based ratiosbetween the two Adriatic sites may, therefore, largely bea dilution effect, in which the increased lithogenic inputat the RF93-30 site leads to a greater dilution of themagnetosome content.Magnetosomes have been identified within zone 4 of

P.ALB94-1E (Gibbs, 2000), where they dominate thesediment magnetic properties. Zone 4 has the largestARM-based ratios measured in the Holocene lakesediments and, unlike zones 1 and 2, the thermomag-netic curve obtained from a zone 4 sample produced asingle, magnetite Curie point (Fig. 6; sample A530 cm).A coarse-grained, soil-derived titanomagnetite phasedominates the magnetic properties of much of theremainder of the P.ALB94-1E Holocene record, withmagnetosomes playing a minor role. The low-tempera-ture susceptibility data suggest that catchment soils alsocontain an ultrafine (SP) ferrimagnetic phase, andmaghemite is the ferrimagnet most commonly producedduring pedogenesis (Schwertmann and Taylor, 1989).Conversely, the stoichiometric nature of the fineferrimagnetic component identified in zone 4 is con-sistent with a bacterial origin (e.g. Bazylinski andMoskowitz, 1997).The stratigraphic variation of magnetic parameters at

the four sites thus defines relative and absolute changesin the magnitudes of these two magnetic components.The lithogenic component provides a measure ofsediment input and thus relates to environmentalconditions in the catchment, while the bacterial compo-nent reflects environmental conditions at the core sites.The cores also display isolated peaks in magneticparameters that can be associated with tephra-richhorizons and, within the lake cores, magnetic peaksthat possibly represent micro-turbidite horizons (in thelatter case, magnetic peaks are accompanied by equallyabrupt decreases in water and organic matter content,Ryves et al., 1996). In addition, environmental condi-tions at the core sites have modified sediment magneticproperties through the sub-oxic process of reductivediagenesis. Redox-driven processes depend on factorsthat include sediment accumulation rate, sedimentorganic matter content, the availability of both ironand sulphate (Berner, 1984), and the grain size andcomposition of the iron minerals (Canfield et al., 1992;Morse and Wang, 1997). These redox processes havebeen active in zone 3 of both lake cores and in theweakly magnetised interval of PAL94-9 (455–515 cm).The latter corresponds to sapropel layer S1 (Oldfieldet al., 1995; Alvisi and Vigliotti, 1996), which is anorganic-rich layer produced during the early Holocene(Ariztegui et al., 2000). Magnetic data from these redox-

influenced zones indicate a relative increase in thecanted-antiferromagnetic content, and an increase inmodal ferrimagnetic grain-size. Coarsening of theferrimagnetic grain-size is a result of the preferentialdissolution of the fine iron fraction, which in these coreslargely consist of pedogenic iron oxides/hydroxides and/or magnetosomes. In magnetic terms, the majorinfluence will be from the decrease in the ferrimagneticcontent, since ferrimagnets will dominate the magnetisa-tion of the fine magnetic fraction. In the case of thesapropel layer, the relative increase in the canted-antiferromagnetic component is partly due to anabsolute increase in this component (see Fig. 9). Anincrease in the canted-antiferromagnetic componentmay reflect a change in sediment composition, perhapsdue to the increased fluvial delivery of terrestrialmaterial during this interval. An increased atmosphericinput of haematite-rich dust seems unlikely in view ofthe greater precipitation of the early Holocene, whichwould tend to have stabilised the dust source areasthrough increased vegetation cover. Alternatively, theincreased HIRM values may reflect the formation ofiron oxyhydroxides through diagenetic modification ofthe sapropel (Higgs et al., 1994). However, some cautionmust be shown in the interpretation of the HIRM data,since it has been revealed that detrital magnetite cancontribute to the IRM region above 300mT (Kruiverand Passier, 2001).

6.2. Comparisons between magnetic parameters and

biological and geochemical proxies

Correlations between magnetic parameters and in-dependent environmental indicators provide a means ofidentifying, or confirming, the links between magneticdata and environmental conditions. For example,Vigliotti et al. (1999) have already noted a strongpositive correlation in the Nemi core between bulkmagnetic parameters and the abundance of Cyclotella

ocellata, a diatom taxon with a low tolerance for theoxygen-poor conditions that are prevalent duringintervals of reductive diagenesis. Identifying similarcorrelations will provide a measure of the utility ofour magnetic records as a proxy for environmentalchange.In the lake cores, zone 4 represents the earliest

