ARTICLE IN PRESS

Quaternary International 161 (2007) 4–21

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

A. Morenoa,Ã, S. Giraltb, B. Valero-Garce´ sa,A.Sa´ ezc, R. Baod, R. Pregoe, J.J. Pueyoc, P. Gonza´ lez-Sampe´ riza, C. Tabernerb

aPyrenean Institute of Ecology—CSIC, Apdo 202, 50080 Zaragoza, Spain bInstitute of Earth Sciences ‘Jaume Almera’-CSIC, C/Lluı´s Sole´ i Sabarı´s s/n, 08028 Barcelona, Spain cFaculty of Geology, University of Barcelona, C/Martı´ Franque´s s/n, 08028 Barcelona, Spain dFaculty of Sciences, University of A Corun˜a, Campus da Zapateira s/n, 15071 A Corun˜a, 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- arid intervals and to compare their timing with Titicaca and Sajama records. Three main components in the lake sediments have been identified: (a) biogenic component, mainly from diatoms (b) volcanics (ash layers) from the nearby Volcano and (c) endogenic carbonates. The correlation between volcanic input in Lago Chungara´ and the total particles deposited in the 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 to any examination of rapid climate fluctuations, are scarce, with diverse proxy records showing numerous Recent research of past climate oscillations has found discrepancies (i.e. Grosjean, 2001). Detailed knowledge of that changes between climate modes during the Holocene the distribution and amplitude of abrupt climate changes in occurred within decades (Mayewski et al., 2004), a period tropical latitudes of the Andean Altiplano is still sparse and of time similar to more recent climate changes (Houghton the processes responsible for climate variability at different et al., 2001). In this context, scientific efforts over the last temporal and regional scales are barely understood. The few years have been directed towards understanding the suggested close link between higher lake levels in the timing and mechanisms of abrupt climate changes during Andean Altiplano and cold sea surface temperatures in the the last millennia. Despite the recent increase in the number Equatorial Atlantic (i.e., Heinrich events, Younger Dryas, of high-resolution paleoclimate records from low latitudes 8.2 kyr event or the Little Ice Age) indicated by the Titicaca (e.g. Hughen et al., 1996, 2004; Kuhlmann et al., 2004), the lake record (Baker et al., 2001a) requires additional records role of the tropics in abrupt Holocene climate changes from tropical South America to confirm this paleoclimate remains a matter of debate. Tropical South America teleconnection between the two hemispheres. exemplifies the complexity of Holocene climate reconstruc- The evolution of temporal and spatial moisture patterns tions, in which high-resolution terrestrial records, essential during the Holocene is one of the main controversies surrounding studies of South American paleoclimate. It ÃCorresponding author. Tel.: +34 976 716118; fax:+34 976 716019. has been generally accepted that the northern-central E-mail address: [email protected] (A. Moreno). Andes were a generally arid region from 7 to 4 kyr BP

1040-6182/$ - see front matter r 2006 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2006.10.020 ARTICLE IN PRESS 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; Tapia et al., 2003) and ice-core records (Thompson et al., 1998; Thompson et al., 2000). This hypothesis is also supported by archeological evidence (Nu´ n˜ ez et al., 2002). In addition to moisture reconstructions, a recent study of long-chain alkenones from Titicaca lake sediments also points to enhanced regional temperatures during the mid-Holocene (Theissen et al., 2005). However, other recent studies support a more complex spatial and temporal pattern, and even 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 understanding both the regional significance and the timing of abrupt humidity changes detected during the mid-Holocene. The overall goal of this study was to document the regional pattern of climatic change for the last 14,000 cal. yr BP using a sedimentary record from Lago Chungara´ (Andean Altiplano, 181S). This paper reports a high-resolution geochemical record from the lake obtained by an X-ray fluorescence (XRF) core scanner together with other paleoenvironmental indicators (i.e. physical proper- ties, mineralogy, opal content and total organic carbon (TOC)). The resulting high-resolution analyses, in tandem with a multi-proxy approach, allowed us not only to infer the paleoclimate signal from the Lago Chungara´ record, but also to contribute to the identification, correlation and understanding of abrupt climate change during the Holocene in tropical regions of South America.

2. Location, climate and limnology of Lago Chungara´

0 0 Lago Chungara´ (18115 S, 69110 W, 4520 m asl) is located Fig. 1. (a) Location of Lago Chungara´ and other paleoclimatic records on in the highest and westernmost fluvio-lacustrine basin in the Northern Chilean Altiplano. (b) Position of sediment cores in Lago the Andean Altiplano (Northern , Fig. 1a). This lake Chungara´ . Isobaths and main inflows are indicated. sits in the central part of a small hydrologically closed subbasin at the northeastern edge of the Cenozoic jet stream as well as the intensification of the Bolivian high- Basin. The intense volcanic activity and, to a lesser extent, pressure system (Garreaud, 2001; Garreaud et al., 2003). the movement of synsedimentary faults are significant Average annual rainfall in the region is about 350 mm. A factors for sedimentation in the Chungara´ subbasin. The significant fraction of the inter-annual variability of Lago Chungara´ subbasin was formed after the collapse of summer precipitation is currently related to the El Nin˜ o the Parinacota Volcano (Fig. 1a), which produced a huge Southern Oscillation (ENSO) (Vuille, 1999). Thus, wet debris avalanche blocking the Paleo- at about summers on the Andean Altiplano are associated with an 15–17 kyr BP (Wo¨ rner et al., 1988; Wo¨ rner et al., 2000; ENSO-related cooling of the tropical Pacific (La Nin˜ a Wo¨ rner et al., 2002). However, the age of this collapse is phase). controversial, and it has been estimated at 8 kyr by other Lago Chungara´ has a maximum water depth of 40 m, a authors (Clavero et al., 2002, 2004). The local vegetation is surface area of 22.5 km2 and a volume of about dominated by tussock-like grasses, shrubs, Polylepsis,a 426 106 m3 (Risacher et al., 2003; Herrera et al., 2006). dwarf tree of the Rosaceae family, as well as extensive The main inflow is the Chungara´ River (300–460 l s1) and soligenous peatlands (‘‘bofedales’’) (Schwalb et al., 1999; several springs on the western margin. Although there is no Earle et al., 2003). surface outlet, groundwater outflow was estimated as Lago Chungara´ is climatically located in the arid Central 0.2 m3 s1 (Montgomery et al., 2003) and the water lost Andes. This region is dominated by tropical summer by potential evaporation measuring about 1230 mm yr1 moisture stemming from the Amazon Basin, and is (Risacher et al., 2003). The lake is polymictic, meso to controlled by the southward migration of the subtropical eutrophic and contains 1.3 g l1 total dissolved solids ARTICLE IN PRESS 6 A. Moreno et al. / Quaternary International 161 (2007) 4–21

