J Paleolimnol (2008) 40:195–215 DOI 10.1007/s10933-007-9151-9

ORIGINAL PAPER

A statistical approach to disentangle environmental forcings in a lacustrine record: the Lago Chungara´ case (Chilean )

Santiago Giralt Æ Ana Moreno Æ Roberto Bao Æ Alberto Sa´ez Æ Ricardo Prego Æ Blas L. Valero-Garce´s Æ Juan Jose´ Pueyo Æ Pene´lope Gonza´lez-Sampe´riz Æ Conxita Taberner

Received: 31 May 2007 / Accepted: 28 August 2007 / Published online: 2 October 2007 Springer Science+Business Media B.V. 2007

Abstract A high resolution multiproxy study (mag- to identify and isolate the main underlying environ- netic susceptibility, X-ray diffraction, XRF scanner, mental gradients that characterize the sedimentary gray-colour values, Total Organic Carbon, Total infill of Lago Chungara´. The first eigenvector of the Inorganic Carbon, Total Carbon and Total Biogenic PCA could be interpreted as an indicator of changes Silica) of the sedimentary infill of Lago Chungara´ in the input of volcaniclastic material, whereas the (northern Chilean Altiplano) was undertaken to second one would indicate changes in water avail- unravel the environmental forcings controlling its ability. The chronological model of this sedimentary evolution using a number of different multivariate sequence was constructed using 17 AMS 14C and statistical techniques. Redundancy analyses enabled 1 238U/230Th dates in order to characterize the us to identify the main provenance of the studied volcaniclastic input and the changes in water avail- proxies whereas stratigraphically unconstrained clus- ability in the last 12,300 cal years BP. ter analyses allowed us to distinguish the ‘‘outsiders’’ Comparison of the reconstructed volcaniclastic as result of anomalous XRF scanner acquisitions. input of Lago Chungara´ with the dust particle record Principal Component Analysis (PCA) was employed from the core suggested that the

S. Giralt (&) R. Prego Institute of Earth Sciences ‘‘Jaume Almera’’ (CSIC), Marine Biochemistry Research Group, Marine Research Lluı´s Sole´ i Sabarı´s s/n, Barcelona 08028, Spain Institute (CSIC), Eduardo Cabello 6, Vigo 36208, Spain e-mail: [email protected] B. L. Valero-Garce´s Á P. Gonza´lez-Sampe´riz A. Moreno Pyrenean Institute of Ecology (CSIC), Apdo 202, Limnological Research Center, Department Zaragoza 50080, Spain of Geology and Geophysics, University of Minnesota, 220 Pillsbury Hall, 310 Pillsbury Drive S.E., Minneapolis, J. J. Pueyo MN 55455-0219, USA Department of Geochemistry, Faculty of Geology, University of Barcelona, Martı´ i Franque`s s/n, R. Bao Barcelona 08028, Spain Facultade de Ciencias, Universidade A Corun˜a, Campus da Zapateira s/n, A Coruna 15071, Spain C. Taberner Shell International Exploration and Production B.V., A. Sa´ez Carbonate Team, Kesslerpark 1, Rijswijk 2288 GS, Department of Stratigraphy, Paleontology and Marine The Netherlands Geosciences, Faculty of Geology, University of Barcelona, Martı´ i Franque`s s/n, Barcelona 08028, Spain 123 196 J Paleolimnol (2008) 40:195–215

