Hydrobiologia, special issue on Palaeolimnology

Palaeolimnology of Lake Hess (Patagonia, Argentina): multi-proxy analyses in short sediment

cores.

P. Guilizzoni1, J. Massaferro2, A. Lami

1, E. Piovano

3, S. Ribeiro Guevara

4, S. Formica

3, R. Daga

4,5

A. Rizzo4,5

and S. Gerli1

1CNR - Istituto per lo Studio degli Ecosistemi Verbania Pallanza, Italy;

2Laboratorio de Biodiversidad Darwin, INIBIOMA/CONICET, Bariloche, Argentina

3Departamento de Quìmica Universidad Nacional de Còrdoba, Argentina

4Laboratorio de Análisis por Activación Neutrónica, Comisión Nacional de Energía Atómica

(LAAN CNEA), Argentina 5also at CONICET, Argentina

p.guilizzoni@ise.cnr.it

Abstract

Compared with North America and Europe, relatively few studies have quantified the

anthropogenic and/or climate impacts in Patagonian lakes. We addressed these issues by analysing

geochemistry, lithology, pigments and chironomid remains from sediment cores collected from

Lake Hess (41° 22´ 20” S, 71° 44´ 0” W) located in the Nahuel Huapi National Park, in northern

Patagonia. The aim of this study is to provide a palaeoenvironmental and climate reconstruction of

the last ca. 5-6 centuries for the cold oligotrophic, quasi-pristine Lake Hess which receives melt

waters from the Tronador ice cap.

Chronology was based on 137

Cs and 210

Pb measurements of the upper core layers and the

sedimentation rate of 23.2 mg cm-2 y

-1 (0.148 cm y

-1) was in good agreement between both

techniques.

Lake Hess was rich in tephra deposits particularly in the lower part of the cores. Peaks of ash layers

were evident through magnetic susceptibility profiles. Tephra layers that were used for core

correlation.

Results from the multiproxy analysis in the longest core of 80 cm allowed us to clearly identify

three main periods. From the bottom up to 40 cm, the sediment shows light-grey clays. Both

pigments and chironomids, suggest a fairly high productivity. From 40 cm to 22-25 cm,

sedimentary record is composed of alternating black and dark clay layers with variable amounts of

macrophyte remains. Chironomid head capsule abundance decreased and pigment concentrations

were also very low. These facies are composed of very fine and plastic sediments with some faint

laminated intervals with a organic matter composition gradually decreasing towards the end of the

zone. We interpreted this period as cold and wet and probably synchronous with the Little Ice Age

(LIA). A sharp change occurs at 25 cm showing a strong increase of organic matter, algal nutrients,

and plant pigments together with a switch in the composition of chironomid assemblages. This,

documents a change in the trophic condition of the lake.

Although there are records of human impact in the study environment, particularly on extended

fires, we believe that owing the lake location, most of the productivity changes in Lake Hess

sediment sequence were firstly related to climate changes, specially to variations in moisture

balance bring about by the westerlies.

Introduction

Compared with North America and Europe, few studies have quantified the anthropogenic

and/or climate impacts on the Patagonian lakes (Massaferro et al., 2005; Ariztegui et al., 1997;

2007;Villarosa et al., 2006). Palaeolimnological research has shown that many remote areas (e.g.

high mountain lakes) have experienced considerable biotic and sedimentary changes in recent

decades (e.g. Lami et al., 2000; Lotter & Birks, 2003). Palaeoenvironmental variability

(hydrological changes) since the Last Glacial Maximum up to the Little Ice Age in Patagonia has

been described from lake sediment cores on both sides of the Southern Andes in Lago Puyehue

(40°S) in the Chilean Lake District (Chapron et al., 2006; Moernaut et al., 2007; Bertrand, 2008;

Charlet et al., 2008; De Batist, 2008) and Laguna Frias (Ariztegui et al., 2007) on the Argentinean

side. In the Patagonian Plateau, the record of Lago Cardiel (49°S; Gilli, 2003; Gilli et al., 2005b;

Markgraf et al., 2003) and Laguna Azul and Laguna Potrok Aike (52°S; Haberzettl, 2006;

Haberzettl et al., 2007; Mayr et al., 2005) contain a detailed climate archive of the environmental

variability since the late Pleistocene.