(Mu¨ hlhauser et al., 1995). Water chemistry is of texture, color and sedimentary structures (Core 11, Fig. 2). Na–Mg–HCO3–SO4 type with an average pH of 9. At Smear slides were described using a Nikon polarizing present, oxic conditions extend to the lake bottom. Primary microscope to estimate the biogenic, clastic and endogenic productivity in the lake is mainly governed by diatoms and mineral content of the defined sedimentary facies. Sub- chlorophyceans. During four sampling periods from 1998 samples were taken every 5 cm for mineralogical, chemical to 1999, biomass values fluctuated from 0.34 to 8.74 mg and biological analyses. After a detailed lithological Chlorophyll a l1 (Dorador et al., 2003). Oscillations in correlation of all cores (Fig. 3 and Sa´ ez et al., 2007), cores both, phytoplanktonic biomass and phytoplanktonic com- 10 and 11 were selected for paleoclimatic and paleoenvir- munity structure seem to be mainly due to changes in water onmental reconstructions. Both cores recorded almost the column temperature and salinity. entire sedimentary infill of the offshore zone, allowing reconstruction of a composite sequence. 3. Materials and methods Total carbon (TC) and total inorganic carbon (TIC) contents were determined by a UIC model 5011 CO2 In November 2002, 15 sediment cores (6.6 cm inner Coulometer, with TOC content then calculated. Samples diameter and up to 8 m long) were retrieved from Lago for X-ray diffraction (XRD) were dried at 60 1Cover24h Chungara´ along the NW–SE and NE–SW transects of the and manually ground using an agate mill. XRD analyses lake (Fig. 1b). The core locations were selected to sample were performed using an automatic Siemens D-500 X-ray the different depositional environments using a modified diffractometer: Cu ka, 40 kV, 30 mA and graphite mono- Kullenberg piston corer from the Limnological Research chromator. Identification and quantification of the differ- Center, University of Minnesota (LRC). The retrieved ent mineralogical species present in the crystalline fraction cores were shipped to the LRC where physical properties were carried out following a standard procedure (Chung, (GRAPE-density, p-wave velocity and magnetic suscept- 1974). The area of the amorphous fraction was calculated ibility) were non-destructively measured every cm using a as total counts using the XRD software. The sample that GEOTEKTM Multi-Sensor Core Logger (MSCL). The showed the highest amorphous area, formed mostly by cores were then split in two halves, scanned for color diatomaceous oozes, was progressively mixed with increas- pictures using a DMT CoreScan digital color line scan ing quantities of pure calcite (5%, 10%, 20%, 40% and camera system, and macroscopically described in terms of 60% of the total weight of the sample) and a logarithmic

Fig. 2. Digital image, lithological column and magnetic susceptibility profile from core 11. Key layers M-1 to M-11 and available 14C dates for this core (not calibrated) are indicated (see Table 2). A detailed facies legend is shown. See text for explanation. ARTICLE IN PRESS A. Moreno et al. / Quaternary International 161 (2007) 4–21 7

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 detected from seismic profiles (Sa´ ez et al., 2007) is also shown. WAF: White ash flow. function was adjusted. This function allowed expressing elemental concentrations failed. Sixty samples were ana- the area of the amorphous fraction as a percentage of the lzsed by inductively coupled plasma-optical emission total sample weight. spectrometry (ICP-OES, Perkin Elmer Optima 3300 RL) Core 11 was analyzed for opal (biogenic silica) content and 30 samples by powder XRF at the University of following an alkaline leaching technique (Mortlock and Bremen (Portable Energy Dispersive Polarization X-ray Froelich, 1989). After leaching, the dissolved silica Fluorescence analyzer, Spectro Xepos). Although there concentration of the resulting extract was measured by was a high correlation between both methods and the the molybdate blue colorimetric method (Hansen and scanner data for some elements, the overall correlation was Grashoff, 1983) using an AutoAnalyser Technicon II. not of sufficient quality to calibrate all elemental data. In addition to MSCLcore logging methods, the XRF Consequently, and consistent with most published studies core scanner was applied. XRF data used in this study were (e.g. Jansen et al., 1998; Ro¨ hl and Abrams, 2000), the produced by the new-generation XRF core scanner at the original XRF core scanner data are expressed in cps. University of Bremen. The core archive halves were Finally, the statistical treatment of the dataset was measured with 2 mm resolution for light (Al, Si, S, K, performed using the R software package (R Development Ca, Ti, Mn and Fe) and 1 cm resolution for heavy (Sr, Zr, Core Team, 2004). Sn and Ba) elements. The measurements were produced The AMS 14C dates from the Chungara sediments were using 60 s count time, 10 kV X-ray voltage (50 kV for heavy obtained from (1) bulk organic matter from the central elements) and an X-ray current of 1 mA to obtain plain cores and (2) aquatic organic macrorests picked from statistically significant measurements. The analyzed sec- littoral cores (Table 2). Several carbonate samples were tions conformed to a composite sequence of cores 10 and also dated with U/Th techniques. 11 (Fig. 3). A detailed description of the applied XRF analysis and system configuration of the XRF core scanner 4. Results and interpretation at the University of Bremen are provided by Jansen et al. (1998) and Ro¨ hl and Abrams (2000). 4.1. Lithostratigraphy The data obtained by the XRF core scanner are expressed as element intensities in counts per second Most facies in Lago Chungara´ sediments (Fig. 2) are (cps). A comparison with other methods employed to massive or laminated diatomaceous ooze (A, B, C, D, E), convert the XRF core scanner measurements to absolute while carbonate-rich facies occur in thin layers or laminae ARTICLE IN PRESS 8 A. Moreno et al. / Quaternary International 161 (2007) 4–21

