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Quaternary International 161 (2007) 4–21

A 14kyr record of the tropical Andes: The Lago Chungara sequence(181S, northern Chilean Altiplano)

A. Morenoa,�, S. Giraltb, B. Valero-Garcesa, A. Saezc, R. Baod, R. Pregoe, J.J. Pueyoc,P. Gonzalez-Samperiza, C. Tabernerb

aPyrenean Institute of Ecology—CSIC, Apdo 202, 50080 Zaragoza, SpainbInstitute of Earth Sciences ‘Jaume Almera’-CSIC, C/Lluıs Sole i Sabarıs s/n, 08028 Barcelona, Spain

cFaculty of Geology, University of Barcelona, C/Martı Franques s/n, 08028 Barcelona, SpaindFaculty of Sciences, University of A Coruna, Campus da Zapateira s/n, 15071 A Coruna, Spain

eDepartment of Marine Biochemistry, Marine Research Institute, CSIC, C/ Eduardo Cabello 6, 36208 Vigo, Spain

Abstract

High-resolution geochemical analyses obtained using an X-ray fluorescence (XRF) Core Scanner, as well as mineralogical data from

the Lago Chungara sedimentary sequence in the northern Andean Chilean Altiplano (181S), provided a detailed reconstruction of the

lacustrine sedimentary evolution during the last 14,000 cal. yr BP. The high-resolution analyses attained in this study allowed to

distinguish abrupt periods, identify the complex structures of the early and mid-Holocene arid intervals and to compare their timing with

Titicaca lake and Sajama ice records. Three main components in the lake sediments have been identified: (a) biogenic component, mainly

from diatoms (b) volcanics (ash layers) from the nearby Parinacota Volcano and (c) endogenic carbonates. The correlation between

volcanic input in Lago Chungara and the total particles deposited in the Nevado Sajama ice core suggests the Parinacota Volcano as the

common source. The geochemical record of Lago Chungara indicates an increase in siliceous productivity during the early Holocene,

lagging behind the rise in temperatures inferred from the Nevado Sajama ice core. The regional mid-Holocene aridity crisis can be

characterized as a number of short events with calcite and aragonite precipitation in the offshore lake zones.

r 2006 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Recent research of past climate oscillations has foundthat changes between climate modes during the Holoceneoccurred within decades (Mayewski et al., 2004), a periodof time similar to more recent climate changes (Houghtonet al., 2001). In this context, scientific efforts over the lastfew years have been directed towards understanding thetiming and mechanisms of abrupt climate changes duringthe last millennia. Despite the recent increase in the numberof high-resolution paleoclimate records from low latitudes(e.g. Hughen et al., 1996, 2004; Kuhlmann et al., 2004), therole of the tropics in abrupt Holocene climate changesremains a matter of debate. Tropical South Americaexemplifies the complexity of Holocene climate reconstruc-tions, in which high-resolution terrestrial records, essential

e front matter r 2006 Elsevier Ltd and INQUA. All rights re

aint.2006.10.020

ing author. Tel.: +34976 716118; fax:+34 976 716019.

ess: amoreno@ipe.csic.es (A. Moreno).

to any examination of rapid climate fluctuations, arescarce, with diverse proxy records showing numerousdiscrepancies (i.e. Grosjean, 2001). Detailed knowledge ofthe distribution and amplitude of abrupt climate changes intropical latitudes of the Andean Altiplano is still sparse andthe processes responsible for climate variability at differenttemporal and regional scales are barely understood. Thesuggested close link between higher lake levels in theAndean Altiplano and cold sea surface temperatures in theEquatorial Atlantic (i.e., Heinrich events, Younger Dryas,8.2 kyr event or the Little Ice Age) indicated by the Titicacalake record (Baker et al., 2001a) requires additional recordsfrom tropical South America to confirm this paleoclimateteleconnection between the two hemispheres.The evolution of temporal and spatial moisture patterns

during the Holocene is one of the main controversiessurrounding studies of South American paleoclimate. Ithas been generally accepted that the northern-centralAndes were a generally arid region from �7 to 4 kyr BP

served.

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Fig. 1. (a) Location of Lago Chungara and other paleoclimatic records on

the Northern Chilean Altiplano. (b) Position of sediment cores in Lago

Chungara. Isobaths and main inflows are indicated.

A. Moreno et al. / Quaternary International 161 (2007) 4–21 5

as observed in lacustrine (Abbott et al., 1997; Baker et al.,2001a; Grosjean et al., 2003; Paduano et al., 2003; Tapiaet al., 2003) and ice-core records (Thompson et al., 1998;Thompson et al., 2000). This hypothesis is also supportedby archeological evidence (Nunez et al., 2002). In additionto moisture reconstructions, a recent study of long-chainalkenones from Titicaca lake sediments also points toenhanced regional temperatures during the mid-Holocene(Theissen et al., 2005). However, other recent studiessupport a more complex spatial and temporal pattern, andeven periods of increased humidity during the mid-Holocene (Holmgren et al., 2001; Latorre et al., 2003;Servant and Servant-Vildary, 2003). Paleoclimate sedimen-tary records possessing a robust and accurate chronologi-cal framework are therefore imperative to understandingboth the regional significance and the timing of abrupthumidity changes detected during the mid-Holocene.The overall goal of this study was to document the

regional pattern of climatic change for the last14,000 cal. yr BP using a sedimentary record from LagoChungara (Andean Altiplano, 181S). This paper reports ahigh-resolution geochemical record from the lake obtainedby an X-ray fluorescence (XRF) core scanner together withother paleoenvironmental indicators (i.e. physical proper-ties, mineralogy, opal content and total organic carbon(TOC)). The resulting high-resolution analyses, in tandemwith a multi-proxy approach, allowed us not only to inferthe paleoclimate signal from the Lago Chungara record,but also to contribute to the identification, correlation andunderstanding of abrupt climate change during theHolocene in tropical regions of South America.

2. Location, climate and limnology of Lago Chungara

Lago Chungara (181150S, 691100W, 4520m asl) is locatedin the highest and westernmost fluvio-lacustrine basin inthe Andean Altiplano (Northern Chile, Fig. 1a). This lakesits in the central part of a small hydrologically closedsubbasin at the northeastern edge of the Cenozoic LaucaBasin. The intense volcanic activity and, to a lesser extent,the movement of synsedimentary faults are significantfactors for sedimentation in the Chungara subbasin. TheLago Chungara subbasin was formed after the collapse ofthe Parinacota Volcano (Fig. 1a), which produced a hugedebris avalanche blocking the Paleo-Lauca River at about15–17 kyr BP (Worner et al., 1988; Worner et al., 2000;Worner et al., 2002). However, the age of this collapse iscontroversial, and it has been estimated at 8 kyr by otherauthors (Clavero et al., 2002, 2004). The local vegetation isdominated by tussock-like grasses, shrubs, Polylepsis, adwarf tree of the Rosaceae family, as well as extensivesoligenous peatlands (‘‘bofedales’’) (Schwalb et al., 1999;Earle et al., 2003).Lago Chungara is climatically located in the arid Central

Andes. This region is dominated by tropical summermoisture stemming from the Amazon Basin, and iscontrolled by the southward migration of the subtropical

jet stream as well as the intensification of the Bolivian high-pressure system (Garreaud, 2001; Garreaud et al., 2003).Average annual rainfall in the region is about 350mm. Asignificant fraction of the inter-annual variability ofsummer precipitation is currently related to the El NinoSouthern Oscillation (ENSO) (Vuille, 1999). Thus, wetsummers on the Andean Altiplano are associated with anENSO-related cooling of the tropical Pacific (La Ninaphase).Lago Chungara has a maximum water depth of 40m, a

surface area of 22.5 km2 and a volume of about426� 106m3 (Risacher et al., 2003; Herrera et al., 2006).The main inflow is the Chungara River (300–460 l s�1) andseveral springs on the western margin. Although there is nosurface outlet, groundwater outflow was estimated as0.2m3 s�1 (Montgomery et al., 2003) and the water lostby potential evaporation measuring about 1230mmyr�1

