Evidence for short-lived oscillations in the biological records from the sediments of Lago Albano...

117Journal of Paleolimnology 23: 117–127, 2000.© 2000 Kluwer Academic Publishers. Printed in the Netherlands.

Evidence for short-lived oscillations in the biological recordsfrom the sediments of Lago Albano (Central Italy) spanningthe period ca. 28 to 17 k yr BP*

Piero Guilizzoni1, Aldo Marchetto1, Andrea Lami1, Frank Oldfield2, Marina Manca1, Claudio A. Belis1, Anna M.Nocentini1, Patrizia Comoli1, Vivienne J. Jones3, Steve Juggins4, Christina Chondrogianni5, Daniel Ariztegui5,John J. Lowe6, David B. Ryves3, Richard W. Battarbee3, Tim C. Rolph7 & Julieta Massaferro8

1C.N.R. Istituto Italiano di Idrobiologia, I-28922 Verbania Pallanza, Italy (E-mail: [email protected])2IGBP PAGES International Project Office, Bärenplatz 2, 3011 Bern, Switzerland3Environmental Change Research Centre, Department of Geography, University College London, 26 BedfordWay, London WC1H 0AP, UK4Department of Geography, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK5Geological Institute, ETH-Zentrum, Sonneggstrasse 5, 8092 Zürich, Switzerland6Centre for Quaternary Research, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK7Department of Geology, University of Newcastle, Callaghan 2308, NSW, Australia8PROGEBA-CONICET, Apartado postal 47, 8400 Bariloche, Argentina

Received 10 February 1998; accepted 7 January 1999

Key words: crater lake, late Pleistocene, pigments, diatoms, ostracods, Cladocera, chironomids, climaticoscillation, Italy

Abstract

We report the results of analyses of pigments (derived from algae and photosynthetic bacteria), diatoms andinvertebrate fossil remains (ostracods, cladocerans, chironomids) in two late Pleistocene sediment cores from LagoAlbano, a crater lake in Central Italy. The record contains evidence for oscillations in lake biota throughout theperiod ca. 28 to 17 k yr BP. The earliest of these are contained in the basal 3.5 m of light olive-gray and yellowish-gray spotted muds sampled in core PALB 94-1E from 70 m water depth. The later oscillations are best representedin the more extended sediment sequence recovered from a second core site, PALB 94-6B, in 30 m water depth.The sediments at site 1E, containing the earlier oscillations (ca. 28–24 k yr BP), predate any sedimentation at theshallower site, from which we infer an initially low lake level rising to permit sediment accumulation at site 6Bfrom ca. 24 k yr onwards. At site 6B, massive silts rich in moss remains are interbedded with laminated silts andcarbonates. These sediments span the period ca. 24 to 17 k yr and are interpreted as representing, respectively, timesof shallow water alternating with higher lake stands, when the lake was stratified and bottom water was stagnant.A range of mutually independent chronological constraints on the frequency and duration of the oscillations recordedin the lake biota indicate that they were aperiodic and occurred on millennial to century timescales. We interpretthem as responses to climate forcing through its impact on lake levels and changing aquatic productivity. The timespan they occupy, their frequency and their duration suggest that at least some of these changes may parallel boththe Dansgaard-Oeschger events recorded in Greenland Ice Cores and the contemporary oscillations in North Atlanticcirculation documented in marine sediment cores.

*This paper was presented at the 7th International Symposiumon Palaeolimnology (1997), held at Heiligkreuztal, Germany

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Introduction

The climate of the Mediterranean region is affectedby both the North Atlantic thermohaline circulation-driven climate system and by a lower latitude mon-soonal climatic regime (Imbrie et al., 1984; Kutzbach& Street-Perrott, 1985). The PALICLAS project(Palaeoenvironmental Analysis of Italian Crater Lakeand Adriatic Sediments) was initiated in part toestablish the extent to which proxy records of past-climate variability have reflected one or the other ofthese regimes. Part of the work has focussed on thesediment records preserved in two crater lakes locatednear Rome in Central Italy; Lago Albano and LagoNemi (Figure 1). Studies of the modern and recent biotaand sediment processes in these two lakes have establishedthat they are extremely sensitive to environmental changes(Masaferro et al., 1993; Niessen et al., 1993; Lami et al.,1994; Guilizzoni & Oldfield, 1996).

