Palaeoenvironmental changes in southern Patagonia during the last millennium recorded in lake...

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Palaeogeography, Palaeoclimatology, Pa

Palaeoenvironmental changes in southern Patagonia during the last

millennium recorded in lake sediments from Laguna Azul

(Argentina)

Christoph Mayr a,*, Michael Fey b, Torsten Haberzettl b, Stephanie Janssen c,

Andreas Lucke a, Nora I. Maidana d, Christian Ohlendorf b, Frank Schabitz c,

Gerhard H. Schleser a, Ulrich Struck e, Michael Wille c, Bernd Zolitschka b

aInstitut fur Chemie und Dynamik der Geosphare, ICG V: Sedimentare Systeme, Forschungszentrum Julich, D-52425 Julich, GermanybGeomorphologie und Polarforschung (GEOPOLAR), Institut fur Geographie, Universitat Bremen,

Celsiusstr. FVG-M, D-28359 Bremen, GermanycSeminar fur Geographie und ihre Didaktik, Universitat zu Koln, Gronewaldstr. 2, D-50931 Koln, Germany

dDepartamento de Biodiversidad y Biologıa Experimental, Universidad Nacional de Buenos Aires- CONICET,

Ciudad Universitaria, C1428EHA. Buenos Aires, ArgentinaeGeoBio-CenterLMU, Ludwig-Maximilians-Universitat Munchen, Richard-Wagner-Str. 10, D-80333 Munchen, Germany

Received 14 October 2004; received in revised form 23 May 2005; accepted 3 June 2005

Abstract

Marked environmental changes in the southern Patagonian steppe during the last 1100 years are detected by a multi-proxy

study of radiocarbon-dated sediment cores from the crater lake Laguna Azul (52805VS, 69835VW). A prominent shift in carbon

isotope records occurred between AD 1670 and AD 1890 induced by a change to cooler climate conditions with a concurrent

lake level rise. A second perturbation of the lake ecosystem started with a fire event around AD 1830. The fire event triggered

increased soil erosion initiating a change of the diatom assemblages. This shift in diatom assemblages may have been enhanced

by shrinkage of littoral habitats and higher nutrient supply in the course of permanent European settlement at the end of the 19th

century. The introduction of neophytes by European sheep farmers is confirmed by the permanent occurrence of Rumex pollen

in the sediment record since the beginning of the 20th century.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Lake sediments; Patagonia; Stable isotopes; Diatoms; Pollen; Geochemistry

0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.palaeo.2005.06.001

* Corresponding author. Tel.: +49 2461 613178; fax: +49 2461

612484.

E-mail address: [email protected] (C. Mayr).

1. Introduction

Due to its unique geographical position as the only

non-glaciated continental land mass south of 478S,

laeoecology 228 (2005) 203–227

C. Mayr et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 228 (2005) 203–227204

Patagonia is a key area for understanding of temperate

climates in the southern hemisphere. Present knowl-

edge of Holocene environmental and climatic changes

in the Patagonian steppes and semi-deserts of Argen-

tina which extend between 418S to 528S east of the

Andes is based on palynological studies at archaeo-

logical sites (e.g., Mancini, 1998, 2002; Prieto et al.,

1998), on pollen and charcoal studies in peat profiles

at the steppe–woodland ecotone in southwestern Pata-

gonia (e.g., Heusser, 1994, 1999; Huber and Mark-

graf, 2003), and on interdisciplinary studies of lake

sediment records from Lago Cardiel (488S, 718W;

Gilli et al., 2001; Schwalb et al., 2002; Markgraf et

al., 2003). The latter site provides comprehensive

insight into Holocene palaeoenvironments and palaeo-

climates. The detection of considerable lake level

fluctuations at Lago Cardiel during the Holocene

demonstrate the sensitivity of Patagonian lacustrine

archives to climatic changes (Stine and Stine, 1990;

Gilli et al., 2001). Recent studies in the Pali Aike

Volcanic Field (PAVF) have shown, that deep crater

lakes located further south bear a high potential for

detailed reconstructions of Holocene palaeoenviron-

mental changes (Haberzettl et al., 2005; Zolitschka et

al., in press).

Here, investigations of radiocarbon-dated gravity

cores recovered from Laguna Azul, a small crater lake

in the PAVF, are presented. After an outline of the

present-day limnology, vegetation, and geomorpholo-

gy, the results of multi-proxy investigations including

diatomological, palynological, isotopic and geochem-

ical methods are presented. Data are interpreted with

regard to climatic and anthropogenic impacts on ter-

restrial and lacustrine ecosystems in this region during

the last ca. 1100 years.

2. Site description

2.1. Climatic setting

Patagonia is situated between the southern flank of

the subtropical high-pressure system and the subpolar

low-pressure trough centred along the Antarctic Circle

(Prohaska, 1976). The main characteristics that allow

delimiting Patagonia as a uniform climatic region are

the prevailing strong westerly winds resulting from

this synoptic constellation. The annual average wind

speeds reach 7.4 m s�1 at Rıo Gallegos (Zolitschka et

al., in press). Average wind speed never drops below 5

m s�1 in any month and increases significantly during

the austral summer. Dominating (62%) wind direc-

tions are NW, W or SW (observation period 1951–

1960 for Rıo Gallegos; Liss, 1979). Due to the rain-

out effect of air-masses passing the Patagonian Andes

a strong rainfall gradient from west to east occurs.

Whereas annual precipitation at the west coast of

South America reaches values up to 3000–5000 mm

(Weischet, 1996) and the eastern slopes of the Andes

still receive 400–900 mm, values decrease to 200 mm

or less in most areas of extra-Andean Patagonia (End-

licher, 1993; Oliva et al., 2001). The southernmost

part of Patagonia including the PAVF is characterized

by a semi-arid, cold-temperate climate with annual

precipitation between 200 and 300 mm. Precipitation

amounts are slightly higher in summer compared to

other seasons (Oliva et al., 2001; Zolitschka et al., in

press). Mean annual temperatures vary around 6–7 8C(Oliva et al., 2001). The lowest monthly temperature

average is observed in July (0.9 8C), the highest in

January (13.4 8C; Zolitschka et al., in press). Daily

temperature extremes at Rıo Gallegos range from 33

8C during summer to �16 8C in winter (data of 1941–

1960; Liss, 1979).

2.2. Geomorphology and geology

Laguna Azul is located near Estancia Monte

Aymond close to the Argentinean–Chilean border,

20 km NW of the Strait of Magellan and 55 km

SSW of the city of Rıo Gallegos (Fig. 1). The lake

fills a volcanic crater in the south-eastern part of the

PAVF, a volcanic area covering 4500 km2 (D’Orazio

et al., 2000). The crater is surrounded by late Pleisto-

cene to Holocene lava flows (Corbella, 2002), one of

which originated from the crater of Laguna Azul. Two

lake basins and adjacent dry craters suggest that

Laguna Azul was formed by several volcanic explo-

sions accompanied by effusive activities. A tuff-ring

partially surrounds the crater complex. The young,

probably Holocene age of the lake is evident through

the rugged lake floor (Fig. 2) and a comparatively thin

sediment infill of less than 6.5 m as determined by 3.5

kHz seismics (Anselmetti and Ariztegui, personal

communication). The present-day lake level is around

100 m a.s.l. and a steep crater rim rises about 60 m

Fig. 1. (A) Location of the research area in southern South America (arrow) and (B) position of Laguna Azul in the Pali Aike Volcanic Field

(=PAVF, grey area adapted from D’Orazio et al., 2000). Also shown are the positions of two other sites with high-resolution terrestrial sediment

records in the area, Laguna Potrok Aike (Haberzettl et al., 2005) and Rıo Rubens Bog (Huber and Markgraf, 2003).