(Risacher et al., 2003). The lake is polymictic, meso toeutrophic and contains 1.3 g l�1 total dissolved solids

ARTICLE IN PRESSA. Moreno et al. / Quaternary International 161 (2007) 4–216

(Muhlhauser et al., 1995). Water chemistry is ofNa–Mg–HCO3–SO4 type with an average pH of 9. Atpresent, oxic conditions extend to the lake bottom. Primaryproductivity in the lake is mainly governed by diatoms andchlorophyceans. During four sampling periods from 1998to 1999, biomass values fluctuated from 0.34 to 8.74mgChlorophyll a l�1 (Dorador et al., 2003). Oscillations inboth, phytoplanktonic biomass and phytoplanktonic com-munity structure seem to be mainly due to changes in watercolumn temperature and salinity.

3. Materials and methods

In November 2002, 15 sediment cores (6.6 cm innerdiameter and up to 8m long) were retrieved from LagoChungara along the NW–SE and NE–SW transects of thelake (Fig. 1b). The core locations were selected to samplethe different depositional environments using a modifiedKullenberg piston corer from the Limnological ResearchCenter, University of Minnesota (LRC). The retrievedcores were shipped to the LRC where physical properties(GRAPE-density, p-wave velocity and magnetic suscept-ibility) were non-destructively measured every cm using aGEOTEKTM Multi-Sensor Core Logger (MSCL). Thecores were then split in two halves, scanned for colorpictures using a DMT CoreScan digital color line scancamera system, and macroscopically described in terms of

Fig. 2. Digital image, lithological column and magnetic susceptibility profile fr

(not calibrated) are indicated (see Table 2). A detailed facies legend is shown.

texture, color and sedimentary structures (Core 11, Fig. 2).Smear slides were described using a Nikon polarizingmicroscope to estimate the biogenic, clastic and endogenicmineral content of the defined sedimentary facies. Sub-samples were taken every 5 cm for mineralogical, chemicaland biological analyses. After a detailed lithologicalcorrelation of all cores (Fig. 3 and Saez et al., 2007), cores10 and 11 were selected for paleoclimatic and paleoenvir-onmental reconstructions. Both cores recorded almost theentire sedimentary infill of the offshore zone, allowingreconstruction of a composite sequence.Total carbon (TC) and total inorganic carbon (TIC)

contents were determined by a UIC model 5011 CO2Coulometer, with TOC content then calculated. Samplesfor X-ray diffraction (XRD) were dried at 60 1C over 24 hand manually ground using an agate mill. XRD analyseswere performed using an automatic Siemens D-500 X-raydiffractometer: Cu ka, 40 kV, 30mA and graphite mono-chromator. Identification and quantification of the differ-ent mineralogical species present in the crystalline fractionwere carried out following a standard procedure (Chung,1974). The area of the amorphous fraction was calculatedas total counts using the XRD software. The sample thatshowed the highest amorphous area, formed mostly bydiatomaceous oozes, was progressively mixed with increas-ing quantities of pure calcite (5%, 10%, 20%, 40% and60% of the total weight of the sample) and a logarithmic

om core 11. Key layers M-1 to M-11 and available 14C dates for this core

See text for explanation.

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Fig. 3. NW–SE stratigraphic cross-section including the six long cores retrieved from Lago Chungara (cores 12, 13 and 15 are projected as indicated in the

figure). Stratigraphic correlations are based on lithostraphic and sedimentological criteria (limits between units, key levels M-1 to M-11) and magnetic

susceptibility profiles. A small fault detected from seismic profiles (Saez et al., 2007) is also shown. WAF: White ash flow.

A. Moreno et al. / Quaternary International 161 (2007) 4–21 7

function was adjusted. This function allowed expressingthe area of the amorphous fraction as a percentage of thetotal sample weight.Core 11 was analyzed for opal (biogenic silica) content

following an alkaline leaching technique (Mortlock andFroelich, 1989). After leaching, the dissolved silicaconcentration of the resulting extract was measured bythe molybdate blue colorimetric method (Hansen andGrashoff, 1983) using an AutoAnalyser Technicon II.In addition to MSCL core logging methods, the XRF

core scanner was applied. XRF data used in this study wereproduced by the new-generation XRF core scanner at theUniversity of Bremen. The core archive halves weremeasured with 2mm resolution for light (Al, Si, S, K,Ca, Ti, Mn and Fe) and 1 cm resolution for heavy (Sr, Zr,Sn and Ba) elements. The measurements were producedusing 60 s count time, 10 kV X-ray voltage (50 kV for heavyelements) and an X-ray current of 1mA to obtainstatistically significant measurements. The analyzed sec-tions conformed to a composite sequence of cores 10 and11 (Fig. 3). A detailed description of the applied XRFanalysis and system configuration of the XRF core scannerat the University of Bremen are provided by Jansen et al.(1998) and Rohl and Abrams (2000).The data obtained by the XRF core scanner are

expressed as element intensities in counts per second(cps). A comparison with other methods employed toconvert the XRF core scanner measurements to absolute

elemental concentrations failed. Sixty samples were ana-lzsed by inductively coupled plasma-optical emissionspectrometry (ICP-OES, Perkin Elmer Optima 3300 RL)and 30 samples by powder XRF at the University ofBremen (Portable Energy Dispersive Polarization X-rayFluorescence analyzer, Spectro Xepos). Although therewas a high correlation between both methods and thescanner data for some elements, the overall correlation wasnot of sufficient quality to calibrate all elemental data.Consequently, and consistent with most published studies(e.g. Jansen et al., 1998; Rohl and Abrams, 2000), theoriginal XRF core scanner data are expressed in cps.Finally, the statistical treatment of the dataset wasperformed using the R software package (R DevelopmentCore Team, 2004).The AMS 14C dates from the Chungara sediments were

obtained from (1) bulk organic matter from the centralplain cores and (2) aquatic organic macrorests picked fromlittoral cores (Table 2). Several carbonate samples werealso dated with U/Th techniques.