Holocene, from B11.3 to B9.5 kyr BP. Bulk magneticparameters in this zone show a decreasing magneticcontent (Fig. 2) combined with an increasing contribu-tion from ultrafine ferrimagnets (Fig. 3, increasingvalues of ARM/SIRM). These data imply a decreasingcontribution from the coarser, catchment-derived mate-rial, which is supported by the decrease in this zone ofboth major and trace elements of lithogenic origin infavour of elements hosted by organic matter (Calanchiet al., 1996). A decrease in catchment material reaching

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the coring site could be a consequence of rising lakelevels, which make the lake margins more distant fromthe site, or stabilisation of the catchment slopes throughincreasing vegetation. There is evidence to support bothmechanisms. Increasing catchment stabilisation is sup-ported by a doubling of the percentage of tree pollenthrough this interval (Lowe et al., 1996), while theobserved increase in zooplanktonic cladocera and non-littoral chironomids (Ryves et al., 1996) and evidence ofincreasing bottom-water anoxia (Ariztegui et al., 2001),are both consistent with rising lake levels. The increasingbottom-water anoxia is not responsible for the gradualdepletion of the zone 4 magnetic content because we donot see the coarsening of magnetic material that wouldresult from the preferential dissolution of the finerfraction by reductive diagenesis. However, the observa-tion that maximum anoxia is established at the top ofzone 4 (Ariztegui et al., 2001), is consistent with theextremely low magnetic content of zone 3.In PAL94-9, the interval 610–530 cm (equivalent to

zone 4) shows an increasing KARM value (and ARM-based ratios), accompanied by a general decrease inHIRM. Thus, a decrease in modal magnetic grain-sizewithin this interval results from an increase in magneto-some concentration together with a decrease in thelithogenic component.The early mid-Holocene (9–6 kyr BP) was a period of

enhanced rainfall for the Mediterranean region (Kallelet al., 1997). This enhanced rainfall generated a largefreshwater influx that produced water-mass stratifica-tion (Ariztegui et al., 2000), leading to widespreadanoxia and the formation of an organic-rich layer,sapropel S1. In core PAL94-9, the interval of decreasedferrimagnetic content is bracketed by calibrated AMS14C dates of 92207130 and 7210760 yr BP, supportedby the palaeomagnetic correlation in Fig. 15. Thesedates are compatible with an estimated duration for S1of 9.0–6.8 kyr BP (Ariztegui et al., 2000), and 9.6–6.4 kyr(Aksu et al., 1999). The increased levels of precipitationduring this interval led to higher lake levels in theMediterranean region and North Africa (cf. Magnyet al., 2002). Higher lake levels usually lead tostratification of the water column, producing bottom-water anoxia, and the dissolution of the magneticminerals seen in both our lake records. In otherEuropean lakes (e.g. Loch Lomond, Turner andThompson, 1981; Lac du Bouchet, Creer et al., 1986;Lago di Mezzano, Brandt et al., 1999), the early mid-Holocene sediments show a similar pattern of magneticchange to that seen here, implying that the environ-mental conditions driving anoxia in Nemi and Albanowere influential over a significant geographic region.In both lakes, zone 2 begins with an abrupt increase in

all bulk magnetic parameters, although in P.NEMI94-1B this behaviour is not fully established until B500 cmdepth, coincident with the unit II–unit I lithological

boundary. The subsequent magnetic peaks in zone 2 aresuperimposed on a gradually decreasing baseline (Fig.2). In P.ALB94-1E, both the peaks and the backgroundtrend correspond with the pattern of variation displayedby the catchment-derived major and trace elements inthe core (Calanchi et al., 1996). Thus, the patterndisplayed by the bulk magnetic parameters in zone 2 canbe related to variation in the delivery of catchmentmaterial at the core site combined with increasing watercontent (Ryves et al., 1996). Geochemical data are notavailable for P.NEMI94-1B but we assume that the zone2 behaviour is caused by the same factors as thoseascribed to P.ALB94-1E. In P.NEMI94-1B, however,the magnetic concentration indices show a greaternumber of abrupt peaks, suggesting episodes of abruptsediment delivery at the site. In P.ALB94-1E, the zone 2behaviour of the kARM data differs subtly from the k