C. Mayr et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 228 (2005) 203–227 205

above the lake surface. The elliptical lake has a max-

imum extension of 560 m in NW–SE direction. The

bathymetry exhibits an up to 56 m deep basin in the

SE and a shallower, up to 33 m deep basin in the NW

(Fig. 2).

2.3. Regional and local vegetation

The west–east climatic gradient in southern Pata-

gonia is expressed in five major vegetation zones

(Moore, 1983; Roig, 1998): Magellanic moorland

and evergreen Magellanic rainforest are distributed

in the humid areas west of the Andes, Andean tundra

dominates above the tree line, deciduous Nothofagus-

forest grows on the foothills of the eastern Andes and

Patagonian (Magellanic) steppe prevails in the semi-

arid part of central and eastern Patagonia. Most of the

PAVF is located in the dry (xeric) Magellanic steppe

with Festuca gracillima as dominant species. Towards

the south and southeast of Laguna Azul, the vegeta-

tion changes into a moister (mesic) Magellanic steppe

which is characterized by a dominance of Festuca

pallescens (Boelcke et al., 1985; Oliva et al., 2001).

Since the first settlement of European sheep farmers in

the late 19th century, the vegetation of the entire

region has altered (Liss, 1979). Today overgrazing

and hence soil erosion are widespread (Aagesen,

2000). Recently published pollen records from the

steppe–forest ecotone in southern Patagonia, however,

raised the question, if European impact on vegetation

has started even earlier during first colonization

attempts in the late 16th century (Huber and Mark-

graf, 2003) indicated by introduction of European

weeds and higher abundances of charcoal particles.

Despite of human influence on lowland vegetation,

there is a close correlation between modern pollen

Fig. 2. Bathymetry and geomorphology of Laguna Azul with positions of sediment cores and of one radiocarbon sample of subfossil aquatic

macrophytes from desiccated lake sediments. The catchment area of Laguna Azul is approximately equivalent to the area confined by the

crater rim.


rain and vegetation zones (D’Antoni, 1991; Mancini,

1993), which offers the possibility to use modern

pollen analogue techniques, as applied for northern

Patagonia (Paez and Schabitz, 2001).

Due to the edaphic setting and the wind-sheltered

more humid microclimate, the local vegetation inside

the crater of Laguna Azul is markedly different from

the vegetation of the surrounding plains. Species with

higher water demands (Hordeum comosum, Rumex

acetosa, Caltha sagittata) grow on desiccated lake

sediments close to the south-western shoreline. The

vegetation on drier, sandy soils of the crater floor is

dominated by Acaena magellanica, Acaena splen-

dens, grasses (Stipa sp., Festuca sp.) and shrubs of

Senecio filaginoides. The foot of the crater slopes and

the slopes itself are covered by Rumex acetosella

herbs, S. filaginoides shrubs and Berberis hetero-

phylla bushes growing on tephra. Vegetation outside

the crater is dominated by regional graminoid steppe

taxa with contributions of herbs (e.g., Perezia recur-

vata, Cerastium arvense and Armeria maritima).

2.4. Limnology

Laguna Azul is a dimictic and holomictic lake.

Water temperatures in 0.5 m depth during a monitoring

18

16

14

12

10

8

6

4

2

0

2/02 5/02

Time (month/ year)

Wat

er te

mpe

ratu

re (

°C)

8/02 11/02 2/03 5/03 8/03 11/03 2/04

0.5 m25 m41 m

Fig. 3. Seasonal variations of water temperatures in different depths (0.5, 25 and 41 m) in Laguna Azul as logged by a chain of thermistors

(2-hourly monitoring intervals) fixed on a mooring during the period from March 7th, 2002 to March 14th, 2004.


interval from March 2002 to February 2004 ranged

from 0.5 8C at the beginning of July 2002 to 17.8 8C in

February 2004 (Fig. 3). During austral summers

(March 2002 and February 2003) pH values in the

epilimnion were 8.3–9.0 and oxygen concentrations of

Fig. 4. Depth profiles of temperature and d13CDIC of Lagu

the water were 11–13 mg l�1 (Zolitschka et al., in

press). In depth profiles of the water column investi-

gated during these summers, temperature, pH and

oxygen concentrations rapidly decrease between 17.5

and 22.5 m. Temperatures in the hypolimnion range

na Azul for March 7th, 2002 and March 14th, 2004.


between 6 and 7 8C, pH values between 7.6 and 8.8

and oxygen concentrations between 2 and 6 mg l�1.

Among the major nutrients, total phosphorus (TP),

silicon (Si) and nitrate (NO3�) concentrations of sur-

face water were determined (Zolitschka et al., in

press). TP concentration of 51 Ag l�1 (March 2002)

and 67 Ag l�1 (February 2003) indicate mesotrophic

to eutrophic conditions according to the classification

of Vollenweider (1979, in Wetzel, 2001). Si concen-

trations were 0.2 and 0.3 mg l�1 for the sampling

periods of 2002 and 2003, respectively. Nitrate con-

centrations in both years were below analytical detec-

tion levels (i.e., b0.05 mg l�1) of the applied method

(ion chromatography). The d13C values of dissolved

inorganic carbon (DIC) were determined for the 2002

and 2004 sampling campaign to 0.3x to 1.3x in the

epilimnion (Fig. 4), typical values for waters contain-

ing high proportions of HCO3�(Meyers, 2003). In the

metalimnion d13CDIC values gradually decreased and

reached values around �2x in the hypolimnion near

the lake bottom (Fig. 4).

3. Material and methods

3.1. Coring and sampling

During February/March 2002, eleven short sedi-

ment cores were recovered from Laguna Azul and

Table 1

AMS radiocarbon dates determined at the Poznan Radiocarbon Laborator

Core/sample no. Sediment depth

(cm)

Standardized sediment

depth (mean, cm)

Sample ty

AZU 02/4 7.0–9.0 24.0 Berberis

AZU 02/11 64.0–66.0 32.8 Charred p

AZU 03/5 42.0–45.0 35.5 Charred p

AZU 02/11 44.0–45.0 20.0 Bulk sedi

AZU 02/11 51.0–52.0 25.1 Bulk sedi

AZU 02/11 54.0–55.0 27.3 Bulk sedi

AZU 02/11 56.0–58.5 28.8 Ephippia

AZU 02/11 67.0–69.0 35.5 Bulk sedi

AZU 02/11 99.0–100.0 67.0 Bulk sedi

AZU 02/11 124.5–126.5 93.0 Bulk sedi

PAIS-48b Aquatic m

PAIS-45 Potamoge

Calibrated ages are median probabilities (bold) and minima and maxima

samples for which the age was derived from the post-modern 14C curvea Date excluded from age–depth model, probably containing reworkedb Aquatic macrophyte remains from desiccated lake sediments. Sample

another eight short cores were taken in February

2003. Cores were obtained with either a modified

ETH-gravity corer (Kelts et al., 1986) or an UWI-

TEC-gravity corer. Immediately after recovery, sedi-

ment cores were sealed gas-tight and transported to the

ODP Bremen Core Repository (Germany), where they

were stored cool and dark until subsampling. Four

cores with well-preserved sediment records were se-

lected for detailed analyses, only these coring locations

are indicated in Fig. 2. Cores were subsampled in

consecutive 1 cm slices, which were divided for the

different analytical procedures. Subsamples from all

four cores were analysed isotopically (d13Corg) which

allowed a detailed correlation of the sediment records

and a transfer of the radiocarbon ages from different

cores to a composite, standardized sediment depth for

all cores. Core AZU 02/11 was studied for diatoms,

pollen, charcoal, isotopes and geochemistry. Magnetic

susceptibility and the distribution of major elements

were determined for core AZU 03/5.