This paper presents some of the results obtainedfrom Lago Albano, and focuses on the analyses ofstratigraphic variations in pigments derived from algaeand from anaerobic photosynthetic bacteria, in diatomassemblages, and in invertebrate fossil remains (ostra-cods, cladocerans, chironomids) from sedimentsspanning the period between ca. 28 and 17 k yr BP. Thetemporal resolution achieved varies between decadaland century scales. The proxy palaeoecological in-dicators studied are known to be sensitive to variationsin lake productivity and pH (Sanger, 1988; Smol, 1990;Guilizzoni et al., 1992). In addition, biostratigraphicalrecords based on these proxy indicators can also be usedto reconstruct ecosystem responses to environmentalvariations, especially with respect to aquatic pro-ductivity and lake catchment surface processes which,if responding synchronously, are likely to reflectregional climatic variations (Bradbury & Dean, 1993;Leavitt et al., 1997).

The study area

Lago Albano occupies a crater in the Albano Hillscaldera, some 25 km to the south-east of Rome. It isone of a number of maar lakes in the region which wereformed by volcanic activity during the Pleistocene(Niessen et al., 1993). The lake is assumed to have beena permanent feature in what is a hydrologically-closedcrater basin, from the time that volcanic activity ceasedin the Albano Hills, ca. 30 k yr ago (Andretta &Voltaggio, 1988; Boni et al., 1995). It has maximum and

mean depths of 175 and 77 m respectively, a surfacearea of 6 km2, a catchment area of 3.7 km2, and it ismainly fed by direct precipitation and by springsbelow the lake surface. It is meromictic, with year-round anoxic conditions below 60 m water depth.More details about the site and an account of thepresent limnological and climate conditions have beengiven elsewhere (Chondrogianni et al., 1996a; Beliset al., 1998).

Field and laboratory methods

A seismic survey (3.5 kHz) was carried out to investigatethe sediment infill of the lake and to select suitable sitesfor coring (Niessen et al., 1993; Chondrogianni et al.,1996b). Sixteen sediment cores up to a length of ca.14 m were recovered from six locations (Figure 1) inJune 1994 using a Kullenberg piston corer (Kelts et al.,1986).

The analyses presented here are based on samplesobtained from two of these cores: PALB 94-1E, whichpenetrated 13.87 m of sediment below a water depthof 70 m (site 1) and PALB 94-6B, which recovered8.40 m of sediment from below 30 m of water (site 6).As will be shown, core 1E extends over the last 28 kyr but only the results of the lowermost 3.5 m arepresented here. Core 6B is limited to the periodbetween ca. 17 and 24 k yr BP (Figure 1). The resultsfrom two separate cores are chosen for presentationhere largely because they offer the highest temporalresolution for the time interval considered. Moreover,the full range of proxy records obtained from them,only some of which are presented here, suggest that,largely because of their different water depths, theywere sensitive to environmental change at differenttimes. Within the context of all the data obtained sofar, for which Guilizzoni & Oldfield (1996) providea preliminary synopsis, they thus provide the bestcombination of sensitivity and temporal resolutionavailable.

Sub-samples of 10–15 g wet mass (sediment slicesof 2 cm, or less, thickness) were removed at ca. 10 cmintervals, with closer sampling over core segmentsdisplaying marked changes in lithology. The organicmatter (OM) content of the sediments was determinedby loss-on-ignition (LOI) at 550 °C (Dean, 1974). Totalchlorophyll derivatives (CD) were extracted using 90%acetone, and aliquots of ca. 10 ml were used for theseparation and measurement of single pigments suchas β-carotene (present in all algae), lutein (specific to

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Figure 1. Bathymetric map of Lago Albano showing core locations (1–6), and correlation between the two studied cores from sites 1 and 6(see text; Ryves et al., 1996; Jones & Juggins, paper in preparation). The location of other relevant study sites cited in this paper, as well ascore depths, and the three ages (one from the basal tephra; two from inferred 14C calibrated dates) on key points for core correlation (see text),are also indicated.