Multi-proxy approaches are important in paleolimnological studies for an overview of the

natural and non-natural events that are taking place in lake sediment. Northern Patagonia is

particularly interesting to perform this kind of studies because it is home to many different aquatic

environments which are in turn, controlled by regional and local climate effects (Villalba, 2007).

For instance, the presence of the westerlies at the latitude of 40º S is a source of change in the

atmospheric moisture coming from the Pacific Ocean (Ariztegui et al., 2007). Patagonia is

particularly sensitive to seasonal atmospheric air-humidity changes driven by the Southern

westerlies and the polar front position in winter (Prohaska, 1976). Temporal changes in the north-

south precipitation gradient is controlled by the latitudinal position of the westerlies, that in turn is

ruled by the strength and latitudinal position of the subtropical anticyclone in the southeast eastern

Pacific and the circum-Antarctic low pressure belt (Markgraf, 1998). The present-day northern limit

of the westerlies influence in the Chilean Pacific coast lies at ca. 27°S. South of 38°S the influence

of westerlies is permanent resulting in high precipitations overall the year, whereas in northern

latitudes mean annual precipitation are comparatively lower and are concentrated during the winter

(Veit, 1996).

Lake Hess is an small lake located in a particular geographical position as it receives the

River Manso Medio carrying melting waters directly from the Tronador glacier. Therefore this

lake is an interesting site to test variations in moisture and/or changes in the glacier mode. Previous

studies in the region have shown that the Tronador ice cap has reacted to climate change during

distinct episodes such as the Late-Glacial-Holocene transition (Ariztegui et al., 1997; Hajdas et al.,

2003) and the Medieval Climatic Anomaly and the LIA with well-identified glacial advances

between AD 1800-1850, and recent push-moraines (Rabassa et al., 1979; Villalba et al., 1990).

In this paper, the issue of climate and palaeoenvironmental changes in a more recent period

(the last ca. 4-5 centuries) has been addressed by analysing, organic matter (by Loss on Ignition,

LOI), nutrients (carbon and nitrogen), photosynthetic pigments and chironomid remains from three

sediment cores collected from Lake Hess located in the Nahuel Huapi National Park in northern

Patagonia. A lithological description has been also carried out.

LOI, nutrients and fossil pigments in lake sediments reflect environmental conditions in

lakes and their catchments at the time of deposition (Guilizzoni et al., 2006). Their sedimentary

composition and abundance can record historical changes in lake trophic status, anthropogenic and

natural (climate) change, adding greatly to our understanding of the development of lakes

As regard the larvae of the aquatic non-biting midges (Diptera: Chironomidae), the long

resistance of their heavily chitinized head capsules to degradation in lake sediments has led to their

widespread use in palaeoecological studies (Brooks and Birks, 2004).

The aims of the research are: (1) to provide additional paleolimnological information from

northern Patagonia lake sediments during the last 200-300 years, and (2) to establish the degree of

human and climate impact within a quasi-pristine lake ecosystem.

Methods

Study area and sampling

Lake Hess (41°21’ 20’’S; 71°44’0’’W) is a small, shallow (max depth 8.3 m ) lake 100 km south to

San Carlos de Bariloche which receives meltwaters from Tronador ice cap carrying by Rio Manso

medio (Fig. 1). The area is far from towns and thus, not direct anthropogenic impacts on the lake

are possible. Records of extended fires in the region for the past 10,000 years have been reported in

Kitzberber et al. (1997) and Veblen et al. (2003).

Surrounded vegetation is composed by a mixed North Patagonian rainforest communities

with mainly evergreen deciduous Nothofagus spp. The area is predominantly a cold-temperate. At

this latitude, the Patagonian region is dominated by air masses coming from the Pacific Ocean and

strong constant west-winds (westerlies) are dominant across the region.

Three short sediment cores (Fig. 2) were extracted in December 2005 using a gravity corer

(Ø PVC liner 6.3 cm). All cores were cut in a longitudinal way, opened, visually inspected, and

sub-sampled every 1 cm. Each sub-sample was freeze-dried and homogenized. Cores were

correlated using magnetic susceptibility, tephra markers and Lithology features.

Core Hess 05-3 was studied at LAAN CNEA for radiometric dating, while cores Hess 05-1

and 05-2 were studied for palaeolimnological indicators.