Parinacota volcano eruptions were the main source of large urban areas where there is an intensive use of dust during the mid and Late rather than the the catchment area (e.g. Constance), making it dry out as has previously been pointed out. The difficult to distinguish the climatic signal from other comparison of the water availability reconstruction of processes. Even lakes located in remote areas have Lago Chungara´ with three of the most detailed demonstrated that they can be markedly affected paleoenvironmental records of the region (Paco by anthropogenic activities (Camarero et al. 1995; Cocha, Lake Titicaca and Salar Uyuni) showed an Carrera et al. 2002). Hence, a detailed paleoenviron- heterogeneous (and sometimes contradictory) tempo- mental reconstruction must clearly identify and ral and spatial pattern distribution of moisture. isolate the different signals. Although the four reconstructions showed a good One way to identify these signals in recent correlation, each lacustrine ecosystem responded sediments is to use a statistical approach. Ordination differently to the moisture oscillations that affected analyses such as Component Analyses have proved to this region. The variations in the paleoenvironmental be effective in highlighting (1) the underlying records could be attributed to the dating uncertaini- environmental processes that trigger the recent ties, lake size, lake morphology, catchment size and marine facies distribution (Hennebert and Lees lacustrine ecosystem responses to the abrupt arid 1991), (2) the periodic change in the marine water events. current regime (Vlag et al. 2004), (3) the mineralog- ical composition of the hypersaline lacustrine Keywords Chilean Altiplano Á Lago Chungara´ Á sediments (Rodo´ et al. 2002; Giralt and Julia` 2003) Holocene Á Statistical analyses Á Principal and (4) the variations in the composition of phyto- Component Analysis Á Redundancy Analysis Á plankton groups (Vink et al. 2003). Ordination Stratigraphically unconstrained cluster analysis Á analyses allow us to arrange samples and variables Water availability reconstruction along axes which are a linear combination of the original variables and represent independent theoret- ical environmental gradients. Consecutive axes explain decreasing percentages of the total variance. Introduction Thus, the first axis (x-axis) would cause the largest variation, the second axis (y-axis) would represent the Lakes are one of the best and most precise continen- second largest source of variation, and so forth. The tal sensors of environmental change (Fritz 1996; ordination analyses are diagrams where the samples Battarbee 2000). These changes provoke variations in are arranged in a space where those that are close the physico-chemical conditions of the lake water together represent similar underlying environmental body, which in turn, induce changes in their biota conditions whereas those spaced apart indicate dis- assemblages, the precipitation of mineralogical similar conditions (ter Braak 1987). phases, and/or variations in the stable isotope values, An understanding of tropical climate circulation among others. Such changes are usually well- and its temporal evolution since the Last Deglaciation preserved in the sediments. Hence, lakes have been is essential in order to characterize the moisture widely used to reconstruct past environmental transport mechanisms towards the extra-tropical changes. This is especially true when reconstructing areas. One way to achieve this is by studying high- past climatic and land use changes in a given region resolution multiproxy lacustrine sequences located in (Pla and Catalan 2005). tropical South America. At present, however, our One of the most important limitations in the use of understanding of the tropical climate variability lacustrine records as past environmental archives is during the Holocene is limited to a small number of the difficulty of differentiating the main forcings high-resolution sedimentary sequences (Marchant (climate, tectonism, volcanism, anthropogenic influ- and Hooghiemstra 2004). ence) that trigger these environmental lacustrine Here, we present a methodology to qualitatively changes. Many lakes are located in highly tectonic characterize and isolate the different forcings that and/or volcanically active areas (e.g. lakes of the have controlled the sedimentary infill of Lago African rift and of the Andean Cordillera), or close to Chungara´ (Chilean Altiplano) for the last 12,300 cal 123 J Paleolimnol (2008) 40:195–215 197 years BP. To this end, it is hoped that a deeper insight input is the Chungara´ River (300–460 l s–1) although into the tropical paleoclimatic variability in the late secondary rivers and streams are present around the Quaternary will be obtained. lake. The main water outputs are evaporation (1,230 mm year–1) and groundwater outflow (esti- mated at about 6.106 m3 year–1) (see Herrera et al. Geographical and geological setting (2006) for further details). The lake has been described as cold-polymictic and from oligomeso- Lago Chungara´ is located at the northeastern edge of trophic to meso-eutrophic, with oxic conditions at the the Basin, in the Chilean Altiplano, at 4,520 m bottom surface (7.6 ppm of dissolved oxygen) above sea level (Fig. 1A). This lake lies in a highly (Mu¨hlhauser et al. 1995). active tectonic and volcanic context (Wo¨rner et al. The sedimentary infill of Lago Chungara´ was 1988; Clavero et al. 2002; Hora et al. 2007). The lake characterized by the lithological description of cores resulted from a debris avalanche during the partial obtained in 2002 (Sa´ez et al. 2007) and by seismic collapse of the volcano, which dammed imagery (Valero-Garce´s et al. 2000). It has a mini- the . The age of this collapse is not well- mum thickness of 10 m, is largely made up of constrained, and it ranges from 18,000 years (Wo¨rner offshore diatomaceous deposits intercalated with et al. 1988; Hora et al. 2007) to 8,000 years BP numerous thin tephra layers (lapilli and ashes from (Clavero et al. 2002). the the eruptions of the Parinacota volcano) and some The lake has an irregular shape with a maximum thin carbonate layers and laminae (Sa´ez et al. 2007). water depth of about 40 m, a water surface area of From the bottom to the top of the offshore core, two about 21 km2 (Fig. 1B), and an approximate water sedimentary units (Units 1 and 2) were identified and volume of 385 Hm3 (for further details see Valero- correlated in the basin mainly using tephra keybeds. Garce´s et al. (2003)). At present, the main water Every lithological unit was, in turn, subdivided in two