4. Results and interpretation

4.1. Lithostratigraphy

Most facies in Lago Chungara sediments (Fig. 2) aremassive or laminated diatomaceous ooze (A, B, C, D, E),while carbonate-rich facies occur in thin layers or laminae

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(F). Volcaniclastic facies (G and H) are particularlyabundant in the upper part of the cores. The presence ofthese tephra layers and their continuity along the basinallows a detailed correlation of the retrieved cores. Tephralayers are labelled from M-1 to M-11 (from bottom to top)and correspond to peaks in magnetic susceptibility (Figs. 2and 3). Additionally, the presence of a white ash-flow(WAF), in spite of the low magnetic susceptibility signal,was used for correlation since it occurs in all cores. Fromstratigraphical correlation and seismic stratigraphy (Saezet al., 2007), two main lithostratigraphic units were defined.Unit 1 was deposited after the volcanic event that createdthe lake and is composed of laminated diatomaceous ooze(Facies A, B and C). Only one 2 cm thick glass-dominatedtephra layer (M1) occurs in Unit 1 (Figs. 2 and 3). Theselamination sets comprise rhythms and cycles of differentmm-sized layers (average 2.6mm) consisting of diatomac-eous ooze with variable types (calcite, aragonite) andquantities of carbonates and amorphous organic matter.Unit 1 is divided in two subunits: Subunit 1a with greenand white laminations and no carbonate (Facies A) andSubunit 1b with brownish to white laminations (Facies B)where endogenic carbonates occur in low concentrations.Carbonates occur in the lighter laminae (Facies F).Towards the upper part of Subunit 1b, laminated facies(Facies B) alternate with intervals of dark green, massive,organic-rich diatomaceous ooze (Facies C). Facies C layersare continuous throughout the basin and were detectedin all cores from the central plain (lithological keybedsin Fig. 3).Unit 2 is mainly composed of massive to slightly banded

diatomaceous ooze (Facies D and E) frequently inter-calated by tephra layers (coarse- and fine-grained ashfalls,Facies G and H). This unit is also divided into twosubunits. Subunit 2a is composed of brownish-red massiveto slightly banded sapropelic diatomaceous ooze (Facies D)with common calcite crystals (silt grain-sized) and carbo-nate-rich layers (Facies F). Facies F (25–55% carbonate)represents up to 5% of the total thickness of Subunit 2a.Specifically, carbonate-rich layers are grouped in discretewhitish to pinkish thin layers, which are composed ofcalcite (fibrous crystals, fusiform aggregates, rice-shapedand euhedral crystals), magnesium calcite, needle-like aragonite and some traces of dolomite. Tephra layersare mainly composed of Ca-rich feldspars and volcanicglass. In particular, the presence of three lapilli layers(Facies G) is highlighted since their lateral continuityallowed considering them as key layers for correlatingthe cores (M-4–M-6 in Fig. 3). Subunit 2b consists ofdark gray diatomaceous ooze with frequent macrophyteremains (Facies E) alternating with massive black tephralayers, mainly composed of plagioclase, glass and maficminerals (Facies H). Volcaniclastic deposits represent50% of the total thickness of this unit (levels from M-8to M-11 in Figs. 2 and 3). A 1-cm-thick rhyolitic WAF in(Figs. 2 and 3) occurs in all cores and is used forcorrelation.

4.2. Mineralogical composition

The sediments of Lago Chungara are mainly composedof two fractions: one crystalline (highlighted by sharpdiffraction peaks), the other amorphous (characterized bythe presence of a broad peak centered between 201 and 2512y angles). In accordance with the previously describedcalculations, the amorphous component was quantified.The amorphous fraction percentages range from 40% (involcaniclastic-rich deposits of Unit 2) to almost 100% (inUnit 1). This fraction represents organic matter, amor-phous silica (from diatoms) and volcanic glass. On theother hand, the percentages of the total crystalline fractionrange between 60% and less than 1% of the total weight ofthe samples, with this fraction composed of Ca-plagioclase,carbonates (calcite and dolomite), biotite, pyrite, quartzand amphibole. In Fig. 4 the main minerals from LagoChungara sediments are represented versus the compositedepth. From the bottom to the top of the sequence, andbased on the dominance of the fraction and the mineralspecies, three zones were defined. These zones broadlycorresponded to the lithostratigraphic units (Fig. 4). Unit 1is dominated by amorphous material (more than 95% ofthe total weight of the samples). Plagioclase, quartz andpyrite are the main mineral species that comprise thecrystalline fraction (o 5%). The presence of silt-sizedcalcite crystals is the main characteristic of Subunit 2a.From a depth of 350 cm upwards, the plagioclasepercentages start to increase whereas the amorphousfraction percentages decrease. This change represents theincrease of volcanic activity in the nearby ParinacotaVolcano. Subunit 2b ranges from a depth of 100 cm to thetop of the sequence. This subunit is characterized byconstant, as well as the highest, percentages of plagioclaseminerals. Plagioclase, amphibole, quartz and biotiteminerals are related to the erosion of catchment rocksand the synsedimentary direct input from eruptions of theParinacota Volcano.

4.3. Geochemical composition

The XRF Core Scanner provided a record of geochem-ical variations for the Chungara sediments. A compositerecord joining the analyzed sections from cores 10(Sections 1–5) and 11 (Sections 4–6) was generated tocover almost the entire sedimentary sequence (Fig. 5).Although above this sedimentary sequence up to 95 cm ofthe Chungara topmost sediments are recorded in othercores (e.g. core 14) (Fig. 3 and Saez et al., 2007), weregarded the top (0 cm) of the studied sequence as the topof core 10 for simplification. The downcore profiles ofheavy and light elements clearly delineated three differentunits in terms of their geochemical composition: an uppertephra-rich unit with maximum values in all elements(Subunit 2b), an intermediate unit with the highest Cavalues (Subunit 2a) and a lower unit characterized by thehighest Si content (Unit 1).

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Calcite (%) Plagioclase (%) Quartz (%) Biotite (%)

Amorphous (%) Dolomite (%) Amphibole (%) Pyrite (%)

Fig. 4. Mineralogical profiles measured by X-ray diffraction (XRD) for the composite sequence of Lago Chungara in percentages versus depth.

Lithological subzones are indicated by dashed lines.

Fig. 5. Light and heavy elements measured by the XRF Core Scanner for the composite sedimentary sequence of Lago Chungara. All the measurements

are in counts per second (cps). Lithological units are indicated.

A. Moreno et al. / Quaternary International 161 (2007) 4–21 9

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Table1

Correlationcoefficientsamongdifferentproxiesusedinthisstudy

Amorphous

Plagioclase

Calcite

Opal

MS

TOC

TC

Al

Si

SK

Ca

Ti

Mn

Fe

Rb

Sr

Zr

Sn

Ba

Amorphous

1.00

Plagioclase

�0.88

1.00

Calcite

�0.42

�0.04

1.00

Opal

0.87

�0.87

�0.18

1.00

MS

�0.60

0.72

�0.09

�0.63

1.00

TOC

0.10

�0.22

0.17

0.17

�0.29

1.00

TC

�0.04

�0.27

0.59

0.11

�0.31

0.85

1.00

Al

�0.38

0.47

�0.08

�0.43

0.68

�0.17

�0.19

1.00

Si

0.28

�0.19

�0.23

0.30

�0.07

0.02

�0.06

0.32

1.00

S0.11

0.02

�0.27

0.06

0.01

�0.18

�0.27

0.44

0.60

1.00

K�0.50

0.62

�0.12

�0.59

0.79

�0.23

�0.27

0.90

0.08

0.27

1.00

Ca

�0.28

0.07

0.47

�0.19

0.14

0.28

0.48

0.32

�0.12

�0.06

0.19

1.00

Ti

�0.57

0.68

�0.09

�0.64

0.78

�0.20

�0.24

0.83

0.02

0.33

0.95

0.20

1.00

Mn

�0.30

0.29

0.11

�0.31

0.16

0.11

0.14

0.11

�0.23

0.00

0.19

0.18

0.26

1.00

Fe

�0.58

0.69

�0.08

�0.65

0.74

�0.19

�0.22

0.80

0.06

0.41

0.91

0.20

0.98

0.27

1.00

Rb

�0.53

0.65

�0.13

�0.62

0.74

�0.21

�0.25

0.78

0.10

0.36

0.88

0.14

0.89

0.19

0.90

1.00

Sr

�0.61

0.62

0.11

�0.64

0.76

�0.07

�0.02

0.80

�0.08

0.17

0.87

0.48

0.89

0.24

0.87

0.86

1.00

Zr

�0.57

0.68

�0.08

�0.64

0.79

�0.17

�0.20

0.79

�0.01

0.27

0.92

0.20

0.96

0.24

0.94

0.94

0.94

1.00

Sn

0.26

�0.23

�0.10

0.25

�0.33

�0.13

�0.09

�0.12

0.37

0.30

�0.26

�0.20

�0.25

�0.06

�0.23

�0.17

�0.30

�0.26

1.00

Ba

�0.52

0.61

�0.08

�0.59

0.78

�0.14

�0.17

0.81

0.00

0.23

0.93

0.23

0.93

0.21

0.89

0.89

0.94

0.97

�0.27

1.00

Correlationsthatareinplaintext(notboldstyle)arenotsignificantafterapplicationoftheBonferronitest(seetext).MSreferstomagneticsusceptibility.