Water content , organic matter (loss-on-ignition at 550 °C), total carbon, sulphur and total

nitrogen (by CHS analyzer) were measured at CNR-ISE. Photosynthetic pigments were extracted in

90% acetone, overnight in the dark, under nitrogen. The extract obtained was used both to quantify

the chlorophylls and their derivatives (Chlorophyll Derivatives Units, CD) and total carotenoids

(TC) by spectrophotometer. Individual carotenoids were detected by Reversed Phase High-

Performance Liquid Chromatography using a Dionex SUMMIT with a DAD-UV detector (Lami et

al., 2000). Carotenoid concentrations were expressed in nanomoles per gram of organic matter

(nmol g-1 OM) and chlorophyll derivates in units per gram of organic matter (U g

-1 OM).

Chironomid analysis were performed on 82 samples, 1 cm interval, along the core Hess 05-

1. Subsamples of 3-4 cm3 wet weight (WW) were deflocculated in 10% KOH and heated to 70 °C

for 20 minutes and sieved through a 212 µm and 95 µm mesh size. Head capsules were picked out

from a Bogorov sorting tray under a stereo microscope at 25-40 x magnification. Larval head

capsules were mounted in Euparal and then identified with reference to Wiederholm (1983).

Chironomid and geochemical data were plotted using C2 software (Juggins, 2003). Zones were

derived by stratigraphically constrained cluster analysis (CONISS).

Chronology

Core dating was performed by 210

Pb and 137

Cs techniques. 210

Pb, 226

Ra (in secular equilibrium with

supported 210

Pb), and 137

Cs specific activity profiles were determined by high resolution gamma ray

spectrometry on core Hess 05-3. The samples were placed in a cylindrical plastic container (46 mm

diameter and 8 mm height), sealed, and counted in close sample-detector geometry. 210

Pb and 226

Ra

specific activities were obtained by measuring the 46 keV (from 210

Pb), 295 keV and 351.9 keV

(from the decay of the 226

Ra short-lived daughters) emissions with a planar HPGe (LO-AX)

detector. 137

Cs specific activity was determined by measuring the 661.7 keV emissions using an

HPGe detector, 30 % relative efficiency.

The Constant Rate of Supply (CRS) model was used for 210

Pb dating (Joshi and Shukla,

1991; Robins and Herche, 1993). Correction of the old date error of the CRS model (Binford, 1990)

was implemented by logarithmic extrapolation of the measurements to complement integration to

infinite depth. For 137

Cs dating, the specific activity profiles was compared with the fallout

sequence determined in this region, associated mainly with South Pacific nuclear tests from 1966 to

1974 (Ribeiro Guevara and Arribére, 2002).

Results and discussion

Chronology

210

Pb, 226

Ra, and 137

Cs specific activity profiles from Lake Hess core 05-3 are shown in Figure 3.

The total 210

Pb specific activity (supported 210

Pb) was significantly higher than 226

Ra only in the

five topmost core layers. Therefore, only a 210

Pb estimation of the sedimentation rate in the upper

core section was achieved. Since there were a few unsupported 210

Pb determinations, with high

uncertainties associated, the measured values of the CRS model were fitted in logarithmic scale and

interpolated values were used. The estimation of the sedimentation rate obtained is 23.2 mg cm-2 y

-1

(0.148 cm y-1). Extrapolated age values have been calculated using the mean post 1950

sedimentation rates of 23.2 mg.cm-2 y

-1 below the 8 cm depth, since from this point downward the

continuity in sedimentation rate cannot be assumed due to the strong changes in sediment density

The 137

Cs fallout sequence in lake sediments from the Nahuel Huapi National Park area has

been determined previously (Ribeiro Guevara and Arribére, 2002; Ribeiro Guevara et al., 2003).