Fig. 1 (A) Location of Lago Chungara´ in South America. (B) Bathymetric map of Lago Chungara´ and location of the cores 123 198 J Paleolimnol (2008) 40:195–215 subunits (Subunits 1a, 1b, 2a and 2b). Basal Unit 1a, the following conditions: 60 s, 10 kV and 1 mA to with a thickness ranging between 2.56 m and 0.58 m, obtain statistically significant data (see Moreno et al. is made up of finely laminated green and whitish (2007) for further details). Samples for X-ray diatomite. Unit 1b (up to 1.87 m in thickness) is diffraction were retrieved every 5 cm, dried at 60C composed of laminated and massive brown diatomite for 24 h and manually ground using an agate mill. with carbonate-rich intervals. Unit 2a (up to 3.44 m X-ray diffractions were performed using an automatic in thickness) is made up of brown massive diatomite Siemens D-500 X-ray diffractometer in the following with carbonate-rich intervals and volcaniclastics. The conditions: Cu ka, 40 kV, 30 mA, and graphite sediments of the uppermost Unit 2b, with a thickness monochromator. The X-ray diffraction patterns ranging between 3 m and 0.86 m, are dark grey to revealed that the samples were composed of two black diatomites with abundant volcaniclastic layers fractions: a crystalline fraction (highlighted by (Sa´ez et al. 2007). different peaks) and an amorphous one (characterised by the presence of a broad peak centered between 20 and 25 2h). The identification and quantification of Materials and methods the different mineralogical species present in the crystalline fraction were carried out following a Fifteen Kullemberg cores (up to 8 m long) were standard procedure (Chung 1974). The area of the recovered from Lago Chungara´ using a raft. Just after amorphous fraction was calculated as total counts the coring all the cores were cut in 1.5 m sections. In using the software attached to the X-ray diffractom- the lab, and prior to the splitting of the sections, eter. A logarithmic function allowed us to convert the physical properties (GRAPE-density, p-wave velocity total counts to area of the amorphous fraction as the and magnetic susceptibility) were measured using a percentage of the total weight of the sample (Moreno GEOTEKTM Multi-Sensor Core Logger (MSCL) et al. 2007). every centimeter. The sections were then split Samples for Total Organic (TOC), Total Inorganic longitudinally and accurate lithological profiles were (TIC) and Total Carbon (TC) determinations were made. retrieved every 5 cm. TC and TIC were determined

According to the results of the stratigraphic cross using a UIC model 5011 CO2 Coulometer, and TOC sections of an earlier study (Sa´ez et al. 2007), cores content was measured by substraction. Samples for 10 and 11, located in the offshore zone, were selected Total Biogenic Silica (TBS) determinations were also for the paleoenvironmental reconstruction. A com- retrieved every 5 cm. TBS was extracted using the posite core was constructed using both cores in order alkaline leaching technique by Mortlock and Froelich to record the whole sedimentary infill in the offshore (1989) and measured by a molybdate blue spectro- zone of the lake. This composite core corresponds to photometric technique (Hansen and Grashoff 1983) the lithological units 1 and 2 defined previously (Sa´ez using an AutoAnalyser Technicon AAII. et al. 2007). This composite core will henceforth be The chronological framework of the sedimentary referred to as core in the text. sequence of Lago Chungara´ was constructed using 17 The best preserved half core of every section was radiocarbonic AMS (Table 1) and one 238U/230Th digitised using a CCD camera. The digital photo- date (Table 2). The radiocarbon dates were per- graphs obtained were intercalibrated and the gray formed in the Arizona Radiocarbon Laboratory curve was calculated using the ImageJ software (USA) and in the Poznan Radiocarbon Laboratory package (Rasband 1997–2004). The gray values were (Poland) whereas the 238U/230Th dates were carried obtained every 0.1 mm. These sections were also out using a ICP-IRMS multicollector at the Univer- employed for X-Ray Fluorescense (XRF) analyses sity of Minnesota (Edwards et al. 1986). The using the new generation XRF core scanner from the Dissolved Inorganic Carbon (DIC) from the surface University of Bremen. Eight light (Al, Si, S, K, Ca, water of the lake was dated at the Beta Analytics Inc. Ti, Mn and Fe) and six heavy (Rb, Sr, Zr, Sn, Te and Laboratory in order to determine the present day Ba) elements were measured. The light elements reservoir-effect, and an apparent age of 2,320 ± were determined every 2 mm and the heavy ones, 20 years BP was obtained. This apparent reservoir- every 10 mm. Each measurement was counted using effect age was corrected to remove the effects of the 123 aelmo 20)40:195–215 (2008) Paleolimnol J