(F). Volcaniclastic facies (G and H) are particularly 4.2. Mineralogical composition abundant in the upper part of the cores. The presence of these tephra layers and their continuity along the basin The sediments of Lago Chungara´ are mainly composed allows a detailed correlation of the retrieved cores. Tephra of two fractions: one crystalline (highlighted by sharp layers are labelled from M-1 to M-11 (from bottom to top) diffraction peaks), the other amorphous (characterized by and correspond to peaks in magnetic susceptibility (Figs. 2 the presence of a broad peak centered between 201 and 251 and 3). Additionally, the presence of a white ash-flow 2y angles). In accordance with the previously described (WAF), in spite of the low magnetic susceptibility signal, calculations, the amorphous component was quantified. was used for correlation since it occurs in all cores. From The amorphous fraction percentages range from 40% (in stratigraphical correlation and seismic stratigraphy (Sa´ ez volcaniclastic-rich deposits of Unit 2) to almost 100% (in et al., 2007), two main lithostratigraphic units were defined. Unit 1). This fraction represents organic matter, amor- Unit 1 was deposited after the volcanic event that created phous silica (from diatoms) and volcanic glass. On the the lake and is composed of laminated diatomaceous ooze other hand, the percentages of the total crystalline fraction (Facies A, B and C). Only one 2 cm thick glass-dominated range between 60% and less than 1% of the total weight of tephra layer (M1) occurs in Unit 1 (Figs. 2 and 3). These the samples, with this fraction composed of Ca-, lamination sets comprise rhythms and cycles of different carbonates (calcite and dolomite), biotite, pyrite, mm-sized layers (average 2.6 mm) consisting of diatomac- and amphibole. In Fig. 4 the main minerals from Lago eous ooze with variable types (calcite, aragonite) and Chungara´ sediments are represented versus the composite quantities of carbonates and amorphous organic matter. depth. From the bottom to the top of the sequence, and Unit 1 is divided in two subunits: Subunit 1a with green based on the dominance of the fraction and the mineral and white laminations and no carbonate (Facies A) and species, three zones were defined. These zones broadly Subunit 1b with brownish to white laminations (Facies B) corresponded to the lithostratigraphic units (Fig. 4). Unit 1 where endogenic carbonates occur in low concentrations. is dominated by amorphous material (more than 95% of Carbonates occur in the lighter laminae (Facies F). the total weight of the samples). Plagioclase, quartz and Towards the upper part of Subunit 1b, laminated facies pyrite are the main mineral species that comprise the (Facies B) alternate with intervals of dark green, massive, crystalline fraction (o 5%). The presence of silt-sized organic-rich diatomaceous ooze (Facies C). Facies C layers calcite crystals is the main characteristic of Subunit 2a. are continuous throughout the basin and were detected From a depth of 350 cm upwards, the plagioclase in all cores from the central plain (lithological keybeds percentages start to increase whereas the amorphous in Fig. 3). fraction percentages decrease. This change represents the Unit 2 is mainly composed of massive to slightly banded increase of volcanic activity in the nearby Parinacota diatomaceous ooze (Facies D and E) frequently inter- Volcano. Subunit 2b ranges from a depth of 100 cm to the calated by tephra layers (coarse- and fine-grained ashfalls, top of the sequence. This subunit is characterized by Facies G and H). This unit is also divided into two constant, as well as the highest, percentages of plagioclase subunits. Subunit 2a is composed of brownish-red massive minerals. Plagioclase, amphibole, quartz and biotite to slightly banded sapropelic diatomaceous ooze (Facies D) minerals are related to the erosion of catchment rocks with common calcite crystals (silt grain-sized) and carbo- and the synsedimentary direct input from eruptions of the nate-rich layers (Facies F). Facies F (25–55% carbonate) Parinacota Volcano. represents up to 5% of the total thickness of Subunit 2a. Specifically, carbonate-rich layers are grouped in discrete 4.3. Geochemical composition whitish to pinkish thin layers, which are composed of calcite (fibrous crystals, fusiform aggregates, rice-shaped The XRF Core Scanner provided a record of geochem- and euhedral crystals), magnesium calcite, needle- ical variations for the Chungara´ sediments. A composite like aragonite and some traces of dolomite. Tephra layers record joining the analyzed sections from cores 10 are mainly composed of Ca-rich feldspars and volcanic (Sections 1–5) and 11 (Sections 4–6) was generated to glass. In particular, the presence of three lapilli layers cover almost the entire sedimentary sequence (Fig. 5). (Facies G) is highlighted since their lateral continuity Although above this sedimentary sequence up to 95 cm of allowed considering them as key layers for correlating the Chungara´ topmost sediments are recorded in other the cores (M-4–M-6 in Fig. 3). Subunit 2b consists of cores (e.g. core 14) (Fig. 3 and Sa´ ez et al., 2007), we dark gray diatomaceous ooze with frequent macrophyte regarded the top (0 cm) of the studied sequence as the top remains (Facies E) alternating with massive black tephra of core 10 for simplification. The downcore profiles of layers, mainly composed of plagioclase, glass and mafic heavy and light elements clearly delineated three different minerals (Facies H). Volcaniclastic deposits represent units in terms of their geochemical composition: an upper 50% of the total thickness of this unit (levels from M-8 tephra-rich unit with maximum values in all elements to M-11 in Figs. 2 and 3). A 1-cm-thick rhyolitic WAF in (Subunit 2b), an intermediate unit with the highest Ca (Figs. 2 and 3) occurs in all cores and is used for values (Subunit 2a) and a lower unit characterized by the correlation. highest Si content (Unit 1). ARTICLE IN PRESS A. Moreno et al. / Quaternary International 161 (2007) 4–21 9

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

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

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. ARTICLE IN PRESS 10 A. Moreno et al. / Quaternary International 161 (2007) 4–21

4.3.1. Correlation analyses and definition of main sediment components Statistical correlation analysis was performed in order to 0.27 1.00

discern the similarities among the entire set of proxies (Table 1). The sampling interval of the different proxies 1.00 was not identical (e.g. 2 mm for XRF and 5 cm for XRD). 0.26 1.00 Subsequently, to highlight the main coarse relationships between the dataset, all proxies were linearly interpolated 1.00 0.30

with a regular spacing of 5 cm, resulting in a dataset of 17 variables (proxies) and 171 cases (samples). The signifi- 1.00 cance of the correlation analysis (p-values) was calculated 0.17 and the p-values adjusted by applying the Bonferroni test (R Development Core Team, 2004). Many of the sig- 0.90 0.87 0.86 0.94 0.94 0.94 0.23 0.89 0.89 0.94 0.97

nificant correlations among the variables have low values, highlighting that they do not have univocal relationships, eptibility. 0.27 1.00 0.19 0.24 0.24 0.06 since these variables could have more than one origin. As 0.21 an example, Ca has a correlation with calcite and TC contents only around 0.48. This is explained by the 0.98 0.89 0.96 0.25 0.93

volcanic source of Ca in the upper part of the sequence (Ca-plagioclase), as well as the sedimentary source of Ca in 0.20 1.00 0.20 0.14 0.20 0.20 the middle part of the sequence (endogenic carbonates). 0.23 Therefore, although the three main components of Chungara´ sediments discussed above (siliceous biogenic, 0.95 0.88 0.87 0.48 0.89 0.92 0.26 0.93