The most intense fallout peaks were registered in 1964 to 1966, as a consequence of the

combination of stratospheric and tropospheric fallout, with the tropospheric contributions coming

from the South Pacific nuclear tests. Another 137

Cs peak, associated mainly with tropospheric

fallout, was detected in ca 1970–1972, with a secondary peak in 1974. No other relevant 137

Cs

fallout were evident not even the Chernobyl nuclear power plant accident in 1986 that produced

little stratospheric 137

Cs fallout in 1987 and 1988, not relevant for dating purposes. The 137

Cs

specific activity profile in upper 6 layers does not follow the fallout sequence, and the reasons could

be the bioturbation produced by different organisms, Cesium mobilization after deposition and/or

sediment mixing. According to the extrapolation of the sedimentation rate estimated by 210

Pb the

base of the sixth layer corresponds approximately to 1965, in coincidence with the 1964 fallout

peak estimated by 137

Cs dating.

Core Hess 05-1 is longer than cores 05-2 and 05-3, and shows a different sedimentation

pattern. Although this core was extracted close by the other cores, the presence of bubbles and other

lithological material interbedded in the sedimentary sequence, suggest processes that had affected

normal sedimentation in the lake. Instead, cores 05-2 and 05-3 appear to have a more similar

sedimentary sequence (Figs. 2, 4).

Lithology and core-to-core correlation

The lithology of the cores show a general common pattern; the upper part it is a unit

composed by mix of black silty clays with high abundance of macrophyte remains (Fig. 2).

Although this zone has a variable thickness in the different cores (from 8 to 22 cm), it .shows

similar low values of dry weight and density and high values of water content (Figs 2, 4). Below

this zone, the sequence is composed by an homogeneous dark grey-brownish silty clay sediment,

with some scattered macrophyte remains. The other remarkable feature is the tephra layers

intercalated in the sequence. Magnetic susceptibility in core 05-2, as well as density and LOI

support very well these ash layers (Fig. 4) Based on LOI a good correlation of cores 05-1, 05-2 and

05-3 was established (Figs. 2, 4).

We chose the longest sequence core Hess 05-1 for lithological description. This sedimentary

sequence consists of three lithological units: I) light gray clays with tephra layers; II) dark gray

clays and III) black massive clays with abundant remains of macrophytes.

Abundant remains of macrophytes are present between 0-6 cm, 12-15.5 cm and 18-22 cm.

The gray clays are composed of very fine and plastic sediment with a faint lamination. This

lithological feature is present below the level of 22 cm and includes reworked tephra layers at 25-

26cm, 32-38 cm, 55-56cm and 58-59cm (see below). Tephra layers are located at different depths in

the three cores (Fig. 2). The detailed observation of the tephra layer located at 24-26 cm depth in

core Hess 05-1 allows the identification of brownish glassy shards, dark and grey fragments (scoria

and rock fragments), remains of vegetables and diatom frustules, among other organic materials,

included in a silty -clay matrix. This layer was related to the tephra located below 19-21 cm in core

Hess 05-3 (Fig. 2). According to this characterization based on macroscopic features, this ash

deposition at different depth on both cores could be associated to the same volcanic event (Fig. 2).

A detailed microscopic and geochemical characterization of the material is needed to achieve a

better understanding of these events. Reagarding the tephra layer identified at 16-17 cm in Hess 05-

2, (located 3 cm up in Hess 05-3) complementary analysis are necessary to ensure that correspond

to the same event.

Other tephra layers were identified in Hess 05-1 at the moment of sub-sampling, at 32-35

cm, 36.5-38 cm and 58-59 cm depths which were not identifiable in core Hess-05-3. Instead in core

Hess 05-3, a coarse tephra layer is visible at 46-47 cm depth (Fig 2). In core Hess 05-2, four

separated strong peaks of magnetic susceptibility, between 60 and 70 cm depth, evidence different

volcanic events (Fig. 4).

Concluding, figure 2 and 4 show these tephra layers in the different cores of Lake Hess and

their tentative correlations although the unfeasibility to assign the time of eruptions without a

geochemical characterization which allow to recognise its source (Daga et al., 2008a). Previous

studies performed on sedimentary sequences from water bodies from Hahuel Huapi National Park

demonstrated that the region was impacted by high frecuency volcanic episodes such as Calbuco

volcano and Puyehue-Cordón Caulle volcanic system but being Calbuco volcano eruptions the most

important affecting the South of the Park (Daga et al., 2006, 2008a).

LOI, nutrients and pigments

As magnetic susceptibility, dry weight profiles are indicative of the presence of tephra layers or

clastic materials derived for example from high erosion or floods. In our case, dry weight and water

content fluctuations are strongly related to tephra layers: three high peaks in core 05-1 are shown at

ca. 57, 43 and 25 cm core depth. Dry weight is negative correlated with organic matter (LOI). The

upper 10-20 cm layer is rich in plant remains.