Table 1 AMS 14C dates carried out in order to construct the chronological framework of the Lago Chungara´ sedimentary sequence Lithological Composite Lab ID Sample Sample material Uncalibrated Calibrated agea Calibrated ageb d13C %PDB units depth (mm) reference 14C (years BP) (calendar years BP) (calendar years BP)

Unit 2 – Beta-188745 – Lake water 2,320 ± 40 – – – 375 Poz-8726 14 A-1, 5 Bulk organic remains 4,620 ± 40 1,850 ± 454 2,050 ± 500 –13.6 ± 0.2 420 Poz-8720 11 A-2, 39 Bulk organic remains 4,850 ± 40 1,964 ± 460 2,185 ± 520 –12.9 ± 0.4 670 AA56904 15 A-2, 48 Aquatic plants 6,635 ± 39 2,590 ± 630 2,900 ± 710 –25.46 951 Poz-8721 11 A-2, 84 Bulk organic remains 7,290 ± 50 3,290 ± 860 3,645 ± 910 –14.8 ± 0.2 2,573 Poo-8723 11 A-3, 2 Bulk organic remains 8,920 ± 50 6,227 ± 1,200 6,555 ± 1,010 –16.1 ± 0.1 3,440 AA56903 15 A-4, 27 Aquatic plants 9,999 ± 50 7,070 ± 1,200 7,505 ± 890 – 4,361 Poz-8724 11 A-3, 86 Bulk organic remains 10,860 ± 60 7,530 ± 1,250 8,775 ± 940 –16.9 ± 0.1 Unit 1 4,909.1 Poz-7170 11 A-3, 123 Bulk organic remains 8,570 ± 50 7,705 ± 1,230 9,900 ± 950 –16.8 ± 0.1 5,504.8 Poz-8647 11 A-4, 10 Bulk organic remains 9,860 ± 60 7,940 ± 1,260 11,290 ± 1,080 –14.1 ± 0.3 6,152 Poz-7171 11 A-4, 63 Bulk organic remains 11,070 ± 70 8,270 ± 1,400 12,490 ± 910 –13.6 ± 0.2 6,650 AA56905 15 A-5, 77 Aquatic plants 4,385 ± 101 ––– 6,750 Poz-8725 13 A-4, 66 Bulk organic remains 8,810 ± 50 –––22.9 ± 0.1 6,962 Poo-11891 11 A-4, 145.5 Bulk organic remains 11,460 ± 60 8,765 ± 1,420 13,120 ± 930 –16.2 ± 0.4 7,422 Poz-13032 11 A-5, 41 Bulk organic remains 10,950 ± 80 9,080 ± 1,540 13,290 ± 910 –22.7 ± 2.3 7,852 Paz-11982 11 A-5, 84 Bulk organic remains 11,180 ± 70 9,400 ± 1,740 13,605 ± 880 –28.7 ± 3.7 8,272 Poz-13033 11 A-6, 41 Bulk organic remains 12,120 ± 80 9,730 ± 2,090 14,155 ± 1,390 –19.6 ± 1.7 8,652 Poz-7169 11 A-6, 79 Bulk organic remains 13,100 ± 80 10,040 ± 2,640 14,795 ± 1,760 –23.1 ± 0.2 Bolded radiocarbon dates were not taken into account in the construction of the chronological framework. This chronological model differs from those previously published elsewhere (Moreno et al. 2007;Sa´cz et al. 2007) since this new one incorporates the post-bomb effects correction. See the text and Moreno et al. (2007) for a detailed discussion about the validity of the radiocarbon dates a Constant reservoir effect of 3,260 years through out all the sequence b No reservoir effect correction for the radiocarbon dates of lithological Unit 1 123 199 200 J Paleolimnol (2008) 40:195–215