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green algae, macrophytes and higher plants), and thefull suite of carotenoids derived from sulphur-utilising,photosynthetic bacteria. These were determined by ion-pairing, reverse-phase HPLC (Lami et al., 1994), andexpressed as nmoles per gram of organic matter(nmoles g–1

OM). Samples for diatom, ostracod, clad-

oceran and chironomid analyses were prepared andcounted following the methods outlined in Manca et al.(1996) and Ryves et al. (1996). These data are ex-pressed as relative percentages (diatoms, chironomids)or, in the case of very simple assemblage structures(ostracods, cladocerans), in absolute counts permilligram of dry sediment mass (No. mg–1

DM).

Correlation between cores and chronology

Developing a sequential interpretation of the recordfrom the two cores depends on establishing a firmbasis for their correlation. Secure and detailed bet-ween-core correlations at each of the two individualsites can be obtained from high resolution magneticsusceptibility measurements (Rolph et al., 1996), butthese do not provide a sufficiently robust basis forcorrelation between the two stations. This has beendone by close comparison of the planktonic diatomrecord from each core. This is because contemp-oraneous sediment horizons in core sequences shouldhave very similar planktonic diatom communities,although benthic communities may be different dueto variations in water depth between the two locations(Ryves et al., 1996). A series of sequential changes in therelative frequency of Fragilaria reicheltii, Asterionellaformosa and several Cyclotella and Stephanodiscusspecies provide eight tie-points, thus permitting closesynchronisation of the record from the two cores (Ryveset al., 1996; Jones & Juggins, paper in preparation).Moreover, the correlations established do not violatethose suggested more tentatively by the suite ofpalaeomagnetic and rock magnetic changes in eachcore (Rolph et al., 1996; L. Vigliotti, pers. commun.).

The chronology for the sequence as a whole isconstrained in the upper part (not considered in thispaper) by the correlation based on diatom (Jones &Juggins, paper in preparation) and magnetic sus-ceptibility profiles (Rolph et al., 1996) with a coretaken in position 3, where 4 AMS radiocarbon dateswere available and, in the lower part, by well datedtephra layers. At the base of core PALB 94-1E, thereis a layer (28 cm in thickness) of coarse tephra,composed of fragments that are too large to have been

derived by airfall. Samples from this layer have beendated by the 40Ar/39Ar method to 26 ± 1 k yr BP (1 σ)(Calanchi & Lucchini, pers. commun.). At the top ofthe sequence lies the Etna Y-1 tephra, radiocarbondated securely elsewhere to 14.2 k yr -uncalibrated(Narcisi, 1993; Calanchi et al., 1996). This occurs ata depth of 717 cm in core 1E, which in turn correlateswell with the top of the sequence preserved in 6B,where the tephra occurs as a lag deposit at the sed-iment surface.

Between these upper and lower dated key horizons,a number of approaches have been employed toestablish the chronology, including PalaeomagneticSecular Variation (PSV) and counting of laminations(representing annual sediment accumulations, i.e.‘varves’). Within the constraints of the tephra dates,the best chronology available so far for core 1E hasbeen derived from a correlation between the PSVrecord and the 14C-dated PSV record from the Lac duBouchet, France (Figure 2; Thouveny et al., 1990).The chronology proposed here differs somewhat fromthat put forward by Oldfield (1996), before the latestage determination on the basal tephra was available.It rests largely on matching declination featuresbetween core 1E and the Lac du Bouchet record(Figure 2) and is now more closely compatible withthe basal date.

Ages for core 6B have been derived by correlationfrom the age-depth model for core 1E. According to thecorrelation with core 1E, 6B consists almost entirely ofsediments that pre-date the Holocene and about 7.7 mof the 8.4 m of sediment accumulated over a period ofsome 7 k yr (Figure 1; see below). Two 14C dates onthe 44–88 mm ‘pollen fraction’ (18.4 cal. k yr BP and21.2 cal. k yr BP, at 2.0 and 4.7 m levels, respectively)appear to be too old relative to the chronology proposed,but only by some 1–2 k yr.

In the chronology finally proposed (Figure 2), the 14Cages transferred to the present sequence have beencalibrated using the analyses given in Bard et al.(1993). However, correction of radiocarbon datesolder than 20 k yr is problematic because of the limitednumber of calibration points currently available. Ourcalibrated chronology for the basal part of core 1Eshould therefore be treated as preliminary.