Figure 2 and 4 summarize the data about core-to-core correlation and impact of the tephra

deposits (the black layers in the cores; Fig. 2) on water content and organic matter. The same effect

of tephra and the dramatic increase in LOI in the upper part is also seen in core 05-2 (Fig. 4).

From ca. ca. 25 cm upwards a remarkable increase in LOI (from ca. 8-9 % d.w. to ca. 40%

d.w.) and plant pigments document a change in the trophic condition of the lake (Fig. 5). Sulphur

after a similar increase, decrease in the very recent time. A long period of very low productivity is

shown from 40 cm to 25 cm of the core 05-1. It is possible that this period coincides with the LIA.

Calcium carbonate content is lower than 5 % d.w.

Similarly to LOI profile, total carbon and nitrogen show the same recent increase from ca.

25 cm onward (Fig. 5). From core bottom to 20 cm C and N show wide fluctuations in part related

to the impact of volcanic tephra deposits.

From the C:N ratio we infer that the source of organic matter is changing over time with a

sharp increase from 25 cm onward associated with evident input of allochthonous materials (C:N >

10). The section comprising 50cm and 25 cm shows a more autochthonous origin of the organic

matter (C:N values around 9-10).

From the different chemical profiles (Figs 5, 6) and similarly to the chironomids

assemblages changes (see below) we can recognize three main zones: zone I (85-40 cm), zone II

(40-25 cm) and zone III (25-0 cm).

The oldest part of the core, zone I (40-85cm), show relatively high value of algal pigment

concentration pointing out to a period of high primary production even if lower compared to that

one observed near the core top (zone III). The phytoplankton composition consist of cyanobacteria

and siliceous algae. Along this period there is also a phase (45-55cm) were cryptophytes carotenoid

shows relatively high value. The presence of cryptophytes is usually associated to high lake water

level. In this core zone is also evident a lower grazing pressure as shown by the generally higher

value of the herborivicity index compared to the rest of the core (Fig. 6).

In section II (40-25 cm), total and specific pigments (Figs 5,6) were barely detectable and

point out to a very low productive phase probably due to cold condition since this coincides with

the low productive phase observed among the chironomid remains. Among the specific carotenoids

there was the only exception of a very short lived event around ca. 30 cm dominated by the

presence of terrestrial vegetation (lutein) and cyanobacteria (echinenone and zeaxanthin).

The recent phytoplankton biomass (25 cm-top), as detected by the pigment analysis (Fig. 5,

6), show higher values in concentration respect to oldest periods (80-25 cm) and is dominated

siliceous algae. The carotenoid composition of this section is also characterized by the presence of

planktonic algae (alloxanthin), particularly abundant between 5-10 cm, and lutein that show the

highest value in this core section. Lutein, as well as chlorophyll b, are specific pigments not only in

algae, but they are also present in terrestrial vegetation and aquatic macrophytes. Therefore their

concentration peaks are very likely associated to the presence of these remains.

Chironomids

Lake Hess chironomid record is represented by a blend of profundal and littoral/fluvial taxa. (Fig.

7) .

The CONISS cluster analysis allowed us to divided the assemblage in three distinct zones: zone

I (from 85 to ~40 cm) is characterised by the coexistence of upperlittoral, littoral and fluvial

elements with profundal taxa. Although profundal taxa are present with high abundances (between

20-40%), the presence of Eukiefferiella (fluvial) and Demicryptochironomus (upperlittoral) together

with other littoral elements is indicating low water level. In Zone II (from 40 to 25cm) the

abundance of heads capsules drops sharply. The dominant and almost sole taxa present in this zone

are Ablabesmya and Parapsectrocladius showing abundance values close to 80%. Littoral and

fluvial elements disappeared. The last zone, zone III, from 25 cm upwards, is characterised by the

increase of Chironomus and Riethia and the revisit of littoral taxa such as Limnophyes and

Cricotopus . It is interesting to notice the total disappearance of Apsectrotanypus and Eukiefferiella

which were very well represented in zone I. Profundal taxa such as the Tribu Tanytarsini are poorly

represented along the whole sequence.