A. Moreno et al. / Quaternary International 161 (2007) 4–2110

4.3.1. Correlation analyses and definition of main sediment

components

Statistical correlation analysis was performed in order todiscern the similarities among the entire set of proxies(Table 1). The sampling interval of the different proxieswas not identical (e.g. 2mm for XRF and 5 cm for XRD).Subsequently, to highlight the main coarse relationshipsbetween the dataset, all proxies were linearly interpolatedwith a regular spacing of 5 cm, resulting in a dataset of 17variables (proxies) and 171 cases (samples). The signifi-cance of the correlation analysis (p-values) was calculatedand the p-values adjusted by applying the Bonferroni test(R Development Core Team, 2004). Many of the sig-nificant correlations among the variables have low values,highlighting that they do not have univocal relationships,since these variables could have more than one origin. Asan example, Ca has a correlation with calcite and TCcontents only around 0.48. This is explained by thevolcanic source of Ca in the upper part of the sequence(Ca-plagioclase), as well as the sedimentary source of Ca inthe middle part of the sequence (endogenic carbonates).Therefore, although the three main components ofChungara sediments discussed above (siliceous biogenic,volcanic and carbonates) can be shown via the highercorrelation values among the related proxies (Table 1),several patterns arise when the records are examined moreclosely.Al, K, Ti and Fe are associated with the allochthonous

component (here, volcanic), increasing with the presence oftephra layers as observed by lithology; heavy elements likeZr, Sr, Sn and Ba are related to tephra layers as well. Onthe other hand, while Ca behaved in similar fashion to theprevious group of volcanic-originated elements along theupper part of the sedimentary succession, it began tobehave independently starting from the uppermost carbo-nate-rich layer (190 cm in depth) and extending towards thebottom of the sequence (Fig. 5). Accordingly, Ca behavedsimilar to Al, K, Ti and Fe from 0 to 190 cm depth, whilethis element exhibited a distinctly different pattern from190 cm to the bottom of the sequence (Fig. 5). The bestexplanation is the increasing abundance of Ca in thecomposition of volcanic minerals such as plagioclase,which dominates volcanic eruptions, thereby influencingChungara sediments mainly in the upper part of the record.The Ca profile changes drastically in Subunit 2a: Caappears as very sharp spikes instead of smooth peakscommon in Subunit 2b. Ca content in the lower part of therecord is very low but it is still possible to recognize abruptCa spikes associated with some white laminae. In the samemanner as Ca, Si is influenced by different components,with volcanic and biogenic silica (diatoms) being the mostimportant. The Si signal thus represents a mixture of bothinfluences. Other elements, as S or Mn, are not stronglyrelated to any of the described patterns. Therefore, the Sprofile does not closely mimic any other profile, but showsa slight correspondence with Si values (0.6 correlation inTable 1) indicating that this pattern most likely reflects an

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interference of influences and origins. In the same way, Mnvalues mark the occurrence of tephra layers and correlatewith some Ca peaks. The presence of Mn in thesesediments is related to changes in redox conditions at thesediment–water interface since Mn forms a highly insolubleoxide where oxic conditions prevail. Thus, solid phases ofMn in lake sediments appear as a result of the upwarddiffusion of dissolved Mn and its posterior precipitation atoxic horizons in the form of Mn oxides (Aguilar andNealson, 1998). In Chungara sediments, Mn background isvery low, usually below 0.5 g kg�1 (1000 cps), although itincreases to 2.5 g kg�1 (5000 cps) during discrete peaks thatare more abundant during Subunit 1b. This behavior mayindicate a close relation with changes in the sedimentaryoxygen content most likely related to biogenic processes orchanges in water circulation due to lake-level variations.The strong volcanic influence in the geochemical

composition of Chungara sediments makes it necessary tonormalize the elemental data to ‘‘volcanic’’ elements like Tito unravel the environmental or climatic meaning of thedifferent profiles. Such high-resolution profiles constrainthe evolution of the three main inputs contributing to theChungara record: siliceous biological remains (mainlydiatom skeletons), volcanic minerals and endogeniccarbonates.

4.3.2. Biogenic component

In Fig. 6, the Si/Ti ratio is represented to discern thebiogenic silica amount compared to the percentage of opal

Fig. 6. Biogenic influences on Lago Chungara sediments: Si/Ti ratio, biogenic

ratio. All proxies are plotted versus core depth (cm), lithological subunits are

extracted by alkaline digestion, as well as with theamorphous fraction obtained from XRD. In addition,TOC and S/Ti profiles are plotted to infer the variability ofthe organic matter content in Lago Chungara since the Scontent reflects the occurrence of pyrite, which generallyincreases when organic matter content is high. The Mn/Tiratio is plotted in the same figure. From a comparison ofthe whole dataset (Fig. 6), some striking similarities can beobserved. Thus, the Si/Ti, as well as the opal andamorphous profiles follow similar trends at some intervals:low values in the upper part tend to increase from 300 to550 cm and remain at intermediate levels until the bottomof the sequence. These results point to a higher lake diatomproductivity (or better, silica preservation) in the laminateddeposits of Subunit 1b and, to a lesser extent, in Subunit 1adeposits (Fig. 6). In addition, the Si/Ti record helps toidentify intervals where the biogenic lacustrine signal isdominant over volcanic layers. This pattern is not so clearin opal or amorphous profiles because of lower spatialresolution.The S/Ti, TOC and Mn/Ti data require careful inter-

pretation (Fig. 6). Microscopic observation of smear slidesand SEM-EDS determinations (Saez et al., 2007) show thepresence of pyrite associated with organic-rich sediments.Thus, the S/Ti ratio could reflect variations in pyrite thatcould be correlated to TOC. Pyrite was detected by XRDalthough always at very low percentages (Fig. 4). However,the very low values of S obtained in some samples by XRF(below 2 g kg�1) prevent an accurate comparison among

opal, amorphous minerals obtained by XRD, S/Ti ratio, TOC and Mn/Ti

indicated.

ARTICLE IN PRESSA. Moreno et al. / Quaternary International 161 (2007) 4–2112

these records. In addition, the TOC record obtained fromupper Chungara sediments is highly variable due to thefrequent macrophyte remains and volcanic input. Organicmatter content increases from the bottom of the sequence(4%) to the top of laminated deposits of Subunit 1b, whereit reaches 8%. The maximum in Mn/Ti is associated withthe maximum in organic matter and diatomaceousproductivity (Subunit 1b). Although an accurate explana-tion requires an understanding of diatom assemblagesduring this interval, since diatoms contribute about 80% ofthe sediment, we would suggest changes in redox condi-tions as a significant factor in Mn variability. More oxygenin the water column caused either by photosyntheticprocesses (related to maximum diatom productivity) orincreased bottom water ventilation would account for theincrease in Mn. In such a scenario, Mn would beprecipitated as oxides and then, preserved in sediments.