3.2. Chronology

The ages of six samples of bulk sediment, one

sample of cladoceran ephippia (Daphnia sp.) and

three samples of terrestrial plant macro-remains

(charred plant remains, twig) from different cores

were obtained by AMS 14C dating (Table 1). Further-

more, modern aquatic macrophytes (Potamogeton sp.)

y, Poland

pe Lab-no. 14C age

(yr BPF1r)Calibrated age

(yr AD)

twig Poz-3575 157F0.5 pMC 1963 or 1969

lant remains Poz-8445 145F30 1851 (1683–1955)

lant remains Poz-8447 215F35 1760 (1647–1955)

ment Poz-1685 280F30 1652a (1511–1800)

ment Poz-893 520F30 1433a (1408–1454)

ment Poz-1684 490F30 1444a (1414–1482)

Poz-1547 370F30 1558a (1463–1636)

ment Poz-894 345F25 1558a (1500–1645)

ment Poz-928 590F30 1404 (1322–1435)

ment Poz-898 1160F30 937 (783–1017)

acrophytes Poz-5071 1325F30 727 (661–846)

ton sp. Poz-3574 105F0.4 pMC 1956 or 2003

of 2r ranges (in brackets), except for Potamogeton and Berberis

(pMC: percent modern carbon).

organic matter.

location indicated in Fig. 2.


collected in 2003 were dated to test for a potential

hardwater effect. Subfossil aquatic macrophyte

remains from lake sediments exposed 2 m above the

present shoreline were dated to determine the age of a

lake level highstand (location in Fig. 2). Radiocarbon

ages were calibrated with the southern hemisphere

calibration curve (SHCal02; McCormac et al., 2002)

using CALIB 4.4 (Stuiver et al., 2003) and are given

as calendar years AD. Dating with 210Pb and 137Cs

was performed on the uppermost samples of core

AZU 02/7 but failed due to very low concentrations

of these radiogenic isotopes.

3.3. Pollen and charcoal analysis

Pollen samples were processed according to stan-

dard palynological techniques including HCl and

KOH treatment and heavy liquid separation with

ZnCl2 followed by acetolysis and ultra-sonic treat-

ment (Faegri and Iversen, 1989). Pollen percentages

of all taxa were calculated from pollen sums that vary

from 300 to 500 grains per sample excluding fern

spores, algae and pollen from water plants. The latter

palynomorph groups are given in percentages of the

terrestrial pollen sum. Pollen concentrations were de-

termined with Lycopodium marker grains (e.g., Moore

et al., 1991). All taxa included in the pollen sum were

used for a CONISS cluster analysis (Grimm, 1987).

Charcoal was counted in two different ways. First-

ly, charcoal particles were counted from pollen slides

including all size fractions N0.02 mm. Secondly, mac-

roscopic charcoal was counted from dispersed and

sieved sediment (method described in Huber and

Markgraf, 2003). Four different size fractions (0.1–1

mm, 1–2 mm, 2–3 mm, 3–4 mm) of sieved charcoal

were counted from a known sediment volume.

3.4. Diatoms

Diatom samples were heated with hydrogen per-

oxide to oxidize organic material and mounted onto

microscope slides following standard procedures (Bat-

tarbee, 1986). Duplicated permanent slides for light

microscopy were prepared with NaphraxR. A mini-

mum of 400 valves per slide was counted in order to

calculate relative frequencies. All permanent slides of

the studied material are deposited in the personal slide

collection of N. Maidana. Identification of the diatom

taxa to species level or variety is based on various

studies (Schmidt et al., 1874-1959; Patrick and

Reimer, 1966, 1975; Simonsen, 1987; Krammer and

Lange-Bertalot, 1986, 1988, 1991a,b; Rumrich et al.,

2000). Nomenclature follows criteria set up by Round

et al. (1990).

3.5. Geochemistry and sediment logging

After determination of dry density and water con-

tent, each volumetric and freeze-dried sediment sam-

ple was homogenized in a mortar. Total nitrogen (TN),

total carbon (TC) and total sulphur (TS) were ana-

lysed with a standard CNS analyser (EuroEA, Euro-

vector). Concentrations of total organic carbon (TOC)

were determined with the same device after successive

treatment with 3% and 20% HCl at 80 8C to remove

carbonates. Total inorganic carbon (TIC) was calcu-

lated as difference between TC and TOC. Biogenic

silica was analysed with an automated leaching meth-

od using a continuous flow system with UV-VIS

spectroscopy (Muller and Schneider, 1993). Magnetic

susceptibility was measured in one centimetre incre-

ments with a Bartington F-point-sensor on a measur-

ing bench developed by the Department of Marine

Geophysics, University of Bremen. Qualitative ele-

ment counts of 13 elements (K, Ca, Ti, Fe, Mn, Sr,

Cu, V, Cr, Co, Ni, Zn, Pb) were obtained in one

centimetre resolution using a CORTEX X-ray fluo-

rescence (XRF) scanning system (Gunn and Best,

1998; Zolitschka et al., 2001) in the ODP core repos-

itory of the University of Bremen and are given in

counts per second (cps).

Hydrogen (HI) and Oxygen (OI) indices were an-

alyzed with a Rock-Eval II instrument according to

the method described by Espitalie et al. (1977, 1985).

3.6. Stable isotopes

Lacustrine sediments, modern samples of aquatic

macrophytes and terrestrial plants, as well as soils

from Laguna Azul and its catchment were investigat-

ed isotopically and geochemically to characterize po-

tential sources of lacustrine sediment organic matter.

Sediment and soil material for isotopic analyses was

freeze-dried, homogenized and sieved with a 200 Amsieve to eliminate macrophyte debris. Nitrogen iso-

tope ratios (d15N) were determined on approx. 4 mg


of bulk sediment weighed into tin capsules and com-

busted at 1080 8C in an elemental analyser (EuroEA,

Eurovector) with automated sample supply linked to

an isotope ratio mass spectrometer (Isoprime, Micro-

mass). Analytical precision is 0.14x.