In zone I, the occurrence of a mix littoral/profundal chironmid assembalges could be related to a

decrease of the precipitations and low water levels. As a consequence, the lake dries becoming a

flow carrying melting waters from Tronador ice cap. The presence of fluvial (Eukieferiella ) and

upperlittoral chironomid remains could be linked to this lotic situation of Lake Hess (Fig. 7).

A long period of low productivity from ca. 40 cm to ca. 20 cm (zone II) coincides in part with a

generally cold and wet period (LIA) as also reported by a study of a nearby lake, Lake Frias

(Ariztegui and Davaud, 2007). During this period chironomid assemblages are represented by

profundal taxa, indicating an increase in the precipitations, increase of the lake water level and

therefore, a totally lentic regime. Other aspect to remark in section II is the decrease in the

concentration of head capsules (total HC; Fig. 7) just after the tephra deposition in coincidence with

the decrease of all biotic parameters studied (Figs 5, 6). Three tephra layers were identified in this

section, recording volcanic events that may affect strongly lake dynamics. Only Ablabesmya and

Parapsectrocladius relative abundance increased in section II, while other taxa decreased

drastically, revealing an advantage of these two genera to grow in this kind of sediment as observed

by Urrutia et al. (2007) in lake Galletué (Chilean Andes).

In zone III, like in zone I, the littoral/profundal chironomid fauna is again showing low water

levels and a decrease of the precipitations. From pigment, and chironomid total abundance profiles

is possible to distinguish a further trophic change in the topmost 10 cm of zone III which could be

linked to the increase of the temperature in recent times. A strong increase in nutrient (C, N) and

sulphur concentration occurs synchronous to these biotic changes. Also, the presence of

macrophytes can be related to a low water level and dry periods which in turn, addresses to an

increase of productivity.

Conclusions

Multi cores, multi-proxy analyses at Lake Hess show a number of palaeolimnological changes

through the core depth which represent ca.5-6 centuries of history.

Lake Hess sediments were rich in tephra deposits particularly in the lower part of the cores.

The lithology includes tephra layers that have been used for cores correlation and are clearly

represented by peaks in the magnetic susceptibility values.

Based on algal biomass, nutrient and chironomid assemblage analyses, major environmental

changes occurred mainly at three levels along core 05-1: at 85-40 cm (zone I), 40-25 cm (zone II)

and 25- top (zone III) (Figs. 5, 6, 7). We believe that these changes are strongly affected by the

physical environment such as water level fluctuations and the lotic/lentic regime of the lake

incoming water from the glaciers associated to dry/wet periods.

Results of a previous paleolimnological study developed in a short core from the small

closed basin of Lake Morenito (Massaferro et al., 2005) showed that chironomid and pigment

changes in the last 200-300 yrs BP were related to productivity changes. Although tephra layers

from volcanic events deposited in the lake may have had an impact on the biota, main biological

changes may have been in response to human-related environmental disturbances in the catchment

area.

A study performed in a sedimentary sequence collected in Lake Toncek (1700 m a.s.l) from

Nahuel Huapi National Park show natural and anthropogenic events occurred in the region during

last 900 years (Rizzo et al., 2007; Daga et al., 2008b; Ribeiro Guevara et al., 2008). The study of

subfossil chironomid assemblages showed from 11th to 17

th centuries the predominance of

Pseudosmittia and cold-stenothermic Podonominae, which decrease in the upper layers (18th

century to present) being replaced by the warm-adapted Tanypodinae. These observations together

with a continuous increase of abundance and diversity indices during 20th century (Rizzo et al.,

2007).agrees with the observations in lake Hess

Summarising, remote oligotrophic lakes Hess and Toncek showed biological changes related

to climate and regional environmental changes. Instead, Lake Morenito, located in a human

disturbed environment, evidenced changes mainly related to the productivity in the last 100 years .

Biological and Chemicals sediment profiles from Lake Hess confirm a cyclic pattern of climate

change. Chironomid assemblages show profundal, littoral, sublittoral, terrestrial and fluvial elements

which may indicate repeated dry/wet periods.