The chronology adopted here implies that the tem-poral resolution achieved in part of the organicchemistry record is roughly decadal and that most ofthe other records are at century scale. In addition,detailed varve counts provide floating chronologies withannual scale temporal resolution of lithostratigraphic

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Figure 2. Chronostratigraphic framework for core PALB 94-1E based on (1) two tephra layers, and (2) on the Palaeomagnetic SecularVariation (PSV) record from Lago Albano and the dated ‘master curve’ from the Lac du Bouchet (Thouveny et al., 1990; Rolph et al., 1996).

variations for those parts of the sequence where thevarves are preserved (Chondrogianni et al., 1996c).

Lithological variations

Each core section considered is characterised by abruptchanges in lithology. Three different lithological unitswere identified from the base of core 1E (10. 5–13.6 m)(Chondrogianni et al., 1996b): light olive-grey spottedmud; yellowish-gray, spotted mud rich in calcite withmoss remains (Limprichtia revolvens, Drepanocladusaduncus and Warnstorfia exannulata); distinct lam-

inations of whitish calcite and olive-grey detrital silts.When analysed at a relatively high stratigraphicalresolution (1–2 cm interval), marked variations ingeochemistry (e.g. organic carbon, nitrogen, biogenicsilica; Ryves et al., 1996; Calanchi et al., 1996) cor-respond with the lithological variations. The sedimentsin core 6B comprise of alternating units of (a) massivesilts, which contain abundant moss remains, and (b)distinctly laminated silts and carbonates (Chondrogianniet al., 1996b). The laminated sediment sections areextremely well preserved, and changed colour whenexposed to the atmosphere.

1919BP BP

Age

(ca

l. 14

C k

yrs

BP

)

Age

(ca

l. 14

C k

yrs

BP

)

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Organic chemistry and the fossil record

The lowermost 3.5 m of the sequence in core PALB 94-1E, and almost the entire sequence in core PALB 94-6B, are characterised by sharply-defined variations inall of the biological proxies, particularly the pigments(Figure 3). The chlorophyll and carotenoid data, forexample, show a number of marked peaks that areconsidered to reflect periods of high productivity byalgae and anaerobic photosynthetic bacteria (Ryveset al., 1996). Primary palaeoproductivity was inde-pendently inferred from both preserved pigments(Guilizzoni et al., 1983) and diatom assemblages(Wunsam & Schmidt, 1995). In fact, a quite largenumber of geographic surveys provided evidences thatconcentrations of sedimentary chlorophyll derivativesand carotenoids reflect algal abundance (see papersquoted in Leavitt, 1993). During the high productivityperiods (eight major rapid events are noted, andrepresented in Figure 3 as grey bands), the sedimentsaccumulated under reducing conditions. This is inferredfrom the presence, often at high concentrations, ofokenone and other carotenoids (not shown) that belongto strictly anaerobic bacteria (Ryves et al., 1996). Thepigment peaks, as well as many organic matter peaks,are matched by sharp increases in fossil abundance andby fossil assemblage variations (see below; Figure 3).

Throughout most of the sequence, the diatomassemblages are dominated by planktonic, centric taxa,notably Cyclotella species, especially Cyclotella sp. 1(this taxon has no known living morphotype, and may bean extinct species – see Ryves et al., 1996). During theepisodes of inferred higher trophic status recorded at thebase of core 1E, analyses of contiguous 1 cm samples(Devoy et al., 1997) show that small Stephanodiscusspecies (namely S. medius and S. parvus) rapidlyreplace Cyclotella sp. 1 in these sections (Figure 3). InLago Albano these species are significant constituentsof the diatom community (Ryves et al., 1996; Jones,1998). Small Stephanodiscus species are today found inmesotrophic and moderately eutrophic lakes (Andersonet al., 1990; Bennion et al., 1995; Wunsam & Schmidt,1995) and peaks in their relative abundance indicateincreased nutrient availability and primary productivity.

The concentration of chironomid remains shows largevariations in the two cores (Figure 3). Head capsules areparticularly abundant in the 1E sequence where, in thelowest section of the core (27–28 k yr BP), the dominanttaxa belong to Tanytarsini (Micropsectra and fewCladotanytarsus larval head capsules). Taxa within theChironomini (Stictochironomus and Chironomus) and

some Orthocladiinae are more sparsely present. Theseare typical of littoral-sublittoral habitats. A veryshallow lake water level (swampy condition) duringthis colonisation phase is inferred from the ecology ofthese taxa and from the ostracod assemblages (Belis etal., 1998; see below). The same happens at the base ofcore 6B, where the very few remains found belong toChironomini. Between 28–24 k yr BP, the dominant taxaare Tanytarsini, with Micropsectra as the most importantgenus. Less important are the Chironomini. The peaksin concentration in core from site 1 correspond quite wellto the grey bands, with the exception of the large increaseat ca. 23 k yr BP (Figure 3).