4.3.3. Volcanic components

Elements directly related to the volcanic influence in theChungara sequence are compared with magnetic suscept-ibility and volcanic minerals, such as plagioclase oramphibole (Fig. 7). It is evident that Unit 1 deposits arealmost free of volcanic input, while Unit 2 is undoubtedlydominated by tephra ashfalls. Mineralogical results in-dicate that the dominant volcanic mineral is plagioclase,likely andesine, with percentages ranging from 5% to 60%.

Fig. 7. Volcanic influences on Lago Chungara sediments: Fe, Ti, K, plagioclas

All proxies are plotted versus core depth, lithological subunits are indicated an

Values of Fe, Ti and K are excellent indicators of volcanicdeposits in the Lago Chungara record (Table 1, Fig. 7).

4.3.4. Carbonate components

To investigate carbonate production variability, the Ca/Ti ratio versus TIC and calcite percentages are plotted(Fig. 8). In addition, the Sr/Ti ratio was plotted to verifywhether the Sr variations were related to carbonateprecipitation (i.e. in aragonite) or to volcanic input. Thehigh correlation between Ca/Ti and Sr/Ti confirms therelation of both elements (Ca and Sr) to the carbonateprecipitation (Fig. 8). Normalization with respect to Ti is avaluable tool for unravelling previously undetected pat-terns. The high-resolution record shows that carbonate inthe offshore central plain of the lake was produced duringvery short intervals and deposited as thin layers. This spikycharacter of carbonates was hidden when working withdiscrete samples at lower sampling resolution (see calciteand TIC profiles, Fig. 8). It is also evident that carbonatedeposits are concentrated in Subunit 2a, thus confirmingthe presence of a carbonate-rich interval (250–450 cm),reaching up to 40% of carbonate. Microscopic (optical andscanning) observation of these carbonate layers indicatesthat they are mainly composed of calcite, in the form ofrice-shaped or euhedral crystals. These types of carbonatecrystals are formed in the offshore central plain epilimnium(without any transport from the vegetated littoral areas or

e and amphibole obtained by XRD and magnetic susceptibility (SI units).

d the correlation lines (M-1 to M-11) marked by arrows (see also Fig. 3).

ARTICLE IN PRESS

Fig. 8. Carbonate in offshore zones at Lago Chungara: Ca/Ti ratio, Sr/Ti ratio, calcite obtained by XRD and TIC. All proxies are plotted versus core

depth, lithological subunits are indicated.

A. Moreno et al. / Quaternary International 161 (2007) 4–21 13

any relation to organisms, such as bivalves or gastropods).Two possible origins of these calcite crystals are: (1) theyare linked to algal blooms capable of altering water CO2concentration (Kelts and Hsu, 1978; Teranes et al., 1999)or (2) they are associated with alterations in waterchemistry, either of volcanic influence or from changes inwater concentration. The apparent absence of correlationbetween the long-term evolution of Ca/Ti (carbonates) andof Si/Ti (mainly diatoms) allowed us to propose that theproductivity blooms are not the main promoter of thecarbonate production. This relation seems to be true onlyin carbonate layers where rice-shaped carbonate crystalsare present (Table 1, Figs. 6 and 8). On the other hand, avolcanic influence cannot be totally discarded since Subunit2a is characterized by the presence of the thickest lapillilayers along the sequence (Fig. 2). However, calciteprecipitation starts earlier (in Subunit 1b) than doesvolcanic activity, which calls into question volcanicinfluence as the main causative force (Fig. 8). Therefore,the most likely mechanism stems from changes in waterconcentration. In current climatic and limnological condi-tions, water concentration is mainly related to the

precipitation–evaporation balance (Valero-Garces et al.,2003). More evaporation or less precipitation (arid period)would imply more concentrated waters and lead toprecipitation of offshore carbonates.

4.4. Chronology

Dating the sediments from Lago Chungara is compli-cated due to the scarcity of terrestrial organic restsresulting from the low vegetation cover. Therefore, AMS14C dates from Chungara sediments were obtained from (1)bulk organic matter from the central plain cores and (2)aquatic organic macrorests picked from littoral cores(Table 2). In Fig. 3, the correlation panel with the coresused to construct the chronological model is presented. Thesimilarities in the sedimentary facies among the cores andthe presence of key tephra layers allow transferring theobtained dates to a single composite depth (Table 2). Asobserved in Table 2, there are five ages that were discarded(shaded samples) and not included in the final age model.Two of them are clearly reversed (15A-5, 76 and 13A-4,66), probably due to depositional reworking of the

ARTIC

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Table 214C AMS radiocarbon ages measured in Chungara cores

Units Composite

depth (cm)

Laboratory ID Sample (core

and depth)

Type of sample Uncalibrated 14C

age (2s) (yr BP)Calibrated age (cal. yr BP) after Heegaard’s

method

d13C (%)

Model 1 Model 2

0 Beta 188745 DIC measured in surface water 2320740 — —

Subunit 2b 42 Poz-8720 11A-2, 39 Bulk organic matter 4850740 28657900 28657900 �12.97 0.4

67 AA56904 15A-2, 48 Aquatic organic macroremains 6635739 44407500 44407500 �25.46

95 Poz-8721 11A-2, 84 Bulk organic matter 7290750 59707600 59707600 �14.870.2

Subunit 2a 258 Poz-8723 11A-3, 2 Bulk organic matter 8920750 �16.170.1

344 AA56903 15A-4, 27 Aquatic organic macroremains 9999750 No data

436 Poz-8724 11A-3, 86 Bulk organic matter 10,860760 �16.970.1

Subunit 1b 490 Poz-7170 11A-3,123 Bulk organic matter 8570750 7325790 9550790 �16.870.1

550 Poz-8647 11A-4, 10 Bulk organic matter 9860760 83807450 114507370 �14.170.3

615 Poz-7171 11A-4, 63 Bulk organic matter 11,070770 92807690 127707390 �13.670.2

Subunit 1a 665 AA56905 15A-5,76 Aquatic organic macroremains 43857100 No data

675 Poz-8725 13A-4, 66 Bulk organic matter 8810750 �22.9

697 Poz-11891 11A-4,145 Bulk organic matter 11,460760 974571000 130007420 �16.270.4

743 Poz-13032 11A-5, 41 Bulk organic matter 10,950780 1011071190 130507420 �22.772.3

785 Poz-11982 11A-5, 84 Bulk organic matter 11,180770 1077571000 134107410 �28.773.7

827 Poz-13033 11A-6, 41 Bulk organic matter 12,120780 1172071225 140957570 �19.671.7

865 Poz-7169 11A-6. 79 Bulk organic matter 13,100780 127107800 148757910 �23.170.2

Depth transposed to composite sequence is indicated. Lithological subunits are also shown. Italic samples were not used for the age model construction (reversed dates, error too high). DIC: dissolved

inorganic carbon (apparent age of surface water). Model 1: constant reservoir age correction; Model 2: no reservoir age correction for Unit 1. See text for details about calibration procedures and Fig. 9.

A.