and SIRM data, most notably between 220 and 300 cm.Here, the kARM data imply a more gradual decrease inthe ultrafine fraction compared with the coarse fraction.The kARM behaviour is clarified by the magnetic ratios,which show that the decrease in catchment material isaccompanied by a small increase in the proportion ofultrafine ferrimagnets (magnetosomes) throughoutP.ALB94-1E zone 2. In P.NEMI94-1B, the magneticratios show a number of maxima and minima, but thebackground is essentially invariant. These maxima andminima in magnetic ratios correlate, respectively, withminima and maxima in bulk magnetic parameters. Thiscorrelation is consistent with the maxima in bulkmagnetic parameters corresponding to an abrupt inputof coarser-grained catchment material, while maxima inthe magnetic ratios correspond to a relative increase inthe magnetosome contribution at times of low catch-ment input. The pollen records for P.ALB94-1E andP.NEMI94-1B (Lowe et al., 1996) show significantdecreases in tree pollen at B500, 450, 390, 250 and210 cm in P.NEMI94-1B, and at B320 and 150 cm inP.ALB94-1E. These tree pollen minima correspond wellwith the peaks in bulk magnetic parameters, illustratinga direct link between catchment destabilisation (devege-tation driven by climate or human activity) andsediment delivery to the core sites. The disparity in thenumber of peaks seen at the two core sites thereforeappears to reflect differences in catchment use.The palaeomagnetic correlation (Fig. 15) puts the

onset of zone 2 in P.NEMI94-1B (550 cm) at slightlyyounger than 5000 yr BP. This is consistent with datesfor the lowering of lake levels in the western Mediterra-nean (5000–4000 yr BP) and northern Africa (5500–4500 yr BP) (Magny et al., 2002). In P.ALB94-1E,palaeomagnetic correlation and the position of the4.1 kyr Avellino Tephra implies that in this core thereis some 500–1000 years of record missing from zone 2.All cores collected from site 1 in Lake Albano show ahigh degree of correspondence between their magnetic

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susceptibility records, indicating that a gap in thesediment record is a characteristic of the site. Compar-ison with the magnetic susceptibility records of bothshallower- and deeper-water Albano cores shows thatthese other sites have a more complete record of zone 3sediment (Rolph et al., 1996). Therefore, it appears thata mid-Holocene interval of non-deposition (or subse-quent erosion) is spatially restricted within the lake. Asnoted above, in P.NEMI94-1B the typical character ofzone 2 is established at 500 cm, giving a date for theonset of deforestation of about 4300 yr BP.We position the zone 3–zone 2 boundary in the

Adriatic at the top of the sapropel interval, rather thanat a depth equivalent to the age of the boundary(B5 kyr) defined in the lakes. We thus define it as anenvironmental, rather than chronological, boundary.Contrasting with the lake cores, in the Adriatic themagnetic character of this zone varies quite smoothly(Figs. 9 and 10), exceptions being narrow intervalsattributed to possible tephra layers. In RF93-30, theinterval equivalent to zone 2 extends from the base ofthe core up to a depth of B180 cm. All bulk magneticparameters decrease towards the top of this interval,which at least in part will reflect the increasing watercontent of the sediment. Two intervals of increasedmagnetosome contribution (higher kARM and kARM-based ratios) occur firstly as a broad zone spanning thelowermost metre of sediment and secondly as a peakcentred at 280 cm. Both peaks are interrupted by narrowminima attributed to the identified tephra at B525 cm,and a possible tephra layer at B295 cm (Fig. 10).Significantly, the stratigraphic pattern displayed by theARM-based parameters is matched very closely by thevariation in total benthic-foram concentration withinthe core (Asioli, 1996). The latter has been linked(Asioli, 1996) to fluctuations in bottom-water oxygena-tion and organic matter load that occur in response to achanging fluvial influence (Jorisen, 1988). Conversely,the stratigraphic pattern displayed by the benthic foramspecies Cassidulina laevigata/carinata matches the varia-tion in the magnitude of the hard IRM component. Thecurrent distribution of C. laevigata/carinata shows apreference for the more eutrophic regions of theMediterranean (de Rijk et al., 2000). Thus, maxima inhard IRM and in C. laevigata/carinata correspond toperiods of depleted oxygenation and increased litho-genic input driven by an increasing fluvial influence atthe site. Conversely, maxima in the total count ofbenthic forams and in the ARM-based parametersreflect intervals of increased bottom-water oxygenationcorresponding to a decreased fluvial influence. Theserelationships show that a decrease in the magnetosomecontent occurs at times of increased fluvial influence,possibly due to an increase in redox-driven dissolutionproduced by organic loading, or to less favourableconditions for the bacteria. The observation that the

bulk ARM also decreases in phase with the benthicforam count suggests another alternative, which is asimple dilution of authigenic components by increasingallochthonous sediment supply. This increased sedimentsupply is coincident with a decrease in the tree pollenconcentration recorded within the core: a decreaseattributed to forest clearance that started in the LateBronze Age (Lowe et al., 1996; Oldfield et al., 2003).Deforestation would have reduced catchment stabilityon the Italian mainland, leading to an increasedsediment flux at the core site (Oldfield et al., 2003).The increase in sediment supply starts at 500 cm,indicated by a rapid rise in %HIRM (Fig. 10). Thepalaeomagnetic correlation (Fig. 15) dates this increaseat B3500 yr BP, some 800 years later than the firstrecorded impact of mid-Holocene deforestation in thelakes.Zone 2 of PAL94-9 starts at B7000 yr BP, and shows