As expected, Orthocladiinae dominate at site 6,particularly Psectrocladius sp. (not shown), a typicallittoral form associated with macrophytes. The fluc-tuations of Orthocladiinae (Figure 3) might be relatedto the redeposition processes active during periods ofhigher lake level. From the chironomid succession andthe pigment profiles in core PALB 94-6B, four mainperiods of higher water level and higher productivitywere inferred (Figure 3). The presence in these fourperiods of species often linked to warmer waters (i.e.Procladius, Polypedilum, Microtendipes) in associationwith Chironomus, also suggest milder environmentalconditions (Levesque et al., 1996). The peaks in bothchironomid and ostracod abundance are not generallysynchronous with the periods of reduced bottom waterinferred from the pigments of photosynthetic bacteria(Figure 3). The contemporary presence of thesepigments and benthic organisms (chironomids andostracods) during a few periods indicate that anoxiawas not permanent.

The pioneer cladoceran community of Lago Albanoappears in the lowermost part of core 1E, and isrepresented by a Chydorus-Alona-Monospilus ass-emblage, with very few remains of Daphnia of the D.pulex group (Manca et al., 1996). The two cores have avery similar taxonomic composition, though the balancebetween chydorids and zooplanktonic Cladocera differs.As expected, the chydorids are most abundant at site6, forming a littoral community, whereas at site 1 thezooplanktonic taxa dominate. In the lowermost part ofcore 1E (> 25 k yr BP), two large peaks in cladoceranconcentrations are observed. In the most recent oneBosmina replaces Daphnia as the dominant zoo-planktonic taxon. The peak below this represents themaximum phase of Daphnia concentration remains andit perfectly matches the pigment peaks (Figure 3).

The peaks in cladoceran concentrations found in themore recent sediments mostly occur alongside the

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Figure 3. The biological multi-proxy records at the base of Lago Albano core PALB94-1E and throughout core PALB94-6B. Dashed greyareas, based on wide peaks that appear in most proxies reflect high productivity (indicated by algal pigments and diatoms) and higher lakewater level intervals. Note the different x-axis scales for cores PALB 94-1E and PALB 94-6B.

indicators of warmer conditions discussed above(Figure 3). Concentrations of both Daphnia andchydorid remains fluctuate in parallel, and there is astriking correspondence between the presence ofherbivorous Cladocera and the pattern of chlorophyllderivatives and other algal pigments. The highestcorrelation coefficient (p < 0.001; see Manca et al.,

1996) is found for CD vs. total Cladocera. The majorpeak in the concentration of chydorids at site 6 (ca.21.5 k yr BP) corresponds to the highest detectedconcentration of pigments of algal and bacteria origin.

Among the ostracod assemblages, Candona neglectais the dominant species throughout most of the levelsanalysed, and especially during the episodes of higher

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lake productivity inferred from the pigment, diatom andinvertebrate records from core 1E discussed above.Fossils of C. neglecta and Cyclocypris sp. are bothabundant in core PALB 94-6B, but alternate throughoutthe sequence as the dominant taxa. Valves of Ilyocyprisbradyi and Potamocypris spp. are restricted to episodesof inferred low lake level and low lake productivity(Manca et al., 1996; Belis et al., 1998). As in the case ofthe chironomids, the peaks in ostracod concentrationsare slightly out of phase with those evident in thepigment data. The response of ostracods to the en-vironmental variations recorded appears to have beenmore in their total concentration rather than in anysignificant assemblage changes. Paucity, even completeabsence of ostracod remains, correspond with phasesof low carbonate content in the sediment record (Beliset al., 1998), and seasonal oxygen stress as indicatedby the presence of anaerobic sulphur photosyntheticbacteria.