Mo

reno

eta

l./

Qu

atern

ary

Intern

atio

na

l1

61

(2

00

7)

4–

21

14

ARTICLE IN PRESSA. Moreno et al. / Quaternary International 161 (2007) 4–21 15

sediments, since one sample comes from a volcanic sandylayer and the second from a reworked peaty interval. Theother three samples (11A-3, 2, 15A-4, 27 and 11A-3, 86)belong to the volcanic-rich Subunit 2a (Fig. 2) and do notshow evidence of depositional reworking. One possibility isthat the strong volcanic influence may have altered the CO2balance among the different sources (atmosphere, soil,runoff and groundwater, volcanic), thus modifying the14C value of these three samples. A similar effect hasbeen documented in other lakes from the Altiplano(Valero-Garces, et al., 1999, 2000) and Easter Island(Butler et al., 2004) with high volcanic influence.Obtaining reliable radiocarbon dates in the sediments of

Lago Chungara is also problematic because of an assumedlarge and variable radiocarbon reservoir effect. Similarproblems occur in the majority of lake deposits from theAndean Altiplano, with very few studies able to properlyevaluate the variations in reservoir effect over time (Geyhet al., 1999; Geyh and Grosjean, 2000; Grosjean et al.,2001). The modern reservoir effect for Lago Chungara wasobtained by dating the dissolved inorganic carbon (DIC) ofthe surface lake water. This resulted in 2320740 14C yr BP(Table 2), a value very similar to that obtained by Geyhet al. (1999) after analyzing DIC lake water (17457160 14Cyr) and living macrophytes (25607245 14C yr) in LagoChungara.However, the reservoir effect in the Altiplano lakes has

proved to be highly variable over time, with the influence oflake water volume being one of the most significant factors(Geyh and Grosjean, 2000). Since no terrestrial organicremains were found in Lago Chungara sediments, we couldnot evaluate the variation of the reservoir effect with timeapplying the methodology of Geyh and Grosjean (2000).Therefore, our approach to correct the dates for thevariable reservoir effect has been based on two assump-tions: (1) the Lago Chungara system during deposition ofSubunit 2b is very similar to that currently found there and(2) the present-day lake level is at the highest in its history.Accordingly, the correction of the reservoir effect was

achieved differently in Unit 1 and Unit 2. It is worth notingthe very different sedimentary patterns recorded whencomparing Units 1 and 2: while the latter is composed ofrather homogenous facies (dark gray massive diatomac-eous oozes in Subunit 2b) characterized by abundant

Table 3238U/230Th ages measured in Chungara cores

Depth (cm) Sample ID Carbonate

material

238U (ppb) 232Th (ppm) d

280 14A-3, 6 Crystal Out of scale — 3

285 13A-2, 45 Crystal 576.4 53.9 3

344 13A-2, 105 Crystal 467.4 35.2 4

374 15A-4, 77 Shell 717.4 203.1 3

Depth transposed to composite sequence is indicated. Italic samples were not us

of 232Th). See text for details about calibration procedures and Fig. 9.

volcanic layers, the former represents a clearly distinctstage of the lake system (laminated sediments, no volcanicinput). This pattern may allow us to consider a constantreservoir effect for the uppermost subunit, since theaverage lake characteristics (depth, water volume) mostlikely did not vary much over time. Therefore, a constantreservoir effect of 2320 years (the present-day one) issubtracted from the Subunit 2b dates. After removing thereversed ages, no 14C AMS data were available fromSubunit 2a (Table 2).It is only possible to hypothesize about the variations

over time of the reservoir effect in Unit 1. In accordancewith the hypothesis set forward by Geyh and Grosjean(2000), the reservoir effect was most likely lower in Unit 1than in Unit 2 since the lake was, on average, shallowerthan during the deposition of the upper unit. This issupported by a seismic study, by the absence of emergedformer lake terraces, and other indicators (sedimentaryfacies, diatoms, carbonate content) (Saez et al., 2007). Wepropose to correct the Unit 1 dates for two possibleextreme reservoir age values: a minimum value of 0 yearsand a maximum of 2320 years. Once corrected byconsidering these two extreme reservoir effect values, anage range of variation for every Unit 1 date is obtained.These dates were calibrated using the most updatedcalibration curve (INTCAL04), which is provided by theCALIB 5.02 software package (Reimer et al., 2004)selecting the mid-point of the 95.4% of the distribution(2s probability interval).In order to fill the gaps of the age model constructed

only by 14C AMS dates, four 238U/230Th measurementswere carried out on calcite crystals that appeared in somethin layers from Chungara cores. These dates were done viathe standard method using an ICP-IRMS multicollector atthe University of Minnesota (Table 3). For a summary ofthis method see Edwards et al. (1986). Only one 238U/230Thdate was finally acceptable for the chronological model dueto their high content of 232Th in the other three samples.Fortunately, this 238U/230Th date belongs to Subunit 2a,the only interval without sound 14C AMS dates.To construct a reliable age-depth model with the

remaining 12 dates (11 14C AMS and 1 U/Th), we usedthe software described in Heegaard et al. (2005) as a usefulinterpolation tool. This software provides a procedure to

234U 230Th/238U 230Th/232Th

(ppm)

Error Calendar

age BP

23.3 — 0 — —

35.1 0.0774 14 1244 4450

13.5 0.1036 23 974 6730

20.2 0.1607 9 3445 7720

ed for age model construction (reversed dates, error too high or high values

ARTICLE IN PRESSA. Moreno et al. / Quaternary International 161 (2007) 4–2116

estimate the age–depth relationship by setting the mid-point value of the calibrated ages in relation to the centraldistributional range. The software provides a final cor-rected age for every calibrated date (Table 2, column‘‘Calibrated age after Heegaard’s method’’). These ages arethen employed for the age-depth model (Fig. 9). Thisapplied correction has led to slight age differences in theboundaries of the subunits with respect to those presentedin Saez et al. (2007) and incorporates two additional tiepoints.In summary, the age model for the sediment sequence of

Lago Chungara presented here is constructed with (1) theavailable dates along Subunit 2b corrected by a constantreservoir effect similar to that currently found, (2) a uniquedate in Subunit 2a acquired by the 238U/230Th procedureand (3) the mid-point value of the obtained range takinginto account the maximum and the minimum reservoir agecorrections for the existing dates along Unit 1. Thus theLago Chungara sequence covers the last 14,000 cal. yr BP

Com

posi

te d

epth

(cm

)

0 2000 4000 6000 8000 10000 12000 14000 160000

100

200

300

400

500

600

700

800

900

Model 1: constant reservoir age correction

Model 2: no reservoir age correction for Unit 1

Model 1 and 2: common reservoir age correction for Unit 2

Calibrated age (cal. years BP) after Heegaard's model

U/Th age

Subunit 2b

Subunit 2a

Subunit 1a

Subunit 1b

Fig. 9. Age control points used for the age model of Lago Chungara. We

represent the ‘‘Calibrated age (cal. yr BP) after Heegaard’s method’’ data

from Table 2. The error bar displayed for every age is a consequence of the

interpolation carried out following Heegaard et al. (2005). Two different

approaches were carried out to correct the reservoir effect in the Lago

Chungara sequence: model 1 (application of a constant reservoir age

correction along the entire sequence, white dots) and model 2 (application

of a constant reservoir age correction for Subunit 2b and no correction for

Unit 1 dates, gray dots). Therefore, for both models the uppermost

interval (Subunit 2b) is common (reservoir age is considered constant,

2320 years, black dots). In Subunit 2a only one U/Th date is available.

Along Unit 1, the calibrated dates range from model 1 to model 2 values,

representing the maximum and minimum reservoir age correction,

respectively. Although the final age model was constructed with the

intermediate values from the represented range (dashed line), the broad

interval is taken into account in the paleoclimate interpretations.

with a sedimentation rate in offshore zones between 0.47and 0.78mmyr�1.