a high bacterial contribution throughout, albeit with asmall, upward-decreasing trend in this component (Fig.10). This gradual decrease in ARM-based ratios isaccompanied by a gradual increase in hard IRM and anincrease in lithogenic major and trace elements (Calan-chi et al., 1996). The palaeomagnetic correlation (Fig.15) links the minor peak-trough-peak in kARM, between300 and 170 cm, with a similar pattern in RF93-30between 520 and 280 cm, although an inverse-correlative(with kARM) HIRM pattern is absent from PAL94-9.This absence of an inverse kARM: HIRM correlationsuggests that factors other than lithogenic flux areimportant in modulating the bacterial component at thePAL94-9 site.In both lakes the zone 2–zone 1 boundary is dated at

B1000 yr BP. Magnetic zone 1 correlates with the upperhalf of pollen sub-zone IIId (Lowe et al., 1996), a sub-zone characterised by an increased contribution ofpollen derived from cultivated species such as Juglans

and Castanea (walnut and chesnut). In P.ALB94-1E, theonset of zone 1 coincides with an abrupt shift to acoarser magnetic grain-size and an increased canted-antiferromagnetic component, while the bulk (ferri)-magnetic parameters have the lowest values outside ofzone 3. Consequently, the interparametric ratios arecloser to the catchment signal than in the preceding zone2. In combination, these data suggest that limitedferrimagnetic dissolution of the finer ferrimagneticcontent has occurred, including both the bacterialmagnetite and the finer fraction of the catchmentcomponent. Reductive diagenesis in zone 1 is consistentwith an average organic content of more than 20% ofdry mass together with increased levels of photosyn-thetic sulphur bacteria that suggest high levels ofproductivity (Ryves et al., 1996). In P.NEMI94-1B, themagnetic data imply little change in magnetic character-istics from the preceding zone, except that the largeamplitude variations in magnetic concentration indices

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common in zone 2 are not present in zone 1. Thisreduction in bulk-parameter variance fits well with thepollen record (Lowe et al., 1996), which for P.NEMI94-1B shows a gradual decrease in tree pollen but withoutthe dramatic minima seen in zone 2. The magnetic datado not support selective dissolution in P.NEMI94-1B,although sulphur bacteria are present at levels higherthan seen in P.ALB94-1E. One explanation may be thelower organic content in P.NEMI94-1B (averagingB10% of dry mass in zone 4), since ultimately redoxprocesses are driven by the availability of labile organicmatter (Berner, 1984). In the uppermost part of zone 1,both lake cores show a gradual increase in magneticconcentration indices. This sediment interval corre-sponds approximately to the last two centuries, andthe narrow peaks seen in the P.NEMI94-1B datamay correspond to turbidite layers, which havebeen identified in the upper B50 cm of Lake Nemisediments (Masaferro et al., 1993). In P.ALB94-1E,the increasing values for magnetic parameters in theuppermost sediment coincide with a small increase inthe ARM-based ratios implying that at least part ofthe increasing magnetic content is provided by magne-tosomes.In the Adriatic cores, zone 1 is associated with an

initial increase in SIRM, kARM, and k (less obvious inthe k data for RF93-30), followed by a gradual decreasein all three parameters in response to increasing watercontent. This decreasing trend is less obvious in RF93-30 because of peaks present at 120 and 50 cm. InPAL94-9, the top 30 cm shows a small increase in the SPfraction (ARM/SIRM remains steady, but kARM/k andSIRM/k both decrease) and in the HIRM fraction. Thisincrease in both the SP and HIRM fractions iscompatible with an increased delivery of soil-derivedmaterial to the site: oxidised soils typically havingenhanced SP fractions and canted-antiferromagneticiron oxide contents (e.g. Mullins, 1977; Maher andTaylor, 1988; Schwertman and Taylor, 1989). Geo-chemical data support this conclusion, with the upperpart of the core showing an increased contribution fromFe2O3, and an increase in the Rb:K2O ratio (Calanchiet al., 1996). The latter ratio provides an indication ofthe degree of weathering intensity of the lithogenicfraction (Ishiga et al., 2000): the increase seen hereimplying the delivery of increasingly weathered materialto the site. In RF93-30, both the ARM-based ratiossuggest a slight increase in magnetic grain-size throughzone 1, although in the lower part of the zone the rate ofdecrease is somewhat greater for kARM/k than forARM/SIRM. This subtle difference in the two ARM-based ratios is consistent with a decrease in magneto-some content accompanied by an increase in the SPcontent. In common with core PAL94-9, we interpretthis as a signal of increased soil input, supported by anincrease in the canted-antiferromagnetic component