volcanic and carbonates) can be shown via the higher correlation values among the related proxies (Table 1), 0.06 0.19 1.00 0.41 0.91 several patterns arise when the records are examined more 0.30 closely. Al, K, Ti and Fe are associated with the allochthonous 0.60 1.00 0.08 0.27 1.00 0.12 0.02 0.33 0.23 0.00 0.19 0.18 0.26 1.00 0.06 0.10 0.36 0.08 0.17 0.01 0.27 0.37 component (here, volcanic), increasing with the presence of 0.00 0.23 tephra layers as observed by lithology; heavy elements like Zr, Sr, Sn and Ba are related to tephra layers as well. On 0.44 0.90 0.32 0.83 0.80 0.78 0.80 0.79 0.12 0.81 the other hand, while Ca behaved in similar fashion to the previous group of volcanic-originated elements along the 1.00 0.19 1.00 0.06 0.32 1.00 0.27 0.27 0.48 0.24 0.22 0.25 0.02 0.20 0.09 upper part of the sedimentary succession, it began to 0.17 behave independently starting from the uppermost carbo- nate-rich layer (190 cm in depth) and extending towards the 0.85 0.17 0.18 0.23 0.20 0.19 0.21 0.07 0.17 0.13 0.14 bottom of the sequence (Fig. 5). Accordingly, Ca behaved similar to Al, K, Ti and Fe from 0 to 190 cm depth, while 0.29 1.00 0.31 0.07 0.02 this element exhibited a distinctly different pattern from 0.33 190 cm to the bottom of the sequence (Fig. 5). The best explanation is the increasing abundance of Ca in the 0.63 1.00 0.11 0.43 0.68 0.59 0.79 0.19 0.14 0.28 0.64 0.78 0.31 0.16 0.11 0.14 0.11 0.65 0.74 0.62 0.74 0.64 0.76 0.64 0.79 0.59 0.78 composition of volcanic minerals such as plagioclase, which dominates volcanic eruptions, thereby influencing 0.180.09 1.00 0.59 0.08 0.23 0.30 0.12 0.27 0.06 0.01 0.47 0.09 0.08 0.13 0.11 0.08 0.10 0.25 Chungara´ sediments mainly in the upper part of the record. 0.08 The Ca profile changes drastically in Subunit 2a: Ca appears as very sharp spikes instead of smooth peaks common in Subunit 2b. Ca content in the lower part of the 1.00 0.040.87 1.00 0.27 0.22 0.17 0.17 0.19 record is very low but it is still possible to recognize abrupt 0.23 Ca spikes associated with some white laminae. In the same manner as Ca, Si is influenced by different components, with volcanic and biogenic silica (diatoms) being the most important. The Si signal thus represents a mixture of both 0.88 0.87 0.42 0.60 0.72 0.04 0.38 0.47 0.50 0.62 0.28 0.07 0.57 0.68 0.30 0.29 0.11 0.58 0.69 0.53 0.65 0.61 0.62 0.57 0.68 0.52 0.61 Amorphous Plagioclase Calcite Opal MS TOC TC Al Si S K Ca Ti Mn Fe Rb Sr Zr Sn Ba influences. Other elements, as S or Mn, are not strongly related to any of the described patterns. Therefore, the S profile does not closely mimic any other profile, but shows a slight correspondence with Si values (0.6 correlation in AmorphousPlagioclase 1.00 Table 1 Correlation coefficients among different proxies used in this study Calcite MS TC Opal TOC 0.10 Al Si 0.28 K S 0.11 0.02 Ca Ti Mn Fe Rb Sr Zr Sn 0.26 Ba Table 1) indicating that this pattern most likely reflects an Correlations that are in plain text (not bold style) are not significant after application of the Bonferroni test (see text). MS refers to magnetic susc ARTICLE IN PRESS A. Moreno et al. / Quaternary International 161 (2007) 4–21 11 interference of influences and origins. In the same way, Mn extracted by alkaline digestion, as well as with the values mark the occurrence of tephra layers and correlate amorphous fraction obtained from XRD. In addition, with some Ca peaks. The presence of Mn in these TOC and S/Ti profiles are plotted to infer the variability of sediments is related to changes in redox conditions at the the organic matter content in Lago Chungara´ since the S sediment–water interface since Mn forms a highly insoluble content reflects the occurrence of pyrite, which generally oxide where oxic conditions prevail. Thus, solid phases of increases when organic matter content is high. The Mn/Ti Mn in lake sediments appear as a result of the upward ratio is plotted in the same figure. From a comparison of diffusion of dissolved Mn and its posterior precipitation at the whole dataset (Fig. 6), some striking similarities can be oxic horizons in the form of Mn oxides (Aguilar and observed. Thus, the Si/Ti, as well as the opal and Nealson, 1998). In Chungara´ sediments, Mn background is amorphous profiles follow similar trends at some intervals: very low, usually below 0.5 g kg1 (1000 cps), although it low values in the upper part tend to increase from 300 to increases to 2.5 g kg1 (5000 cps) during discrete peaks that 550 cm and remain at intermediate levels until the bottom are more abundant during Subunit 1b. This behavior may of the sequence. These results point to a higher lake diatom indicate a close relation with changes in the sedimentary productivity (or better, silica preservation) in the laminated oxygen content most likely related to biogenic processes or deposits of Subunit 1b and, to a lesser extent, in Subunit 1a changes in water circulation due to lake-level variations. deposits (Fig. 6). In addition, the Si/Ti record helps to The strong volcanic influence in the geochemical identify intervals where the biogenic lacustrine signal is composition of Chungara´ sediments makes it necessary to dominant over volcanic layers. This pattern is not so clear normalize the elemental data to ‘‘volcanic’’ elements like Ti in opal or amorphous profiles because of lower spatial to unravel the environmental or climatic meaning of the resolution. different profiles. Such high-resolution profiles constrain The S/Ti, TOC and Mn/Ti data require careful inter- the evolution of the three main inputs contributing to the pretation (Fig. 6). Microscopic observation of smear slides Chungara´ record: siliceous biological remains (mainly and SEM-EDS determinations (Sa´ ez et al., 2007) show the diatom skeletons), volcanic minerals and endogenic presence of pyrite associated with organic-rich sediments. carbonates. Thus, the S/Ti ratio could reflect variations in pyrite that could be correlated to TOC. Pyrite was detected by XRD 4.3.2. Biogenic component although always at very low percentages (Fig. 4). However, In Fig. 6, the Si/Ti ratio is represented to discern the the very low values of S obtained in some samples by XRF biogenic silica amount compared to the percentage of opal (below 2 g kg1) prevent an accurate comparison among

Fig. 6. Biogenic influences on Lago Chungara´ sediments: Si/Ti ratio, biogenic opal, amorphous minerals obtained by XRD, S/Ti ratio, TOC and Mn/Ti ratio. All proxies are plotted versus core depth (cm), lithological subunits are indicated. ARTICLE IN PRESS 12 A. Moreno et al. / Quaternary International 161 (2007) 4–21 these records. In addition, the TOC record obtained from Values of Fe, Ti and K are excellent indicators of volcanic upper Chungara´ sediments is highly variable due to the deposits in the Lago Chungara´ record (Table 1, Fig. 7). frequent macrophyte remains and volcanic input. Organic matter content increases from the bottom of the sequence 4.3.4. Carbonate components (4%) to the top of laminated deposits of Subunit 1b, where To investigate carbonate production variability, the Ca/ it reaches 8%. The maximum in Mn/Ti is associated with Ti ratio versus TIC and calcite percentages are plotted the maximum in organic matter and diatomaceous (Fig. 8). In addition, the Sr/Ti ratio was plotted to verify productivity (Subunit 1b). Although an accurate explana- whether the Sr variations were related to carbonate tion requires an understanding of diatom assemblages precipitation (i.e. in aragonite) or to volcanic input. The during this interval, since diatoms contribute about 80% of high correlation between Ca/Ti and Sr/Ti confirms the the sediment, we would suggest changes in redox condi- relation of both elements (Ca and Sr) to the carbonate tions as a significant factor in Mn variability. More oxygen precipitation (Fig. 8). Normalization with respect to Ti is a in the water column caused either by photosynthetic valuable tool for unravelling previously undetected pat- processes (related to maximum diatom productivity) or terns. The high-resolution record shows that carbonate in increased bottom water ventilation would account for the the offshore central plain of the lake was produced during increase in Mn. In such a scenario, Mn would be very short intervals and deposited as thin layers. This spiky precipitated as oxides and then, preserved in sediments. character of carbonates was hidden when working with discrete samples at lower sampling resolution (see calcite 4.3.3. Volcanic components and TIC profiles, Fig. 8). It is also evident that carbonate Elements directly related to the volcanic influence in the deposits are concentrated in Subunit 2a, thus confirming Chungara´ sequence are compared with magnetic suscept- the presence of a carbonate-rich interval (250–450 cm), ibility and volcanic minerals, such as plagioclase or reaching up to 40% of carbonate. Microscopic (optical and amphibole (Fig. 7). It is evident that Unit 1 deposits are scanning) observation of these carbonate layers indicates almost free of volcanic input, while Unit 2 is undoubtedly that they are mainly composed of calcite, in the form of dominated by tephra ashfalls. Mineralogical results in- rice-shaped or euhedral crystals. These types of carbonate dicate that the dominant volcanic mineral is plagioclase, crystals are formed in the offshore central plain epilimnium likely andesine, with percentages ranging from 5% to 60%. (without any transport from the vegetated littoral areas or