5. Paleoclimate implications

Selected proxies from the Chungara sequence wereplotted versus age and compared with published recordsfrom the nearby Nevado Sajama ice core and Lake Titicaca(Fig. 10). Accordingly, any discussions addressing thepaleoclimate implications should take into account therange of possible ages along Unit 1 (Fig. 9). Additionalimprovements to the age model of this sequence (more238U/230Th dates or dating tephra layers) will allow moreextensive and refined interpretations of paleoclimatedynamics.Close correlation among Fe elemental intensities and the

amount of coarse particles in the Sajama ice core indicatesthat the volcanic signal detected in the Chungara recordhas a regional character and that most of the dust particlesin the Sajama ice core are of volcanic origin. Very likely,this volcanic signal is related to the Parinacota Volcanoeruptions since it is the only one in the area with recenteruptive activity (Worner et al., 1988). Due to the shortdistance (30 km) between Lago Chungara and NevadoSajama (Fig. 1), one can easily infer that the source forvolcaniclastic particles in both records was the same.Therefore, we suggest that the variations found in thecoarser particles in the Sajama ice core are mainlycontrolled by the main volcanic eruptions of ParinacotaVolcano and are not due to (1) atmospheric dust input or(2) the greater availability of salt minerals during dryperiods in the Altiplano, as was proposed by Thompsonet al. (1998). The Fe record from Lago Chungara and theSajama record indicate that explosive activities of Parina-cota Volcano increased dramatically at 5800 cal. yr BP(Fig. 10). The period with less explosive volcanic activityis also coincident in both records from 14,000 to7000–8000 cal. yr BP (Fig. 10).A comparison of the Lago Chungara record with other

paleoclimate reconstructions from the nearby Cotacotanibasin becomes necessary to assess the Chungara recordwithin a regional context. Pollen stratigraphy obtainedfrom an outcrop at Laguna Seca (Fig. 1) indicates agradual transition towards drier and warmer climates sincethe late Pleistocene (Baied and Wheeler, 1993). Anincreased desiccation between 8000 and 6500 14C yr BP,as well as warmer conditions until about 5000 14C yrBP arealso suggested by that study. These paleoclimate inter-pretations are supported by an isotopic study carried out ina sediment core obtained from Laguna Seca (Schwalbet al., 1999). Moreover, a transition towards higher lakelevels from the mid-to-late Holocene was also postulated(Schwalb et al., 1999).The high-resolution geochemical study of Lago Chun-

gara provides new data on two main paleoclimatic topics:the glacial–interglacial transition and the aridity crisesduring the Holocene.

Fig. 10. Selected records (Si/Ti, Ca/Ti, Mn/Ti and Fe) from Lago Chungara plotted versus age. Represented tie points (black rhombuses) are the mid-

point values of model 1 and 2 dates (Table 2). The d18O and total particles coarser than 63mm from the Nevado Sajama ice core record (data from the

World Data Center for Paleoclimatology, Boulder and the NOAA Paleoclimatology Program) and the benthic diatoms from Lake Titicaca (Baker et al.,

2001a) are plotted for comparison. Summer insolation at 181S is indicated (reversed axis). An arrow marks the productivity increase along the early

Holocene. Both maximums in aridity in Lago Chungara and Titicaca are indicated. The complete arid phase for Lake Titicaca is also indicated).

A. Moreno et al. / Quaternary International 161 (2007) 4–21 17

5.1. Glacial–interglacial transition—early holocene

The glacial–interglacial transition can be observed inseveral northern Andean Altiplano records as a sharpchange towards drier conditions associated with minimumsummer insolation (E11,000 cal. yr BP, Fig. 10) followingthe wet Tauca phase when the largest lakes and the highestlake levels occurred (Argollo and Mourguiart, 2000; Bakeret al., 2001a; Mourguiart and Ledru, 2003). Although theend of the Tauca phase has not been well established, it isbelieved to have ended about 14,900 cal. yr BP (Baker et al.,2001b; Fornari et al., 2001). In some records, a short wetevent (13–11.5 kyr BP; ‘‘Coipasa phase’’) has been observedand correlated with the Younger Dryas in the northernhemisphere (Baker et al., 2001a).One of the main transitions in the Lago Chungara

record corresponds with the boundary between Subunits 1aand 1b, which depending on the age model occurredbetween 9500 and 12,500 cal. yr BP (Fig. 9). In spite of thechronological uncertainty, this boundary is likely torepresent the glacial–interglacial transition and the onsetof the Holocene. During that transition, Chungara sedi-ments reflect the change from green-white laminations(Subunit 1a) to brown-white laminations (Subunit 1b).

Although the meaning of these laminations cannot bediscerned with our geochemical proxies, we suggest anincrease in productivity, probably related to favoreddiatom productivity, at the beginning of the Holocene(11,500 cal. yr BP) lasting until 7500 cal. yr BP. A detaileddiatom study (Bao et al., in preparation) and organicgeochemical analyses are in progress to understand thesignificance of the different laminations and their cycli-cities. As well as the sedimentary facies changes, severalproductivity proxies point to a general increase (TOC, Si/Ti and S/Ti) after the end of the glacial period. Highvalues are maintained throughout the early Holocene(Figs. 6 and 10). This change correlates with the Sajamaice core isotopic change towards warmer temperatures(Thompson et al., 1998).In contrast to the glacial–interglacial transition, the

relevant geochemical indicators do not show any clearindication of climate changes associated with the ‘‘Coipasaphase’’ or Younger Dryas event (Fig. 10). However, in theSajama ice core (Thompson et al., 1998) and the Titicacarecord (Baker et al., 2001a), a decrease in temperature and anincrease in humidity have been postulated for that period.The high sampling resolution attained in this study

allowed focusing on short and dramatic climate events that

ARTICLE IN PRESSA. Moreno et al. / Quaternary International 161 (2007) 4–2118

otherwise would not be observable by discrete sampling. Inthe Chungara record, a very prominent peak in the Mn/Tiprofile occurs at around 10,000 cal. yrBP ranging from 8400to 11,400 cal. yrBP (Fig. 10). As outlined above, Mn isassumed to be associated with redox front variations whenoxic conditions develop following an interval in which ananoxic environment dominated. Therefore, we propose thatan increase in oxygen in the bottom waters occurred at thattime. The most probable mechanism that would producesuch an increase in oxygen is a lack of long periods of waterstratification, e.g., sufficiently low lake levels could causewinds to mix the entire water column. This mechanism maybe reinforced by an increase in diatom productivity, as aconsequence of such a mixing, and thus in oxygen supply.This potentially more arid scenario (lower water levels),when compared with previous sediment deposits would besupported by an increase in benthic diatoms during thisinterval as indicated by preliminary data (Saez et al., 2007;Bao et al., in preparation). In addition, peaks in calcitecontents, Ca/Ti and Sr/Ti would further strengthen thishypothesis (Figs. 8 and 10).