For analyses of carbon isotope ratios of sediments,

soils and water plants samples were decarbonized

with HCl (5%) for 6 h in a water bath at 50 8C,afterwards centrifuged, rinsed repeatedly with deio-

nized water to neutral pH and freeze-dried. Carbon

isotope ratios (d13C) were determined on approx. 1

mg of sample with a Carlo Erba elemental analyser

linked to an Optima isotope ratio mass spectrometer

or with the system described above for nitrogen iso-

tope analyses. Analytical precision for d13C determi-

nations is 0.08x. Isotope ratios are reported as dvalues in per mil according to the equation:

d ¼ Rs=Rst � 1ð ÞT1000 ð1Þ

with Rs and Rst as isotope ratios (13C / 12C, 15N/ 14N)

of samples and international standards (VPDB for

carbon, AIR for nitrogen), respectively.

d13CDIC values were determined from samples

poisoned with NaN3 in a similar way as described

by Atekwana and Krishnamurty (1998) with a semi-

automatic sampling supply and an isotope ratio mass

spectrometer. DIC samples of 2002 were analysed

with an AP 2003 (Analytical Precision) isotope ratio

mass spectrometer, those of 2004 with a Thermo

Finnigan GASBENCH II coupled online to a Thermo

Finnigan Delta plus isotope ratio mass spectrometer.

4. Results

4.1. Sedimentology

The sediments of Laguna Azul consist of homoge-

neous brown diatom ooze with intercalations of dark

layers. In all four cores the frequency of dark inter-

calations increase towards the top of the core and the

upper part (upper 65 cm in core AZU 02/11) was

darker than the lower part of the core. The fresh

sediments have high water contents between 90%

and 98% (cf. Fig. 8B). Water contents increase and

hence dry densities of the sediments gradually de-

crease from 0.12 g cm�3 at the core bottom to 0.06

g cm�3 at the core top.

4.2. Correlation of sediment cores

A characteristic, macroscopically visible charcoal

layer was found in all cores and serves as a distinct

marker horizon. This horizon was found in different

depths in the respective cores (AZU 02/11: 64–66 cm,

AZU 03/5: 42–44 cm, AZU 02/7: 32–34 cm, AZU 02/

4: 17–19 cm). In the sediment immediately above this

horizon a characteristic mass occurrence of cladoceran

ephippia (Daphnia sp.) was observed. A compilation

of all d13Corg records from Laguna Azul reveals a

high inter-core consistency (Fig. 5A). Relevant

d13Corg wiggles can be matched between all cores.

The comparison of isotope records and marker hor-

izons of the cores exhibits locally different sedimen-

tation rates within the lake basin, likely an effect of

the uneven crater floor.

Comparison of data, as well as integration of ra-

diocarbon dates from different cores requires a stan-

dardization of sediment depths of the individual cores.

Sediment depths below the charcoal horizon were

transformed to the depth scale of AZU 02/11 by

means of d13C-wiggle-matching (Fig. 5B). In the

upper part depths of the shortest core AZU 02/7

were taken as standardized depths, because this core

has the best preservation of the sediment–water inter-

face as it was sub-sampled in the field. Further on, all

data are reported with regard to this standardized

sediment depth.

4.3. Age–depth model

The calibrated AMS 14C dates of the sediment

cores provide a time-scale from AD 940 to AD

1969 (Table 1). The age–depth plot (Fig. 6) demon-

strate that several ages from bulk sediment as well as

the ephippia age are older than those of terrestrial

organic material dated in the same section. Since14C dating of modern aquatic plants revealed a post-

modern age (105 pMC), a hard-water effect as possi-

ble source of this bias can be excluded. A contami-

nation with older, reworked carbon is the most likely

reason for erroneous ages in this respective sediment

section. Following a conservative approach, these

dates were rejected and a chronology that only con-

siders the youngest dates was constructed. Linear

interpolation between median probabilities of the

remaining 14C dates provides sedimentation rates of

Fig. 5. A) Correlation of d13Corg records along a NW–SE transect. Hatched areas indicate the charcoal marker horizon and the sections with

abundant Daphnia ephippia. Lines between records demonstrate corresponding d13Corg wiggles. B) Correlation between sediment depth of

individual sediment cores and standardized sediment depth. Further explanations see text.


Fig. 6. Calibrated AMS 14C dates from different sediment cores

plotted vs. standardized sediment depth. 2r ranges of calibrated

ages are given as bold lines. Symbols represent the medians of

probability. Different types of dated material are indicated by

different symbols. The age–depth model used is given by a

dashed line.


0.6 to 0.8 mm yr�1 below 24 cm sediment depth and

an increased sedimentation rate of 6.5 mm yr�1 for

the uppermost 24 cm (Fig. 6). This apparent increase

is the result of less compaction of the sediment, as

evidenced by increasing water contents in this section

(Fig. 8B).

4.4. Pollen and charcoal

Pollen taxa were grouped into Patagonian Steppe

Taxa (PST), which represent the local and regional

vegetation, and into Andean Forest Taxa (AFT),

which were transported by strong westerly winds

over long distances from the Andean forests (Fig.

7). AFT constitute 25–40% of the pollen sum in the

pollen record of AZU 02/11 which is remarkably

high, as the modern tree-line is situated 160 km

west of Laguna Azul. The abundance of AFT in the

sediment core is consistent with high proportions of

AFT in modern pollen samples from the soil surface at

the shoreline of Laguna Azul (Schabitz et al., 2003).

Pollen of AFT almost exclusively belong to the

Nothofagus dombeyi-type, whereas the Nothofagus

obliqua-type, Misodendron, Podocarpus and Gun-

nera only occur in small quantities (Fig. 7A).

In spite of high amounts of long-distance trans-

ported Nothofagus pollen, PST are always the domi-

nant group in the pollen spectrum. Percentages of PST

exhibit highest values at 57 cm and lowest at 30 cm

sediment depth. Most abundant among these autoch-

thonous steppe elements are Poaceae. Maximum

values of around 50% occur between 67 and 52 cm

and significantly lower values between 42% and 24%

prevail in stratigraphic levels above 40 cm. Asteraceae

(Tubuliflorae) and Ericaceae are abundant Patagonian

steppe taxa, but contribute much lower percentages

(b15%) than Poaceae.

The pollen record of Rumex is noteworthy. Rumex

is almost absent below 23 cm sediment depth but

continuously present above this level. The distribution

of Asteraceae (Liguliflorae) is similar, although not as

distinct compared to Rumex. Charcoal is continuously

present in the AZU 02/11 record with concentrations

between 1000 and 19,600 particles ml�1 (size

fractionN0.02 mm) and between 120 and 920 particles

ml�1 (size fractionN0.1. mm, Fig. 7B). A single layer

between 32 and 33 cm, however, exhibits an extremely

high charcoal concentration in both size fractions

(51,700 and 2960 particles ml�1 for the size fractions

N0.02 and N0.1 mm, respectively) reflecting an excep-

tionally strong fire event. The first division of the

dendrogram of CONISS cluster analysis indicates a

marked change in pollen assemblage immediately

above this charcoal level, too (Fig. 7A).

4.5. Diatoms

The diatom assemblages of AZU 02/11 are char-

acterized by a relatively low species number (31)

with only few taxa appearing in large quantities

(Fig. 7B). Stratigraphic levels of AZU 02/11

below 27 cm exhibit a dominance of Staurosira

construens var. venter reaching 60–82% (Fig. 7B).

From 30 to 27 cm values decrease to 32% and

remain below 40% in the upper part of the core.