In conclusion, owing the remote location of this lake, the observed changes in the biological

and chemical parameters are likely related to climate variations. This high resolution short core

multiproxy study ,allow important conclusions about changes in moisture balance bring about by

the westerlies and occurrence of volcanic events in the area. Further research is needed in

Patagonia, particularly to test climate changes came about during the last millennium, such as

ENSO, LIA and other short oscillation climate periods in southern South America

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Figure Captions

Fig. 1. Map showing location of the study Lake Hess (Northern Patagonia, Argentina). Aerial view

of Lake Hess is also shown.

Fig. 2. Core correlations based on lithology and tephra layers (t) from Lake Hess. Dotted line

correlate the top layer rich in macrophytes. Sedimentological units are also indicated.

Fig. 3. 137

Cs specific activity profile 210

Pb and 226

Ra (in secular equilibrium with supported 210

Pb of

Lake Hess (core 05-3).

Fig. 4. Lake Hess, cores 05-1, 05-2 and 05-3. Depth distribution of dry weight (DW, % w.w.),

density (g m-3), magnetic susceptibility (MS, in SI 10

-5) and organic matter content (LOI, % d.w.).

Magnetic susceptibility in core 05-2, and density put in well evidence the presence of tephra layers.

Core correlation between cores 05-1 and 05-2, based on LOI, is shown by numbers. Asterisk show

core correlation between cores 05-3 and 05-2 based on DW. Details about the three cores and their

correlation based on lithology and tephra layers are also shown in figure 2.

Fig. 5. Nitrogen (N), carbon (C), their ratio, sulphur (S), in per cent of dry weight, total chlorophyll

derivatives (CD, Units g-1 LOI), total carotenoids (TC; mg g

-1 LOI) in core 05-1 of Lake Hess.

Zones I-III resulting from cluster analysis (CONISS) are also indicated.

Fig. 6. Selected specific carotenoids and chlorophylls in core 05-1 of Lake Hess. Zones I-III

resulting from cluster analysis (CONISS) are also indicated.

Fig. 7. Chironomid percentage diagram showing the most abundant taxa found in the Lake Hess

sediment core (Hess 05-1). The taxa are grouped according to littoral (L), profundal (P) and

fluvial (F) habitats. Zones I-III resulting from cluster analysis (CONISS) are also indicated.

Fig. 1

Fig. 2.

-1,6

-1,4

-1,2

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0 10 20 30 40 50

137

Cs

detection limit

depth (g.cm

-2)

0 20 40 60 80 100

specific activity (Bq.kg-1)

Total 210

Pb

226

Ra

Fig. 3

Fig. 4.

0 20 40 60

CD

0.0 0.2 0.4

TC

0.0 0.5 1.0 1.5

N

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

0 10 20

C

7.2 9.6 12.0

C:N

0.0 0.2 0.4 0.6

S

cm

% d.w.

I

II

III

% d.w. % d.w. U g LOI-1 mg g LOI-1

2005

1986

1952

1870

1770

1600

year A.D.Age

Fig. 5.

0 100 200 300

Chlorophyll a

0

Chlorophyll b

20

Alfa-carotene

0 20 40

beta-Caroten

e

0 200 400

Fucoxanthin

0 20 40

Diatoxanthin

0 20 40

Alloxanthin

0 40 80

Lutein

0 20 40

Echinenone

0

Zeaxanthin

all taxa Diatom-Chrysophyceae Cryptophyceae Chlorophyceae Cyanobacteria

nMoles g LOI- 1

10 20 100 0 100 200 300

Pheophythin

tot.

0 100

Pheophorbid

es tot.

0 100

Herborivicity

index

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85 0 10

2005

1986

1952

1870

1770

1600~

~

~

year A.D.Age

III

II

I

Fig. 6.

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

040

80

Ablabesmya

040

80

Parapsectrocladius

040

Apsectrotanypus

0

Tanytarsini 1A

0

Tanytarsini 1B

040

80

Paratanytarsus

0

Labrundinia

040

80

Chironomus

040

80

Riethia

0

Limnophyes

040

80

Polypedilum0

40

80

Microtendipes

0

Dicrotendipes

0

Cricotopus

0

Eukiefferiella (fluvial)

0

Demicryptochironomus

060120180

Total HC

profundal

littoral +

fluvial

I II

III

cm

2005

1986

1952

1870

1770

1600

year A

.D.

Age

~~~

Fig. 7

.