Fig. 7. Volcanic influences on Lago Chungara´ sediments: Fe, Ti, K, plagioclase and amphibole obtained by XRD and magnetic susceptibility (SI units). All proxies are plotted versus core depth, lithological subunits are indicated and the correlation lines (M-1 to M-11) marked by arrows (see also Fig. 3). ARTICLE IN PRESS A. Moreno et al. / Quaternary International 161 (2007) 4–21 13

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.

any relation to organisms, such as bivalves or gastropods). precipitation–evaporation balance (Valero-Garce´ s et al., Two possible origins of these calcite crystals are: (1) they 2003). More evaporation or less precipitation (arid period) are linked to algal blooms capable of altering water CO2 would imply more concentrated waters and lead to concentration (Kelts and Hsu¨ , 1978; Teranes et al., 1999) precipitation of offshore carbonates. or (2) they are associated with alterations in water chemistry, either of volcanic influence or from changes in 4.4. Chronology water concentration. The apparent absence of correlation between the long-term evolution of Ca/Ti (carbonates) and Dating the sediments from Lago Chungara´ is compli- of Si/Ti (mainly diatoms) allowed us to propose that the cated due to the scarcity of terrestrial organic rests productivity blooms are not the main promoter of the resulting from the low vegetation cover. Therefore, AMS carbonate production. This relation seems to be true only 14C dates from Chungara´ sediments were obtained from (1) in carbonate layers where rice-shaped carbonate crystals bulk organic matter from the central plain cores and (2) are present (Table 1, Figs. 6 and 8). On the other hand, a aquatic organic macrorests picked from littoral cores volcanic influence cannot be totally discarded since Subunit (Table 2). In Fig. 3, the correlation panel with the cores 2a is characterized by the presence of the thickest lapilli used to construct the chronological model is presented. The layers along the sequence (Fig. 2). However, calcite similarities in the sedimentary facies among the cores and precipitation starts earlier (in Subunit 1b) than does the presence of key tephra layers allow transferring the volcanic activity, which calls into question volcanic obtained dates to a single composite depth (Table 2). As influence as the main causative force (Fig. 8). Therefore, observed in Table 2, there are five ages that were discarded the most likely mechanism stems from changes in water (shaded samples) and not included in the final age model. concentration. In current climatic and limnological condi- Two of them are clearly reversed (15A-5, 76 and 13A-4, tions, water concentration is mainly related to the 66), probably due to depositional reworking of the ARTICLE IN PRESS 14 A. Moreno et al. / Quaternary International 161 (2007) 4–21 . Fig. 9 0.4 ) 0.2 0.1 0.1 0.1 0.3 0.2 0.4 2.3 3.7 1.7 0.2 % 7 7 7 7 7 7 7 7 7 7 7 7 C( 12.9 25.46 14.8 16.1 16.9 16.8 14.1 13.6 22.9 16.2 22.7 28.7 19.6 23.1 13 d 370 390 420 420 410 570 910 900 500 600 90 7 7 7 7 7 7 7 7 7 7 7 on (reversed dates, error too high). DIC: dissolved t for details about calibration procedures and 1190 13050 1000 13410 1225 14095 800 14875 900 2865 500 4440 600 5970 90 9550 450 11450 690 12770 1000 13000 7 7 7 7 7 7 7 7 7 7 7 Calibrated age (cal. yr BP)method after Heegaard’s Model 1 Model 2 C 14 60 70 9280 60 9745 80 10110 70 10775 80 11720 80 12710 ) (yr BP) 40 — — 40 2865 39 4440 50 5970 50 50 No data 50 7325 60 8380 50 100 No data 7 7 7 7 7 7 7 s 7 7 7 7 7 7 7 7 7 7 age (2 Type of sample Uncalibrated and depth) Laboratory ID Sample (core depth (cm) 0 Beta 188745 DIC measured in surface water 2320 67 AA56904 15A-2, 48 Aquatic organic macroremains 6635 95 Poz-8721 11A-2, 84 Bulk organic matter 7290 258 Poz-8723 11A-3, 2 Bulk organic matter 8920 436 Poz-8724 11A-3, 86 Bulk organic matter 10,860 344 AA56903 15A-4, 27 Aquatic organic macroremains 9999 550 Poz-8647 11A-4, 10 Bulk organic matter 9860 615 Poz-7171 11A-4, 63 Bulk organic matter 11,070 675 Poz-8725 13A-4, 66 Bulk organic matter 8810 665 AA56905 15A-5,76 Aquatic organic macroremains 4385 697 Poz-11891 11A-4,145 Bulk organic matter 11,460 743 Poz-13032 11A-5, 41 Bulk organic matter 10,950 785 Poz-11982 11A-5, 84 Bulk organic matter 11,180 827 Poz-13033 11A-6, 41 Bulk organic matter 12,120 865 Poz-7169 11A-6. 79 Bulk organic matter 13,100 C AMS radiocarbon ages measured in Chungara cores Table 2 14 Units Composite Subunit 2b 42 Poz-8720 11A-2, 39 Bulk organic matter 4850 Subunit 2a Subunit 1b 490 Poz-7170 11A-3,123 Bulk organic matter 8570 Subunit 1a Depth transposed to composite sequenceinorganic is carbon indicated. (apparent Lithological age subunits of surface are water). also Model shown. 1: Italic constant samples reservoir were age not correction; Model used 2: for no the reservoir age age model correction constructi for Unit 1. See tex ARTICLE IN PRESS A. Moreno et al. / Quaternary International 161 (2007) 4–21 15 sediments, since one sample comes from a volcanic sandy volcanic layers, the former represents a clearly distinct layer and the second from a reworked peaty interval. The stage of the lake system (laminated sediments, no volcanic other three samples (11A-3, 2, 15A-4, 27 and 11A-3, 86) input). This pattern may allow us to consider a constant belong to the volcanic-rich Subunit 2a (Fig. 2) and do not reservoir effect for the uppermost subunit, since the show evidence of depositional reworking. One possibility is average lake characteristics (depth, water volume) most that the strong volcanic influence may have altered the CO2 likely did not vary much over time. Therefore, a constant balance among the different sources (atmosphere, soil, reservoir effect of 2320 years (the present-day one) is runoff and groundwater, volcanic), thus modifying the subtracted from the Subunit 2b dates. After removing the 14C value of these three samples. A similar effect has reversed ages, no 14C AMS data were available from been documented in other from the Altiplano Subunit 2a (Table 2). (Valero-Garce´ s, et al., 1999, 2000) and Easter Island It is only possible to hypothesize about the variations (Butler et al., 2004) with high volcanic influence. over time of the reservoir effect in Unit 1. In accordance Obtaining reliable radiocarbon dates in the sediments of with the hypothesis set forward by Geyh and Grosjean Lago Chungara´ is also problematic because of an assumed (2000), the reservoir effect was most likely lower in Unit 1 large and variable radiocarbon reservoir effect. Similar than in Unit 2 since the lake was, on average, shallower problems occur in the majority of lake deposits from the than during the deposition of the upper unit. This is Andean Altiplano, with very few studies able to properly supported by a seismic study, by the absence of emerged evaluate the variations in reservoir effect over time (Geyh former lake terraces, and other indicators (sedimentary et al., 1999; Geyh and Grosjean, 2000; Grosjean et al., facies, diatoms, carbonate content) (Sa´ ez et al., 2007). We 2001). The modern reservoir effect for Lago Chungara´ was propose to correct the Unit 1 dates for two possible obtained by dating the dissolved inorganic carbon (DIC) of extreme reservoir age values: a minimum value of 0 years the surface lake water. This resulted in 2320740 14CyrBP and a maximum of 2320 years. Once corrected by (Table 2), a value very similar to that obtained by Geyh considering these two extreme reservoir effect values, an et al. (1999) after analyzing DIC lake water (17457160 14C age range of variation for every Unit 1 date is obtained. yr) and living macrophytes (25607245 14C yr) in Lago These dates were calibrated using the most updated Chungara´ . calibration curve (INTCAL04), which is provided by the However, the reservoir effect in the Altiplano lakes has CALIB 5.02 software package (Reimer et al., 2004) proved to be highly variable over time, with the influence of selecting the mid-point of the 95.4% of the distribution lake water volume being one of the most significant factors (2s probability interval). (Geyh and Grosjean, 2000). Since no terrestrial organic In order to fill the gaps of the age model constructed remains were found in Lago Chungara´ sediments, we could only by 14C AMS dates, four 238U/230Th measurements not evaluate the variation of the reservoir effect with time were carried out on calcite crystals that appeared in some applying the methodology of Geyh and Grosjean (2000). thin layers from Chungara´ cores. These dates were done via Therefore, our approach to correct the dates for the the standard method using an ICP-IRMS multicollector at variable reservoir effect has been based on two assump- the University of Minnesota (Table 3). For a summary of tions: (1) the Lago Chungara´ system during deposition of this method see Edwards et al. (1986). Only one 238U/230Th Subunit 2b is very similar to that currently found there and date was finally acceptable for the chronological model due (2) the present-day lake level is at the highest in its history. to their high content of 232Th in the other three samples. Accordingly, the correction of the reservoir effect was Fortunately, this 238U/230Th date belongs to Subunit 2a, achieved differently in Unit 1 and Unit 2. It is worth noting the only interval without sound 14C AMS dates. the very different sedimentary patterns recorded when To construct a reliable age-depth model with the comparing Units 1 and 2: while the latter is composed of remaining 12 dates (11 14C AMS and 1 U/Th), we used rather homogenous facies (dark gray massive diatomac- the software described in Heegaard et al. (2005) as a useful eous oozes in Subunit 2b) characterized by abundant interpolation tool. This software provides a procedure to