Towards a palaeoenvironmental interpretation

The combined palaeobiological and lithological datasuggest that there were eight major oscillations in lakelevel and in lake productivity, as inferred from the algalremains, during the ca. 11 k yr represented by thesediments considered here (duration of between 0.262and 0.790 k yr). Within these major oscillations othershort-lived oscillations can be noted. The sedimentsstudied span the Last Glacial Maximum (LGM), anddemonstration of such high variability, especially if itcan be linked to climate, may perhaps be used toreconstruct and model environmental conditions duringthe LGM by treating the period as one experiencingrelatively uniform conditions. In evaluating further thelikelihood that the changes recorded here reflectclimatic changes, we consider first the evidence for amajor rise in water level around 23–24 k yr BP.

Lack of sedimentation prior to ca. 24 k yr at site 6,in 30 m of water, suggests that prior to this period lakelevels were below the site of deposition. We infer thata marked increase in the level of Lago Albano at ca.23–24 k yr BP initiated the deposition of sediments atthe location of core PALB 94-6B. The top of thesediments in this sequence are presently 40 m abovethe sediment-water interface at the location from whichcore PALB 94-1E was obtained.

All the oscillations recorded in the sediment from site6 thus occurred in a deeper and enlarged lake comparedto that within which the oscillations at the base of

PALB94-1E occurred. For the period of sedimentaccumulation at site 6, the extended scale of depositionrelative to that recorded for the same time interval inthe deeper cores from site 1, and the greater sensitivityof the record they contain to environmental changes,is thought to reflect the different water depths at whichsediment accumulation occurred at the two sites. Theonset of lacustrine sedimentation at site 6 thus reflectsa significant rise in water level within the basin as awhole. There is no evidence either in the history ofvolcanicity in the region or in the seismic survey of thesedimentary in fill at the site to suggest that some eventin the evolution of the caldera system was responsiblefor the inferred rise in water level. Our favouredhypothesis is therefore that the rise that brought site 6into the zone of lacustrine sedimentation for the firsttime reflected a change in the precipitation/evaporationbalance, though until this hypothesis can be tested atother sites that are both hydrologically and seismicallyindependent from the present one, this cannot beconfirmed.

We now turn to the changes documented by shiftsin biotic assemblage at each site both before and afterthe inferred rise in water level and especially to aconsideration of the oscillations recorded at the baseof core 1E.

The two most likely explanations for the observedshifts in biotic assemblages are: (a) endogenetic pro-cesses, such as geothermal activity; and (b) exogeneticprocesses – i.e. regional climatic changes. Under theformer hypothesis, the marked changes in lithology,between massive and laminated sediment units couldreflect periodic geothermal warming of the lake withincreased lake productivity and the formation oflaminated sediments, alternating with episodes of lowergeothermal heating and seismically-induced instabilityin the catchment, leading to the high influx of clasticmaterial which dilutes pollen concentrations (Lowe etal., 1996) and increases the concentration of magneticminerals (Rolph et al., 1996).

The following observations are offered in Oldfield(1996) in refutation of the endogenetic, geothermalhypothesis:

1. Sudden pulses of clastic, allochthonous sedimentinput resulting from seismic activity would haveled initially to sharp, high amplitude, in-phasereductions in diatom concentrations, as well as inother aquatic biological remains in the lake sed-iment sequence. There is no evidence for any suchchanges. For example, there are no marked changesin the diatom concentrations, which have been

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analysed and the concentrations calculated oncontiguous 0.5 cm slices between 1270 and 1301cm in the 1E sequence, with at least decadaltemporal resolution (Devoy et al., 1998).

2. The interplay of geothermal and seismic influenceswould have created lithological alternations con-sistently in phase with the linked biological andgeochemical responses discussed above. Althoughlithological, biological and organic geochemicalchanges are frequently in phase in both cores, in thesediments from 1E (which was subjected to the highresolution analysis, between 1205 and 1325 cm)major lithological changes in carbonate precipitationare strongly out of phase with the other linkedcatchment and aquatic system variables.

3. The pollen and within-lake-ecosystem changes arenot simply linked through in-phase changes in pollenconcentration and inferred aquatic productivity, aswould be expected if geothermal and seismicinterplay generated the responses. Instead, there aresignificant changes in the relative proportions ofdry-land pollen taxa (for example Pinus andArtemisia) that reflect successional changes interrestrial vegetation in phase with, but clearlyindependent from, the changes in the aquatic biota(Lowe et al., 1996). From these observations,based on an overview of all the relevant evidencepresented in Guilizzoni & Oldfield (1996), we inferthat the changes in lake biota outlined here mayreflect, in part at least, climate forcing on centuryand millenial timescales, mediated through effectson lake level and aquatic productivity. This in-terpretation is strengthened by the findings in Lagodi Monticchio (Southern Italy) where, for example,the high resolution diatom analysis shows a patternvery similar to that found in Lago Albano over thesame time interval (Nimmergut et al., 1998).