that sees core-maximum %HIRM values at the top ofzone 1. In RF93-30, there is a gradual decrease in thepercentage of tree pollen throughout zone 1 (Lowe et al.,1996), suggesting that deforestation is driving increasingsediment delivery to both Adriatic sites. This increasedfluvial delivery is accompanied by a decrease in thebenthic foram content (Asioli, 1996), consistent with thefluvial–benthic foram correlation displayed in zone 2.An increase in C. laevigata/carinata towards the top ofzone 1 indicates an increased trophic status, implyingthat increasing nutrient delivery accompanies theincreased fluvial component.

7. Conclusions

Mineral magnetic data indicate that the lacustrine andmarine sediment cores used in this study contain asignificant and varying remanence contribution frombacterial magnetite that coexists with a lithogenicmagnetic component derived from catchment erosion.This is consistent with suggestions that magnetotacticbacteria are ubiquitous in aquatic environments (Bazy-linski and Moskowitz, 1997). Geomagnetic secular-variation curves obtained from the cores have providedan opportunity to compare the terrestrial and marineenvironmental records within a common time frame. Wehave established clear linkages between stratigraphicchanges in biological environmental proxies and mineralmagnetic data, which confirm that the magnetic para-meters are responding to environmental and anthropo-genic controls on erosion rates and biologicalproductivity. This response takes the form of amodulation of both the biogenic and catchment-derivedmagnetic components. For example, there is a strongcorrelation between pollen records of deforestationepisodes and magnetic records of increased sedimentdelivery at the core sites. The established palaeomag-netic correlations indicate that anthropogenic treeclearance was initiated in the lake catchments atB4300 yr BP, within the Early Bronze Age, some 800years before the signals of deforestation are seen in coreRF93-30. This time lag, together with the ability of thelake records to resolve discrete deforestation events,reflects the very different sensitivity of the two catch-ment scales, with the larger and more varied catchmentof RF93-30 recording a broader pattern of changingsediment delivery. We also note that the higherprecipitation of the early to mid-Holocene in theMediterranean region is recorded as intervals ofsediment anoxia in both the lake records and inPAL94-9. In the lakes the anoxic conditions terminatedat B5000 yr BP, some 2000 years later than at theAdriatic site, and coincident with the timing of dramaticdecreases in lake levels within the Mediterranean region(Magny et al., 2002).

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Finally, our magnetic remanence data show that thecore-average inclination in the lakes is some 10� belowthe GAD value, but that inclination shallowing is absentfrom the Adriatic cores. We propose that the lack ofinclination shallowing in the Adriatic cores is a result ofthe greater contribution that magnetosomes make to theAdriatic NRMs. Magnetosome chains are designed toproduce a passive alignment of the bacterium with thelocal geomagnetic field, and their introduction to thesediment some time after its deposition will reduce theinfluence of compaction. However, their post-deposi-tional introduction to the sediment will tend to smooththe palaeomagnetic record, where the degree of smooth-ing will depend on the relative contribution that themagnetosomes make to the sediment NRM and thelength of time that magnetotactic bacteria colonise (andperhaps re-colonise, if redox conditions change) aparticular sediment interval. The accumulation rateand organic content of the sediment will largelydetermine this last factor. This smoothing effect maymean that, contrary to the general perception, sedimentswith high ARM-based ratios may be unsuitable forstudies of higher-frequency geomagnetic secular change.However, such sediments may be ideally suited topalaeomagnetic studies focussed on obtaining accuratepalaeolatitudes, in which the smoothing of secularvariation can be advantageous.

Acknowledgements

The results presented here were produced as part ofthe European Union funded PALICLAS project (Pa-laeoenvironmental Analysis of Italian Crater Lake andAdriatic Sediments; EU Contract No. 5V-CT 93-0267).The authors gratefully acknowledge the many colleagueswho participated in the project and whose results havecontributed to the interpretations presented here. Theauthors are particularly grateful to K. Van der Post andF. Alvisi, who contributed significantly to the magneticcomponent of the PALICLAS project. LV alsoacknowledges support for a study visit to Australiafrom the Italian CNR ‘‘short-term mobility’’ program.Finally, the authors recognise the constructive reviewsprovided by Prof. M.E. Evans and Dr. S.G. Robinson,which helped to improve the paper.

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