Table 3 238U/230Th ages measured in Chungara cores

Depth (cm) Sample ID Carbonate 238U (ppb) 232Th (ppm) d234U 230Th/238U 230Th/232Th Error Calendar material (ppm) age BP

280 14A-3, 6 Crystal Out of scale — 323.3 — 0 — — 285 13A-2, 45 Crystal 576.4 53.9 335.1 0.0774 14 1244 4450 344 13A-2, 105 Crystal 467.4 35.2 413.5 0.1036 23 974 6730 374 15A-4, 77 Shell 717.4 203.1 320.2 0.1607 9 3445 7720

Depth transposed to composite sequence is indicated. Italic samples were not used for age model construction (reversed dates, error too high or high values of 232Th). See text for details about calibration procedures and Fig. 9. ARTICLE IN PRESS 16 A. Moreno et al. / Quaternary International 161 (2007) 4–21 estimate the age–depth relationship by setting the mid- with a sedimentation rate in offshore zones between 0.47 point value of the calibrated ages in relation to the central and 0.78 mm yr1. distributional range. The software provides a final cor- rected age for every calibrated date (Table 2, column 5. Paleoclimate implications ‘‘Calibrated age after Heegaard’s method’’). These ages are then employed for the age-depth model (Fig. 9). This Selected proxies from the Chungara´ sequence were applied correction has led to slight age differences in the plotted versus age and compared with published records boundaries of the subunits with respect to those presented from the nearby Nevado Sajama ice core and Lake Titicaca in Sa´ ez et al. (2007) and incorporates two additional tie (Fig. 10). Accordingly, any discussions addressing the points. paleoclimate implications should take into account the In summary, the age model for the sediment sequence of range of possible ages along Unit 1 (Fig. 9). Additional Lago Chungara´ presented here is constructed with (1) the improvements to the age model of this sequence (more available dates along Subunit 2b corrected by a constant 238U/230Th dates or dating tephra layers) will allow more reservoir effect similar to that currently found, (2) a unique extensive and refined interpretations of paleoclimate date in Subunit 2a acquired by the 238U/230Th procedure dynamics. and (3) the mid-point value of the obtained range taking Close correlation among Fe elemental intensities and the into account the maximum and the minimum reservoir age amount of coarse particles in the Sajama ice core indicates corrections for the existing dates along Unit 1. Thus the that the volcanic signal detected in the Chungara´ record Lago Chungara´ sequence covers the last 14,000 cal. yr BP has a regional character and that most of the dust particles in the Sajama ice core are of volcanic origin. Very likely, this volcanic signal is related to the Parinacota Volcano Calibrated age (cal. years BP) after Heegaard's model eruptions since it is the only one in the area with recent 0 2000 4000 6000 8000 10000 12000 14000 16000 eruptive activity (Wo¨ rner et al., 1988). Due to the short 0 distance (30 km) between Lago Chungara´ and Nevado Subunit 2b Sajama (Fig. 1), one can easily infer that the source for 100 volcaniclastic particles in both records was the same. Therefore, we suggest that the variations found in the 200 coarser particles in the Sajama ice core are mainly Subunit 2a 300 controlled by the main volcanic eruptions of Parinacota U/Th age Volcano and are not due to (1) atmospheric dust input or 400 (2) the greater availability of salt minerals during dry periods in the Altiplano, as was proposed by Thompson 500 et al. (1998). The Fe record from Lago Chungara´ and the Subunit 1b Sajama record indicate that explosive activities of Parina- Composite depth (cm) 600 cota Volcano increased dramatically at 5800 cal. yr BP 700 (Fig. 10). The period with less explosive volcanic activity is also coincident in both records from 14,000 to 800 Subunit 1a 7000–8000 cal. yr BP (Fig. 10). 900 A comparison of the Lago Chungara´ record with other Model 1 and 2: common reservoir age correction for Unit 2 paleoclimate reconstructions from the nearby Cotacotani basin becomes necessary to assess the Chungara´ record Model 1: constant reservoir age correction within a regional context. Pollen stratigraphy obtained Model 2: no reservoir age correction for Unit 1 from an outcrop at Laguna Seca (Fig. 1) indicates a gradual transition towards drier and warmer climates since Fig. 9. Age control points used for the age model of Lago Chungara´ .We the late (Baied and Wheeler, 1993). An represent the ‘‘Calibrated age (cal. yr BP) after Heegaard’s method’’ data 14 from Table 2. The error bar displayed for every age is a consequence of the increased desiccation between 8000 and 6500 C yr BP, 14 interpolation carried out following Heegaard et al. (2005). Two different as well as warmer conditions until about 5000 C yr BP are approaches were carried out to correct the reservoir effect in the Lago also suggested by that study. These paleoclimate inter- Chungara´ sequence: model 1 (application of a constant reservoir age pretations are supported by an isotopic study carried out in correction along the entire sequence, white dots) and model 2 (application of a constant reservoir age correction for Subunit 2b and no correction for a sediment core obtained from Laguna Seca (Schwalb Unit 1 dates, gray dots). Therefore, for both models the uppermost et al., 1999). Moreover, a transition towards higher lake interval (Subunit 2b) is common (reservoir age is considered constant, levels from the mid-to-late Holocene was also postulated 2320 years, black dots). In Subunit 2a only one U/Th date is available. (Schwalb et al., 1999). Along Unit 1, the calibrated dates range from model 1 to model 2 values, The high-resolution geochemical study of Lago Chun- representing the maximum and minimum reservoir age correction, respectively. Although the final age model was constructed with the gara´ provides new data on two main paleoclimatic topics: intermediate values from the represented range (dashed line), the broad the glacial–interglacial transition and the aridity crises interval is taken into account in the paleoclimate interpretations. during the Holocene. A. Moreno et al. / Quaternary International 161 (2007) 4–21 17

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 63 mm 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).