5.2. Aridity crisis during the early and mid-Holocene

Apart from the event detected in the Lago Chungarasediments around 10,000 cal. yr BP in the Mn/Ti record,other intervals were observed during the early-to-midHolocene that may indicate lower lake levels as well(Fig. 10). In general, periods of high lake levels in theAndean Altiplano are interpreted in terms of increasingsummer precipitation in the southern hemisphere, with theITCZ occupying a more southern position, while dryperiods are related to a northward displacement of theITCZ (i.e. Argollo and Mourguiart, 2000). In the nearbyLaguna Seca record, a transition from carbonate-rich,laminated lacustrine sediments to peaty sediments occurredat about 70307245 14C yr BP (Baied and Wheeler, 1993).In spite of the uncertainties with the Laguna Seca agemodel, this change from a lake towards a peatbog points toarid conditions for the mid-Holocene in the LagoChungara area.Enhanced carbonate precipitation in the offshore zones

of Lago Chungara was observed along Subunit 2a (Figs. 8and 10). Carbonate production in the lake requires thepresence of Ca in the water, which is assured throughoutnearly the entire sequence: Ca is provided by leaching ofCa-rich volcanic ashes or by the increase of volcanic inputthat becomes more important in Subunit 2a (Saez et al.,2007). Some of the carbonate-rich levels occur in intervalswhere Ca-rich tephra layers are more frequent, suggestingthat volcanism plays a role. However, carbonate formationpredates the Ca-rich volcanic interval (Subunit 2b), andcarbonate production halts before the Ca-rich volcanicinput to the lake stops, suggesting that other factors couldalso control carbonate formation. Therefore, the presenceof endogenic carbonate deposits in offshore cores is apotential indicator of more concentrated waters, e.g., a

scenario involving lower lake levels with an increase inlittoral areas. The increase in benthic diatoms in thecarbonate-rich levels supports such a hypothesis (Saezet al., 2007; Bao et al., in preparation).However, fluctuations in biological activity, as well as

changes in inflowing water composition could also haveplayed a role in making the lake waters more conducive tocarbonate formation. Several periods of increased carbo-nate formation in littoral areas from Lago Chungara,related to increased charophyte productivity, have oc-curred during the last 4000 years (Valero-Garces et al.,1996, 2003). In nearby Laguna Seca, periods of carbonateformation seem to have occurred throughout the Holocene(Baied and Wheeler, 1993; Schwalb et al., 1999), althoughin this case they seem to be related to travertine depositionand spring activity. Detailed diatom studies and statisticalanalyses to more precisely determine the volcanic input arein progress.Considering the offshore formation of carbonate as an

indicator of more concentrated lake water, the periodranging from 8600 to 6400 cal. yr BP could be proposedas the most arid one (taking into account the establishedage ranges, the starting point of this interval varies from7300 to 9500 cal. yr BP). This period does not preciselycoincide with the dry period detected in Lake Titicacausing benthic diatom abundances which is dated between8.5 and 4.5 cal. kyr BP, with an extremely dry period from 6to 5 cal. kyr BP (Baker et al., 2001a; Tapia et al., 2003,Fig. 10). The main difference with the Lake Titicaca recordis the timing of the dry period, ending after thecorresponding period of the Chungara record (Fig. 10).As detected in other studies, Holocene aridity crises werenot synchronous in the Andean Altiplano (Betancourtet al., 2000; Holmgren et al., 2001; Abbott et al., 2003;Grosjean et al., 2003; Latorre et al., 2003). According tothe age model, these findings are consistent with theproposed N–S gradient, and therefore support the complextiming and structure of the early-to-mid Holocene ariditycrisis.Finally, it is worth noting that dry conditions were not

constant during the arid period in the Chungara region, butcharacterized by a series of short and rapid dry spells(Fig. 10). Considering the Ca/Ti record as an aridityindicator, several abrupt arid periods (of less than 100years duration) some of them coincident with Mn/Ti peaksare detected (Fig. 10). As stated before, both Ca/Ti andMn/Ti ratios point towards climate scenarios with lesswater availability in the Chilean Altiplano. The fact thatseveral arid events alternate with less arid periods during arelatively short time interval indicates that conditions wereunstable and highly changeable.Although these events cannot be precisely correlated

with other paleoclimate archives in the region due to agemodel uncertainties, they support the argument that thearidity postulated to have occurred since the earlyHolocene in South American tropical records is notcontinuous but rather represents a succession of short

ARTICLE IN PRESSA. Moreno et al. / Quaternary International 161 (2007) 4–21 19

dry/wet events. Despite the fact that the forcing mechan-isms of these abrupt changes in moisture availabilityremain partially unknown (Ruter et al., 2004), modelreconstructions should take into consideration all possiblefactors that could account for the presence of abrupt andshort dry/wet events over this generally arid period.

6. Conclusions

High-resolution geochemical profiles were produced forthe sedimentary succesion recovered from Lago Chungaraat 181S. These results, compared with physical, miner-alogical and sedimentological data obtained from the samecores, provide the first high-resolution lacustrine sequenceof the Chilean Andean Altiplano during the last14,000 cal. yr BP. The exhaustive analysis of the meaningof the elementary signatures, taking into account theirpotential origins before inferring a paleoclimatic signal,allows recognition of three main components in LagoChungara sediments: (1) lacustrine biological remainsmainly from diatoms, (2) volcanic minerals, and (3)endogenic offshore carbonates. Siliceous productivity wasdeduced from Si/Ti, opal and amorphous material whilethe volcanic supplies were inferred from Fe, Ti and Kelemental intensities, variation in magnetic susceptibilityand occurrences of plagioclase and amphibole minerals.Volcanic contributions were closely correlated to the totalamount of coarse particles recovered in the Nevado Sajamaice core, thus pointing to the same volcano source for both,the Lago Chungara and Sajama records, most likely theParinacota Volcano main explosive eruptions. Offshorecarbonate production was asserted from Ca/Ti and Sr/Tiratios as well as from calcite and TIC contents.The transition from greenish to brownish laminated

diatomites identifies the glacial–interglacial transition inChungara sediments that should be synchronous with theSajama temperature increase. In addition, productivityproxies show a tendency towards higher values, suggestingan increase in diatoms that was maintained throughout theearly Holocene. The use of continuously measured proper-ties allowed investigating the timing of abrupt and shortevents that would remain undetectable with a discretesampling methodology. The interval between 8600 and6400 cal. kyr BP appears to have been the driest episode,starting and ending before the Titicaca extreme dry period(6–5 cal. kyr BP). Additionally, several short and abruptarid intervals were detected from the carbonate content,with the ca. 10,000 yr event being the most pronouncedone. For that period, an increase in oxic conditions in thelake bottom waters is postulated to have occurredsynchronously to the sharp Mn/Ti peak. We suggest thata lowered lake level may have caused more efficient deep-water ventilation and, consequently, the formation of Mnoxides. A study of biological proxies, mainly diatoms, iscurrently underway to further strengthen our hypothesisregarding lake-level variability during the last 14,000 years.

Acknowledgements

We are indebted to the Limnological Research Centerstaff who participated in the field expedition (D. Schnur-remberger, M. Shapley and A. Myrbo) and collaboratedduring the initial core descriptions, as well as the CONAF(Corporacion Nacional Forestal, Chile) for the facilitiesprovided in Chungara. We acknowledge C. Herrera(Universidad Catolica del Norte, Chile) for his help duringthe field expedition. We are very grateful to L. Edwards(University of Minnesota) and R.O. Gibert (University ofBarcelona) for the ICP-IRMS U/Th dating and to S. Fritzfor providing the Titicaca diatom data. The University ofBremen, particularly U. Rohl, F. Lamy, M. Kolling, H.Pfletschinger and H. Kuhlmann are acknowledged fortechnical assistance with the XRF-Core Scanner, powderXRF and ICP-OES analyses. We also acknowledge M.Grosjean and N. Piotrowska for their advice in age modelconstruction and R. Rycroft for the English correction.The Paleostudies programme (European Science Founda-tion) provided the necessary funding to carry out theanalyses at the University of Bremen. This study issupported by the projects BTE2001-3225 and BTE2001-5257-E and CGL2004-00683/BTE funded by the CICYT,the Spanish Ministry of Science and Technology. A.Moreno and P. Gonzalez-Samperiz are the recipients of aCSIC research contract (I3P postdoctoral programme). S.Giralt acknowledges the Spanish Ministry of Science andTechnology for his postdoctoral contract in the Ramon y

Cajal programme.

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