Fig. 7. A) Palynomorph percentages of core AZU 02/11 vs. standardized sediment depth and time-scale. Hatched area indicates the charcoal marker horizon. B) Summary diagrams of

percentages of the most abundant diatom taxa and of charcoal concentrations of core AZU 02/11 vs. standardized sediment depth and time. Charcoal concentrations are given for

particles N0.02 mm (counted from pollen slides) and for the size fraction N0.1 mm (counted from sieved samples).

C.Mayr

etal./Palaeogeography,Palaeoclim

atology,Palaeoeco

logy228(2005)203–227

213

Fig.7(continued).



Simultaneous to the decrease of S. construens var.

venter the planktonic diatom Stephanodiscus parvus

rises from almost absence to 45–65% in the upper-

most 27 cm. Other diatom taxa only appear in small

quantities (b5%) with the exception of Staurosirella

pinnata and Cyclotella agassizensis. S. pinnata

shows a similar distribution pattern as S. construens

var. venter with lower percentages (1–8%) in the

uppermost 27 cm compared to 6–25% in the lower

parts. In contrast, C. agassizensis exhibits a single

peak with a maximum of 9% at 27 cm.

4.6. Geochemistry and stable isotopes

Results of geochemical and stable isotope analyses

of the sediment core AZU 02/11 are compiled in Fig.

8A, B. The d13Corg record exhibits several fluctua-

tions ranging between �25.9x and �22.5x below

70 cm and increase to highest values of the record

(�22.9x to �21.8x) between 65 and 44 cm. Im-

mediately thereafter, d13Corg values decrease in two

steps and reach lowest values of �27.6x at 30 cm.

Although absolute values between AZU 02/11 and the

other three cores differ, the carbon isotope patterns of

all cores from Laguna Azul are similar (Fig. 5A)

pointing to undisturbed sedimentation and demon-

strating that the observed changes in sedimentary

parameters affected the entire lake basin. In contrast

to the large variations in d13Corg, those of d15N are

almost negligible. Hydrogen and oxygen indices ex-

hibit only little variation throughout the sediment

record (Fig. 8B) and range between 90–120 and

350–390, respectively.

Rather high values of TOC and TN ranging from

12.8% to 17.3% and 1.6% to 2.2%, respectively, were

determined. TIC is only present in the uppermost 20

cm with less than 1.4% (not plotted). TOC and TN

show a significant positive correlation (R2=0.86).

TOC, TN, and TS exhibit minima around 30 cm

and maxima around 55 cm, coinciding with extreme

d13Corg values and implying a common cause for

variations of these parameters in the upper part of

the core. However, in the lower part correlation be-

tween carbon isotope ratios and these parameters dis-

appears. A mass occurrence of ephippia of

cladocerans (Daphnia sp.) between 32 and 27 cm

coincides with minima of d13Corg, TOC, and TN.

The TOC/TN ratios of AZU 02/11 vary between

7.1 and 8.9 and gradually increase from the bottom

of the core towards a maximum at 31 cm. Biogenic

silica values are rather high and vary between 40%

and 54%. Biogenic silica mainly consists of diatom

frustules and spicules of freshwater sponges.

4.7. XRF scanning and magnetic susceptibility

Element analyses derived from XRF scanning of

core AZU 03/5 show significant variations of Fe, Ti,

Mn and Ca counts (Fig. 8B). Other elements either

show little variation (Cu) or signals are too low for

interpretation (b20 cps for K, V, Cr, Co, Ni, Zn, Sr,

Pb) and thus are not shown. Fe and Ti records of the

core are positively correlated (r =0.49), as well as Fe

and Mn (r =0.56). Ca is correlated significantly with

Mn (r=0.63), but to a lesser extent with Fe (r =0.39)

and Ti (r =�0.22). Fe and Ti records both start with

elevated values around 85 cm. Between 75 and 35 cm

values of both elements are permanently low. At the

same level in which charcoal is macroscopically ob-

servable and d13Corg values decline (around 35 cm) Fe

and Ti counts increase rapidly and reach maxima at 30

cm, coinciding with minimum d13Corg values. Above

that level, Ti and Fe exhibit several oscillations. The

Fe and Ti curves compare well with the magnetic

susceptibility record. Magnetic susceptibility drops

immediately after a short excursion in the lowermost

section and remains low with the exception of a

distinct maximum between 35 and 27 cm coinciding

with highest Fe and Ti counts and a minor peak

around 22 cm. Mn counts change to higher values

above the charcoal horizon and remain high in the

upper part of the core, whereas Ca increases above 20

cm core depth.

4.8. Isotopic and geochemical characterization of

potential organic matter sources and lake sediments

We evaluated d15N, d13C, TOC/TN, HI and OI of

non-algal sources of organic matter to estimate their

possible contributions to the sediments of Laguna Azul

(Fig. 9). Terrestrial plants around Laguna Azul show

high TOC/TN ratios between 23 and 142 (n =10),

d13C values between �28.3x and �24.5x (n =10)

and d15N values ranging between �2.5x and 2.3x(n =8), with the exception of one value of 10.8x.

Submerged aquatic macrophytes from Laguna Azul

Fig. 8. A) Stable isotope (d13Corg, d15N) and geochemical analyses (TC, TOC, TN, TOC/TN, TS, biogenic silica) of core AZU 02/11 vs.

standardized sediment depth. Charcoal marker horizon indicated by hatched area. B) Hydrogen index (HI), oxygen index (OI), magnetic

susceptibility (j), water content (WC) and counts per second of selected elements (Fe, Ti, Mn, Ca) vs. standardized sediment depth. All data are

from core AZU 03/5, except WC which is from AZU 02/7. Charcoal marker horizon indicated by hatched area.


Fig. 9. Scatter plots of (A) d13C vs. TOC/TN, (B) d13C vs. d15N and (C) HI vs. OI of modern soils, aquatic macrophytes, terrestrial plants and

littoral sediments from Laguna Azul and its surroundings. For comparison, values of the uppermost centimetres of four sediment cores situated

along a transect in the lake (see Fig. 2) are given by different symbols. Note axis breaks and one d15N outlier for terrestrial plants.



had TOC/TN ratios of 26 to 38 (n =4), d13C values

between �14.8x and �7.1x (n =4), which indicate

assimilation of HCO3�(Keeley and Sandquist, 1992),

and a large range of d15N values from �16.5x to

3.5x (n =6). Soils exhibit rather homogeneous

TOC/TN between 10 and 14 (n =7), d13Corg values

ranging between �26.6x to �23.8x (n =7) and

d15N values from 2.4x to 6.8x (n=7). In compar-

ison to these potential sources of organic matter,

TOC/TN ratios of the sediment core AZU 02/11

are between 7.1 and 8.9, d13Corg values are in the

range of �27.6x to �21.8x and d15N values are

between 3.8x and 5.0x.