5.1. Glacial–interglacial transition—early holocene Although the meaning of these laminations cannot be discerned with our geochemical proxies, we suggest an The glacial–interglacial transition can be observed in increase in productivity, probably related to favored several northern Andean Altiplano records as a sharp diatom productivity, at the beginning of the Holocene change towards drier conditions associated with minimum (11,500 cal. yr BP) lasting until 7500 cal. yr BP. A detailed summer insolation (E11,000 cal. yr BP, Fig. 10) following diatom study (Bao et al., in preparation) and organic the wet Tauca phase when the largest lakes and the highest geochemical analyses are in progress to understand the lake levels occurred (Argollo and Mourguiart, 2000; Baker significance of the different laminations and their cycli- et al., 2001a; Mourguiart and Ledru, 2003). Although the cities. As well as the sedimentary facies changes, several end of the Tauca phase has not been well established, it is productivity proxies point to a general increase (TOC, Si/ believed to have ended about 14,900 cal. yr BP (Baker et al., Ti and S/Ti) after the end of the glacial period. High 2001b; Fornari et al., 2001). In some records, a short wet values are maintained throughout the early Holocene event (13–11.5 kyr BP; ‘‘Coipasa phase’’) has been observed (Figs. 6 and 10). This change correlates with the Sajama and correlated with the Younger Dryas in the northern ice core isotopic change towards warmer temperatures hemisphere (Baker et al., 2001a). (Thompson et al., 1998). One of the main transitions in the Lago Chungara´ In contrast to the glacial–interglacial transition, the record corresponds with the boundary between Subunits 1a relevant geochemical indicators do not show any clear and 1b, which depending on the age model occurred indication of climate changes associated with the ‘‘Coipasa between 9500 and 12,500 cal. yr BP (Fig. 9). In spite of the phase’’ or Younger Dryas event (Fig. 10). However, in the chronological uncertainty, this boundary is likely to Sajama ice core (Thompson et al., 1998) and the Titicaca represent the glacial–interglacial transition and the onset record (Baker et al., 2001a), a decrease in temperature and an of the Holocene. During that transition, Chungara sedi- increase in humidity have been postulated for that period. ments reflect the change from green-white laminations The high sampling resolution attained in this study (Subunit 1a) to brown-white laminations (Subunit 1b). allowed focusing on short and dramatic climate events that ARTICLE IN PRESS 18 A. Moreno et al. / Quaternary International 161 (2007) 4–21 otherwise would not be observable by discrete sampling. In scenario involving lower lake levels with an increase in the Chungara´ record, a very prominent peak in the Mn/Ti littoral areas. The increase in benthic diatoms in the profile occurs at around 10,000 cal. yr BP ranging from 8400 carbonate-rich levels supports such a hypothesis (Sa´ ez to 11,400 cal. yr BP (Fig. 10). As outlined above, Mn is et al., 2007; Bao et al., in preparation). assumed to be associated with redox front variations when However, fluctuations in biological activity, as well as oxic conditions develop following an interval in which an changes in inflowing water composition could also have anoxic environment dominated. Therefore, we propose that played a role in making the lake waters more conducive to an increase in oxygen in the bottom waters occurred at that carbonate formation. Several periods of increased carbo- time. The most probable mechanism that would produce nate formation in littoral areas from Lago Chungara´ , such an increase in oxygen is a lack of long periods of water related to increased charophyte productivity, have oc- stratification, e.g., sufficiently low lake levels could cause curred during the last 4000 years (Valero-Garce´ s et al., winds to mix the entire water column. This mechanism may 1996, 2003). In nearby Laguna Seca, periods of carbonate be reinforced by an increase in diatom productivity, as a formation seem to have occurred throughout the Holocene consequence of such a mixing, and thus in oxygen supply. (Baied and Wheeler, 1993; Schwalb et al., 1999), although This potentially more arid scenario (lower water levels), in this case they seem to be related to travertine deposition when compared with previous sediment deposits would be and spring activity. Detailed diatom studies and statistical supported by an increase in benthic diatoms during this analyses to more precisely determine the volcanic input are interval as indicated by preliminary data (Sa´ ez et al., 2007; in progress. Bao et al., in preparation). In addition, peaks in calcite Considering the offshore formation of carbonate as an contents, Ca/Ti and Sr/Ti would further strengthen this indicator of more concentrated lake water, the period hypothesis (Figs. 8 and 10). ranging from 8600 to 6400 cal. yr BP could be proposed as the most arid one (taking into account the established 5.2. Aridity crisis during the early and mid-Holocene age ranges, the starting point of this interval varies from 7300 to 9500 cal. yr BP). This period does not precisely Apart from the event detected in the Lago Chungara´ coincide with the dry period detected in Lake Titicaca sediments around 10,000 cal. yr BP in the Mn/Ti record, using benthic diatom abundances which is dated between other intervals were observed during the early-to-mid 8.5 and 4.5 cal. kyr BP, with an extremely dry period from 6 Holocene that may indicate lower lake levels as well to 5 cal. kyr BP (Baker et al., 2001a; Tapia et al., 2003, (Fig. 10). In general, periods of high lake levels in the Fig. 10). The main difference with the Lake Titicaca record Andean Altiplano are interpreted in terms of increasing is the timing of the dry period, ending after the summer precipitation in the southern hemisphere, with the corresponding period of the Chungara´ record (Fig. 10). ITCZ occupying a more southern position, while dry As detected in other studies, Holocene aridity crises were periods are related to a northward displacement of the not synchronous in the Andean Altiplano (Betancourt ITCZ (i.e. Argollo and Mourguiart, 2000). In the nearby et al., 2000; Holmgren