Three main factors, either acting alone or in com-bination, may have caused the recurrent increases inhigher aquatic productivity inferred from the evidencepresented here:

(i) Increased input of dissolved nutrients by surfacewater flow from the catchment under conditions ofincreased leaching;

(ii) Warming of the local climate, reducing the durationand severity of ice cover;

(iii) Enhanced nutrient supply from sediments at thesediment-water interface during periods of anoxia– itself likely to be in part a positive feedbackmechanism from increased aquatic productivity. It

is possible that during the meromictic-like con-ditions the nutrients would be retained in thehypolimnion although, as seen in some Italianlakes (e.g. Mergozzo; Guilizzoni et al., 1981), thepresence of laminations in the sediments does notnecessary imply meromictic conditions and theabsence of seasonal overturn.

Thus, the increased productivity phases were probablyresponses to shifts to warmer and moister climate anddeeper water, while the minima in fossil remains,pigments and, in most cases, carbonate concentrationsprobably reflect cooler, drier periods with shallowerwater (Ryves et al., 1996). These inferences are fullyconsistent with both the pollen (Lowe et al., 1996) andlithological data (Chondrogianni et al., 1996b) from thesite and from the Adriatic Sea cores (Guilizzoni &Oldfield, 1996). Similarly, two recent studies onminerogenic and biogenic accumulation rates byRamrath et al. (1998a and b) on another nearby maarlake, Lago di Mezzano, (cf. Figure 1), showed the samecyclic fluctuations of environmental conditions duringthe time interval investigated. Follieri et al. (1988) andNarcisi et al. (1992) also infer periodic dry periods aroundthe time of the LGM from the evidence from the LagoValle di Castiglione, and Watts et al. (1996) conclude thatthe record from Lago Grande di Monticchio (SouthernItaly) also contains evidence for alternations betweencool arid periods and slightly warmer, more humidepisodes over the same time interval.

Given the time interval covered by the evidencepresented from Lago Albano, the duration and per-iodicity of the oscillations recorded and the inferenceof climatic forcing, it is tempting to propose cor-relations between the LGM record from Lago Albanoand both the Greenland Ice Core (Dansgaard et al.,1993; Grootes et al., 1993; Stuiver et al., 1995) andAtlantic sediment (Bond et al. 1993; Bond & Lotti,1995) records for the same time interval. Whilst itseems highly likely that the pattern of variabilitypresented here reflects the same sequence of broadscale climate forcing as that documented in the ice andmarine records, the scope for precise correlation islimited by the chronological uncertainties alreadynoted. This limitation will only be overcome by moresecure age/depth modelling, more direct and quan-titative proxies for climate variability than are dis-cussed here, and a more sophisticated algorithm forstatistically testing any proposed sequential tele-connections between the Albano record and those fromGreenland and the Atlantic than is justified by thepresent data. Only at that stage will it be possible to

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develop interpretative models directly linking thesequence of changes recorded in Lago Albano withthe rapid changes in ocean circulation and northernhemisphere temperature known to have occurred overthe time interval studied.

The significance of our results lies, for the present,in their demonstration of the sensitivity of the Albanosediment record to environmental change, the qualitativedocumentation of high levels of climatically forcedvariability around the time interval of the LGM, andthe clear indications that this variability includeselements that are broadly comparable in timing,duration and frequency with those from the ice core andhigh resolution marine records.

Acknowledgements

This study formed part of, and was largely financedthrough, the PALICLAS (Palaeoenvironmental Analysisof Italian Crater Lake and Adriatic Sediments) Project.It was funded for 2 yrs (April 1st, 1994 to March 31st,1996) by the European Union as part of the Framework3, Climate and Environment Programme, contract No.EV5V-CT93-0267. We are grateful to Mrs LucianaCorbella for help with the chironomid analyses. Weappreciate the time and trouble taken by two anonymousreviewers in their efforts to improve the paper.

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Evidence for short-lived oscillations in the biological records from the sediments of Lago Albano...

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