5. Discussion

5.1. Origin of sediment organic matter

The small catchment area of Laguna Azul (0.24

km2), the absence of any tributary, and high contents

of biogenic silica support an autochthonous and algal

origin of sediment organic matter. A high amount of

terrestrial organic matter is unlikely under prevailing

semi-arid conditions, except for wind-transported ter-

restrial organic matter or erosion due to surface run-

off during rare heavy rainfall events. Nevertheless,

input of terrestrial and non-algal lacustrine organic

matter has to be excluded before any conclusions

from isotopes and organic geochemistry can be

drawn. To determine the origin of sediment organic

matter different approaches were applied. TOC/TN

ratios and d13Corg values are frequently used to char-

acterize the sources of organic matter in lake sedi-

ments (e.g., Meyers, 1994; Meyers and Lallier-Verges,

1999; Hassan et al., 1997; Herczeg et al., 2001).

Another approach uses the hydrogen index (HI) and

the oxygen index (OI) of kerogen for organic matter

characterization (e.g., Tissot and Welte, 1978; Talbot

and Livingstone, 1989).

The observed sediment values of the TOC/TN

weight ratios are entirely within the range of organic

matter originating from phytoplankton (4–10 accord-

ing to Meyers, 1994, 2003), hence input of non-

planktonic organic matter generally can be considered

as low. The only distinct increase in TOC/TN during

the last millennium occurs in conjunction with the

charcoal peak (Fig. 8A), but the relatively slight

shift from 7.7 to 8.9 demonstrates, that the proportion

of terrestrial organic matter even in this layer is neg-

ligible. Input of soil organic matter (with low TOC/

TN ratios) into the lake cannot be excluded by means

of TOC/TN and isotope ratios, as the range of values

for soils is close to or overlaps with those of sediment

organic matter (Fig. 9A, B). However, HI and OI

values show almost no variation within the profile

evidencing no major changes in organic matter com-

position. HI values of all non-algal sources are lower

than those of the sediments (Fig. 9C). High contribu-

tions of algal organic matter for littoral sediments

have to be considered, as their HI values overlap

with those of the core sediments.

In conclusion, all available evidences point to an

algal origin of sedimentary organic matter of Laguna

Azul, and variable input of non-algal organic matter

thus is excluded as reason for observed d13Corg

variations.

5.2. Hydrological changes reflected by d13Corg

records

As detailed above, isotopic and geochemical sur-

veys clearly demonstrate that d13Corg variations must

be attributed to environmental changes within the

lake. Possibly d13Corg variations reflect changes in

the contribution of algal sediments from littoral

zones to the pelagic coring sites. This hypothesis is

supported by a comparison of d13Corg records along a

transect across the lake (Fig. 10). The transect reaches

from the shallow NW basin (AZU 02/4) to the SE of

the central deep basin (AZU 03/5). The sediment core

AZU 02/4 is close to the broad littoral zone along the

NW shore, whereas AZU 03/5 is situated closer to the

steep southern shore of Laguna Azul (Fig. 2). The

surface sediments of the four sediment cores along the

transect differ considerably in absolute isotope values

(up to 2.8x). With increasing proximity to the shal-

low water basin d13Corg values of surface sediments

tend towards those of littoral sediments in Fig. 9A, B

indicating that the proximity to extensive littoral habi-

tats apparently influences the absolute d13Corg values

of the sediment. It is thus likely that relative d13Corg

variations in the sediment profile were also induced

by changing littoral influence. Shifts in littoral versus

pelagic influence on sedimentation could have been

triggered by lake level variations. In periods with


lower lake levels, littoral habitats advance towards the

basin centre resulting in higher d13Corg values com-

pared to periods with high lake levels when littoral

habitats shift away from coring locations. Markedly

lower lake levels than today could also have ham-

pered water exchange between the NW basin and the

central basin resulting in a higher divergence between

absolute d13Corg values of both basins, as it is ob-

served in the isotope records from AD 1450 to AD

1670 (Fig. 10).

Additionally, d13Corg variations could reflect

changes in the isotopic composition of the DIC

pool. The DIC pool of a lake is permanently refreshed

by CO2 diffusing from the atmosphere into the lake

water. With the beginning of industrialization, the

d13C values of atmospheric CO2 have been changed

anthropogenically by release of CO2 from fossil fuel

combustion (e.g., Francey et al., 1999). This bfossilfuel effectQ results in 1.7x more negative values of

d13C of atmospheric CO2 since the beginning of

industrialization (i.e., since AD 1850; McCarroll and

Loader, 2004), and thus can explain only to a minor

degree the observed shift to ~ 4.5x more negative

d13Corg values since the second half of the 17th

century and the d13Corg minimum of the late 19th

century at Laguna Azul. Thus other effects must

Fig. 10. Differences in absolute d13Corg values for sediment cores along a t

the southern part of the central basin (AZU 03/5) of Laguna Azul plotted v

the core closest to the littoral zone and lowest values for the core from th

have played a much more important role on the

isotopic composition of DIC of Laguna Azul. The

carbon isotope composition of DIC is controlled by

a variety of parameters such as concentration of dis-

solved CO2, the carbon source used for primary pro-

duction (CO2 or HCO3�), photosynthesis and

respiration rates of aquatic organisms, and the type

of organic matter in the watershed (Brenner et al.,

1999). The last argument can be excluded for changes

in the d13Corg record of Laguna Azul, as the pollen

data indicate no basic changes in vegetation type.

Shifts in lake-internal productivity are thus more prob-

able. Increased photosynthetic activity in lakes with

low buffering capacity like Laguna Azul may change

epilimnic pH to even higher values and thus lower the

availability of CO2 in favour of HCO3� (Lampert and

Sommer, 1999). Increasing photosynthetic uptake of

HCO3� by phytoplankton should in turn lead to more

positive d13C values of sediment organic matter. Cli-

matic changes as well as variations in nutrient supply

influence lacustrine bio-productivity. Major changes

in nutrients prior to AD 1830, such as shifts in phos-

phorous, nitrogen and silicon concentrations, can be

largely ruled out for Laguna Azul, as they should be

reflected in diatom composition, d15N and biogenic

silica as well. Thus climatic changes causing shifts in

ransect (see Fig. 2) from the NW shallow water areas (AZU 02/4) to

s. standardized sediment depth. Note the highest d13Corg values for

e southern central basin.


d13CDIC are an explanation for the d13Corg variations

in the sediments of Laguna Azul.

Hollander and McKenzie (1991) reported changes

in the carbon isotope fractionation between dissolved

inorganic carbon and particulate organic carbon

(d13CDIC–d13CPOC) varying between 18.0x and

26.8x for Lake Greifen (Switzerland), depending

on the availability of CO2 in different seasons. The

CO2 reservoir in the epilimnion of the softwater lake

Laguna Azul certainly is limited during summer

months, as indicated by the highly enriched d13CDIC

values of the epilimnion (Fig. 4). Regeneration of the

DIC reservoir happens during mixing of the water

body in spring and autumn (Fig. 3). Cooler climatic

conditions in the region during the period AD 1480–

1900 are evidenced by a TIC record from Laguna

Potrok Aike (Haberzettl et al., 2005). The cooler

climate during that period could have caused lower

photosynthesis rates of phytoplankton and aquatic

macrophytes resulting in less consumption of the

DIC pool. At the same time, less DIC consumption

leads to higher CO2 concentrations with the effect of

higher discrimination against 13C by photosynthesiz-

ing organisms during carbon uptake. In consequence,

this results in lower d13Corg values of sedimenting

organic matter. On the contrary, high DIC demands,

e.g., in periods with extended growing seasons could

result in higher d13Corg values.