et al., 2001; Abbott et al., 2003; Laguna Seca record, a transition from carbonate-rich, Grosjean et al., 2003; Latorre et al., 2003). According to laminated lacustrine sediments to peaty sediments occurred the age model, these findings are consistent with the at about 70307245 14CyrBP(Baied and Wheeler, 1993). proposed N–S gradient, and therefore support the complex In spite of the uncertainties with the Laguna Seca age timing and structure of the early-to-mid Holocene aridity model, this change from a lake towards a peatbog points to crisis. arid conditions for the mid-Holocene in the Lago Finally, it is worth noting that dry conditions were not Chungara´ area. constant during the arid period in the Chungara´ region, but Enhanced carbonate precipitation in the offshore zones characterized by a series of short and rapid dry spells of Lago Chungara´ was observed along Subunit 2a (Figs. 8 (Fig. 10). Considering the Ca/Ti record as an aridity and 10). Carbonate production in the lake requires the indicator, several abrupt arid periods (of less than 100 presence of Ca in the water, which is assured throughout years duration) some of them coincident with Mn/Ti peaks nearly the entire sequence: Ca is provided by leaching of are detected (Fig. 10). As stated before, both Ca/Ti and Ca-rich volcanic ashes or by the increase of volcanic input Mn/Ti ratios point towards climate scenarios with less that becomes more important in Subunit 2a (Sa´ ez et al., water availability in the Chilean Altiplano. The fact that 2007). Some of the carbonate-rich levels occur in intervals several arid events alternate with less arid periods during a where Ca-rich tephra layers are more frequent, suggesting relatively short time interval indicates that conditions were that volcanism plays a role. However, carbonate formation unstable and highly changeable. predates the Ca-rich volcanic interval (Subunit 2b), and Although these events cannot be precisely correlated carbonate production halts before the Ca-rich volcanic with other paleoclimate archives in the region due to age input to the lake stops, suggesting that other factors could model uncertainties, they support the argument that the also control carbonate formation. Therefore, the presence aridity postulated to have occurred since the early of endogenic carbonate deposits in offshore cores is a Holocene in South American tropical records is not potential indicator of more concentrated waters, e.g., a continuous but rather represents a succession of short ARTICLE IN PRESS A. Moreno et al. / Quaternary International 161 (2007) 4–21 19 dry/wet events. Despite the fact that the forcing mechan- Acknowledgements isms of these abrupt changes in moisture availability remain partially unknown (Ruter et al., 2004), model We are indebted to the Limnological Research Center reconstructions should take into consideration all possible staff who participated in the field expedition (D. Schnur- factors that could account for the presence of abrupt and remberger, M. Shapley and A. Myrbo) and collaborated short dry/wet events over this generally arid period. during the initial core descriptions, as well as the CONAF (Corporacio´ n Nacional Forestal, Chile) for the facilities provided in Chungara´ . We acknowledge C. Herrera 6. Conclusions (Universidad Cato´ lica del Norte, Chile) for his help during the field expedition. We are very grateful to L. Edwards High-resolution geochemical profiles were produced for (University of Minnesota) and R.O. Gibert (University of the sedimentary succesion recovered from Lago Chungara´ Barcelona) for the ICP-IRMS U/Th dating and to S. Fritz at 181S. These results, compared with physical, miner- for providing the Titicaca diatom data. The University of alogical and sedimentological data obtained from the same Bremen, particularly U. Ro¨ hl, F. Lamy, M. Ko¨ lling, H. cores, provide the first high-resolution lacustrine sequence Pfletschinger and H. Kuhlmann are acknowledged for of the Chilean Andean Altiplano during the last technical assistance with the XRF-Core Scanner, powder 14,000 cal. yr BP. The exhaustive analysis of the meaning XRF and ICP-OES analyses. We also acknowledge M. of the elementary signatures, taking into account their Grosjean and N. Piotrowska for their advice in age model potential origins before inferring a paleoclimatic signal, construction and R. Rycroft for the English correction. allows recognition of three main components in Lago The Paleostudies programme (European Science Founda- Chungara´ sediments: (1) lacustrine biological remains tion) provided the necessary funding to carry out the mainly from diatoms, (2) volcanic minerals, and (3) analyses at the University of Bremen. This study is endogenic offshore carbonates. Siliceous productivity was supported by the projects BTE2001-3225 and BTE2001- deduced from Si/Ti, opal and amorphous material while 5257-E and CGL2004-00683/BTE funded by the CICYT, the volcanic supplies were inferred from Fe, Ti and K the Spanish Ministry of Science and Technology. A. elemental intensities, variation in magnetic susceptibility Moreno and P. Gonza´ lez-Sampe´ riz are the recipients of a and occurrences of plagioclase and amphibole minerals. CSIC research contract (I3P postdoctoral programme). S. Volcanic contributions were closely correlated to the total Giralt acknowledges the Spanish Ministry of Science and amount of coarse particles recovered in the Nevado Sajama Technology for his postdoctoral contract in the Ramo´ny ice core, thus pointing to the same volcano source for both, Cajal programme. the Lago Chungara´ and Sajama records, most likely the Parinacota Volcano main explosive eruptions. Offshore carbonate production was asserted from Ca/Ti and Sr/Ti References ratios as well as from calcite and TIC contents. Abbott, M.B., Seltzer, G.O., Kelts, K., Southon, J., 1997. Holocene The transition from greenish to brownish laminated paleohydrology of the tropical Andes from Lake Records. Quaternary diatomites identifies the glacial–interglacial transition in Research 47, 70–80. Chungara´ sediments that should be synchronous with the Abbott, M.B., Wolfe, B.B., Wolfe, A.P., Seltzer, G.O., Aravena, R., Sajama temperature increase. 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