At present, we cannot decide whether variable

input of littoral sediment or changes in DIC induced

by climatic changes are the main reasons for the

isotopic variations observed in the sediment records.

Probably a combination of both effects led to the

extreme shift towards more negative d13Corg values

between AD 1670 and AD 1890. We suggest that an

increase of the lake level during cooler conditions

caused less input of littoral organic matter to the

coring positions and lower lacustrine bio-productivity

during this period.

This interpretation is supported by geomorphic

evidences that indicate considerable variations of the

lake level in the past. Water levels higher than today

are testified by carbonate crusts on the rocks around

the lake, by ancient shore lines (Fig. 2) and by desic-

cated lake sediments containing aquatic macrophyte

remains (dated to AD 730, Table 1). A most recent

lake level drop of about 1–2 m in the last four decades

is documented by photographs (Fig. 11).

5.3. Other parameters indicating environmental

changes before AD 1880

Apart from high d13Corg values, the period be-

tween AD 1400 and AD 1700 is characterized by a

maximum of PST relative to AFT in the Laguna

Azul record. Pollen index curves calculated as ra-

tios between PST and AFT and between the two

most important representatives of both groups, the

Poaceae and the N. dombeyi-type pollen, are given

in Fig. 12. During the 15th and 16th century twice

as much Poaceae than N. dombeyi-type pollen were

found, whereas in the periods before and thereafter

both groups are represented by equal amounts.

Similar variations are documented for the PST/

AFT ratio. At present, Poaceae encounter excellent

growth conditions on desiccated shore areas around

Laguna Azul. Thus the rise in Poaceae pollen

during the 15th century might be explained by

increased local input of pollen from grass vegeta-

tion that grew on more extended desiccated areas

close to the shore during a period with low lake

levels.

5.4. Evidence for increased fire intensity during the

period of early European impact

Both pollen index curves decline rapidly during the

18th century (Fig. 12). At the end of this decline, the

largest charcoal peak of the past millennium marks a

distinct fire event around AD 1830 in the vicinity of

Laguna Azul. The proximity of the fire to Laguna

Azul is indicated by the presence of high charcoal

concentrations in every size class up to several mm.

At present, fires ignited by lightning are rather un-

common for southern Patagonia (Miller, 1976) and

therefore charcoal layers in this region are assumed to

have mainly an anthropogenic origin (Heusser, 1995;

Huber and Markgraf, 2003). Similar to the record

from Laguna Azul, charcoal particles are abundant

in the youngest sections of other southern Patagonian

sediment records, but almost absent during the centu-

ries before (Heusser, 1987, 1995; Huber and Mark-

graf, 2003). The 1200 year long charcoal record from

Rıo Rubens Bog (52808VS, 71852VW; Huber and

Markgraf, 2003) exhibits no local fire events prior

to AD 1600, but a distinct charcoal maximum shortly

thereafter. A tendency to higher charcoal deposition

Fig. 11. Photographs of Laguna Azul from the 1960s (A) and from 2004 (B). Photos were taken from the eastern crater rim towards the south

and demonstrate a lake level drop during recent times (see also hatched areas in Fig. 2). The area in the foreground, still flooded during the

1960s, is completely desiccated and covered with vegetation at present. Photo (A) taken by W. Roil with kind permission to publish from F.

Roil, Rıo Gallegos.


since AD 1600 was also observed in the sediment

record of nearby Laguna Potrok Aike (Haberzettl et

al., 2005). For young sediments from Patagonia char-

coal layers were interpreted as time markers for the

beginning of European exploration at the end of the

19th century (Heusser, 1987) or as indicators for early

European influence on hunting techniques of the in-

digenous population (Huber and Markgraf, 2003).

During the 19th century the exploration of Pata-

gonia was accelerating. First permanent settlements

evolved at the coast, whereas the interior was not

inhabited by Europeans until the arrival of sheep-

farmers towards the end of the 19th century

(Alvarez, 2000).

European impact seems to be a likely reason for the

single prominent charcoal peak of the Laguna Azul

Fig. 12. Sediment parameters of cores AZU 03/5 (magnetic susceptibility) and AZU 02/11 (all other parameters) exhibiting significant changes

during the interval AD 1700–1900. Charcoal marker horizon indicated by hatched area. Pollen indices are given as ratios of Patagonian Steppe

Taxa to Andean Forest Taxa (PST/AFT) and Poaceae to Nothofagus dombeyi-type.


record around AD 1830. Although European explora-

tion mainly was restricted to coastal areas at that time

(Martinic, 1997, 1999), the adoption of horses by the

native indigenous population, the southern Tehuelche,

led to substantial changes in their hunting traditions

(Prieto, 1997). The Tehuelche began trading with

guanaco skin-cloaks with the first colonial settlers in

the 19th century and there is evidence from travelo-

gues that they used fire for hunting of guanacos

(Lama guanicoe) and rheas (Pterocemia pennata pen-

nata) during that time (Musters, 1870 in Prieto, 1997).

Thus the hypothesis that European impact on hunting

traditions is the reason for increased fire activity in the

early time of European exploration in Patagonia

(Huber and Markgraf, 2003) may be supported by

our record, but our data exhibit a charcoal maximum

200 years later than at Rıo Rubens Bog.

5.5. Post-fire sedimentary and ecological changes

The charcoal layer predates the permanent occur-

rence of Rumex pollen, a reliable indicator of Euro-

pean settlement activities (Heusser, 1987; Mancini,

2002; Huber and Markgraf, 2003) by about 100


years. Although determination to species level is dif-

ficult, Rumex pollen observed in our pollen record

most probably belongs to R. acetosella, a species

presently growing in considerable quantities on the

crater slopes of Laguna Azul, whereas other Rumex

species are rare in the regional flora. R. acetosella

occurs frequently in areas of Patagonia, where the

vegetation is heavily disturbed (Mancini, 1998; Prieto

et al., 1998), e.g., in areas with overgrazing (Schabitz,

1999) or after fire events (Ghermandi et al., 2004).

Above the charcoal layer lowest d13Corg values

are reached around AD 1890. At the same time, Ti

and Fe, as well as magnetic susceptibility reach

maxima (Fig. 8B). These three parameters are gen-

erally interpreted as a measure of minerogenic input

(e.g., Thompson et al., 1975; Peterson et al., 2001).

As the rocks surrounding Laguna Azul have relative-

ly high Ti and Fe contents (D’Orazio et al., 2000),

the increases in Ti, Fe and magnetic susceptibility

after the fire event point to a period of enhanced

erosion. It is conceivable that erosion was triggered

by the fire event. Distinct maxima are documented

by elevated Ti, Fe and magnetic susceptibility

around AD 1880 and towards the end of the 20th

century. Increases in magnetic susceptibility due to

fire-induced erosion were also reported by Mill-

spaugh and Whitlock (1995) for lakes with steep-

sided water sheds in Yellowstone National Park

(USA).

During the time when minerogenic input reached

its first maximum, the diatom assemblage changed

within a few decades from a S. construens var.

venter-dominated assemblage to a predominance of

S. parvus. The transition zone between both species

is marked by a distinct peak of Cylotella agassizen-

sis. Repeated mass occurrences of this species were

reported from laminated Late Glacial lake sediments

of NW Argentina after pronounced input of Fe-rich

mud (Trauth and Strecker, 1999). At Laguna Azul, C.

agassizensis occurred also during a period of higher

input of Fe-rich allochthonous material to the sedi-

ments. However, the ecological significance of this

species is as yet too poorly understood to allow

further conclusions on its occurrence. The plankton-

ic/periphytic diatom S. construens var. venter indi-

cates meso- to eutrophic water conditions (Whitmore,

1989) and an extended littoral habitat, whereas the

small planktonic Stephanodiscus species stand out as

low Si/P specialists with very low Si requirements,

but relatively high P demands (Mechling and Kilham,

1982; Kilham, 1984; van Donk and Kilham, 1990;

Interlandi et al., 1999). Therefore the main change of

the diatom assemblage may indicate a change in the

nutritional budget of the lake or changes in the

lacustrine habitats after AD 1850. Increased phospho-

rus concentration in aquatic ecosystems after fire

events due to aerial deposition of ash and higher

erosion rates in the watershed were reported (Spencer

et al., 2003; Enache and Prairie, 2000). Philibert et al.

(2003) found a response of the diatom assemblage to

wildfires in a boreal Canadian lake. However, in all of

these studies the impact of fires on nutrient concen-

trations and/or diatom assemblages was short-lived

and generally lasted less than a few decades. In con-

trast, the post-fire diatom assemblage of Laguna Azul

persists until today. Thus increased nutrient levels due

to erosion after the fire event may well explain the

change in diatom assemblage after about AD 1880,

but not the dominance of S. parvus during the fol-

lowing 150 years. The persistence of the S. parvus

dominance until today might also have been enabled

by increased nutrient loadings due to sheep farming

starting around AD 1880 in that region.

Changing diatom habitats are an additional expla-

nation for the observed shift in the diatom assem-

blage (e.g., Douglas et al., 1994). A loss of littoral

habitats could explain the decline of S. construens

var. venter and the rise of S. parvus. Indications for

a change of the lake ecosystem already before the

fire event are provided by the d13Corg records. Now-

adays shallow water habitats are restricted to com-

paratively small areas along the W and NE shore

(see bathymetric map in Fig. 2). Considering the

morphology of the crater, more extensive shallow

water areas than today could arise from a lake

level drop which would result in enlarged habitats

for aquatic macrophyte populations and periphytic

diatoms like S. construens var. venter.

The lake level rise in the 18th century postulated

from the d13Corg records may have been an addi-

tional reason for the change in diatom assemblages.

We hypothesize that lower lake levels during the

period AD 1450–1670 offered enlarged habitats for

S. construens var. venter and resulted also in higher

littoral influence on sediment organic matter as

reflected in the d13Corg records. With an increase


in effective moisture in the 18th century the lake

level rose with the consequence of rapidly shrinking

shallow water areas. One consequence was a loss in

littoral habitats for S. construens var. venter. En-

hanced nutrient supply by fire-induced erosion and

the beginning land-use by sheep farmers shortly

thereafter led to the described change in the diatom

assemblage with a dominance of the planktonic spe-

cies S. parvus.

5.6. Comparison with other environmental records

from Patagonia

Reconstructions of Patagonian glacier fluctuations

derived from radiocarbon and dendrochronologically

dated wood, ascertain maximum extensions of gla-

ciers in the 17th and 18th centuries (Mercer, 1982;

Villalba et al., 1990; Marden and Clapperton, 1995;

Aniya, 1996). Glasser et al. (2004) terms this glacier

advance Neoglacial Advance IVand attributes it to the

bLittle Ice AgeQ implying isochronous climatic

changes between both hemispheres.

Records from lakes situated in the Patagonian

steppe point to severe drought conditions in Mediae-

val times. Radiocarbon dated organic detritus and

drowned shrub and tree stumps indicate drought con-

ditions before AD 1020–1230 at Lago Cardiel and

Lago Argentino followed by a more humid period

(Stine and Stine, 1990; Stine, 1994). In the PAVF,

however, the sediment record of Laguna Potrok Aike

(51858VS, 70824VW) implies drought conditions from

the middle of the 13th century to the end of the 15th

century and a lake level rise thereafter until the be-

ginning 20th century (Haberzettl et al., 2005). Within

the dating uncertainties of the records from Laguna

Azul and Laguna Potrok Aike, the onset of a cool

period can be testified for the time between AD 1500

and AD 1700. Both records concordantly show that

this cool period approximately ended when European

settlers started to inhabit central southern Patagonia.

6. Conclusions

During the last millennium the most severe

palaeoenvironmental changes at Laguna Azul are

recorded for the period AD 1700 to AD 1900

affecting terrestrial as well as lacustrine ecosystems.

High values of d13Corg from AD 1400 to AD 1700

are attributed to a stronger littoral influence on the

pelagic lacustrine sediments and enhanced lacustrine

productivity coinciding with a relative maximum in

the abundance of Poaceae pollen. These results were

interpreted in terms of low lake levels due to warm-

er climatic conditions and more extended desiccated

areas around the lake. From AD 1700 to AD 1900

the lake level rose markedly, as indicated by a drop

in d13Corg values, in the course of climatic cooling.

High charcoal concentrations at the end of that

period possibly reflect changing hunting traditions

of the native Tehuelche population due to first Eu-

ropean impact. Enhanced erosion started shortly

after this fire event as indicated by peaks of Fe,

Ti and magnetic susceptibility pointing to higher

amounts of allochthonous minerogenic input. A per-

sistent shift in the diatom assemblage from a dom-

inance of the littoral species S. construens var.

venter to a predominance of the planktonic species

S. parvus is interpreted as being the result of in-

creasing anthropogenic impact. Around AD 1900

the first direct evidence of European impact on

vegetation is documented by the permanent presence

of Rumex pollen.

Probably the lake level rise between AD 1700 and

AD 1900 was induced by higher effective moisture

due to cooler and/or moister climatic conditions. This

reconstruction is in agreement with data from glacio-

logical and lacustrine archives from southern Patago-

nia that point to a change to cooler and wetter climatic

conditions during the global b Little Ice AgeQ.

Acknowledgements

We are much indebted to Holger Wissel and

Werner Laumer for technical assistance with stable

isotope analyses, Sabine Stahl, Barbara Kuck and

Philipp Bluszcz for assistance with sampling and

geochemical analyses, Franz Leistner for Rock Eval

analyses, and Michael Lindner and Dominik Tallarek

for pollen preparations. We thank Alexius Wulbers,

Heike Pfletschinger, Ursula Rohl and Walter Hale for

storage of sediment cores and for providing technical

equipment at the ODP Core Repository in Bremen.

We are much obliged to Thomas Frederichs and

Christian Hilgenfeldt for access to their magnetic


susceptibility measuring bench and to David Living-

stone for providing us with thermistors. Cristobal

Kennard and Jorge Moreteau are acknowledged for

logistic support, Hugo Corbella for introducing us to

the local geology and providing aerial photographs,

Juan C. Paggi for determination of cladoceran ephip-

pia and Sherilyn Fritz for comments on an early draft

of this manuscript. The manuscript has greatly

benefited from suggestions of M. Mancini and an

anonymous referee. This is a contribution to the

German Climate Research Program DEKLIM (grants

01 LD 000034 and 000035).

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