Table 2 238U/230Th date carried out in order to construct the chronological framework of Lago Chungara´ sequence Depth Sample Carbonate 238U (ppb) 232Th (ppm) d234U measured 230Th/238U 230Th/232Th Calendar age (mm) reference type (years BP)

3,440 13 A-2, 105 Crystalline 467.4 35.2 413.5 0.1036 23 6,728 ± 974 thermonuclear bomb tests carried out during the 60 wt%, is composed of Ca-, carbonate 1950s and 1960s (see below for a detailed discus- (calcite and dolomite), muscovite, pyrite, and sion). The calibration of the radiocarbon dates was an amphibole (probably riebeckite). Three zones performed using the CALIB 5.02 software and the were defined on the basis of compositional criteria INTCAL98 curve (Stuiver et al. 1998; Reimer et al. and labelled from the bottom to the top of the 2004a). The software described in Heegaard et al. sequence (Fig. 2) as follows: (2005) was employed to construct a reliable age- • Zone 3: Ranges from the bottom of the core to depth model, furnishing a final corrected age for each 3,500 mm of core depth. It is 5,000 mm thick, calibrated date. The uppermost sediments of the Lago and is dominated by amorphous material (more Chungara´ were not retrieved due to the Kullemberg than 95% of the total weight of the sample). coring system. This have provoked that the last Ca-plagioclase, quartz and pyrite are the main approximately 1,000 years BP were not studied in the mineral species present in the crystalline fraction. present work (see Sa´ez et al. (2007) for further • Zone 2: Ranges from 3,500 mm to 1,000 mm of details). core depth, and is characterized by the highest The statistical treatment of the datasets was percentages of calcite. From 2,000 mm of core performed using the R software package (R Devel- depth towards the top, the percentages of the opment Core Team 2007) together with the packages plagioclase increase whereas those of the amor- ‘‘gclus’’ (Hurley 2004) for clustering purposes and phous fraction decrease. ‘‘vegan’’ (Oksanen et al. 2005) for Redundancy • Zone 1: Ranges from 1,000 mm of core depth to analyses. At each stage, distances between clusters the top. This zone is characterised by the highest were recomputed by the Lance-Williams dissimilarity percentages of Ca-plagioclase. update formula according to the particular clustering method used (Kaufman and Rousseeuw 1990). The clustering method employed was the complete link- The geology of the Lago Chungara´ catchment (see age method which uses the largest dissimilarity Sa´ez et al. (2007) for further details) indicates that between a point in the first cluster and a point in , quartz, amphibole and muscovite have the second cluster (furthest neighbor method) to build two main provenances: (a) the erosion of former the dissimilarity tree. volcanic rocks and of previous deposited fallout material from the catchment, and (b) the ash fallout from the Parinacota volcano. The macroscopic char- Results acterization of the tephra layers in the sediments of Lago Chungara´ revealed that they consist on ash and Mineralogical (XRD) and geochemical (XRF) lapilli. Hence, the input of volcaniclastic material composition of the sediments eroded from former volcanic rocks could constitute a minor percentage with respect to the direct fall of The offshore sediments of Lago Chungara´ are volcaniclastic material from the Parinacota volcanic composed of an amorphous and a crystalline fraction. eruptions. On the other hand, the origin of the The amorphous fraction, made up of organic matter carbonates is not so obvious. The presence of large and diatoms, range from 40 wt% (towards the top of amounts of Ca of volcanic origin dissolved in the composite core) to almost 100 wt% (from the the water of the lake together with an important lake lower two thirds of the composite core towards its level decrease, evidenced by a sudden increase of bottom) of the total weight of the sample. The benthic diatoms (Sa´ez et al. 2007), could promote the crystalline fraction, ranging between \1 wt% and precipitation of these carbonates. 123 aelmo 20)40:195–215 (2008) Paleolimnol J

Fig. 2 Mineralogy (expressed as percentages on total dry weight), magnetic susceptibility (expressed as standard units), Total Biogenic Silica (TBS), Total Carbon (TC), Total Organic Carbon (TOC) (expressed as percentages) and gray-colour curves of the Lago Chungara´ sediments 123 201 202 J Paleolimnol (2008) 40:195–215

The downcore profiles of heavy and light ele- in order to retain the maximum information from the ments analyzed by XRF scanner mark three different XRF, and were normalised in order to avoid problems zones in terms of their geochemical composition arising from their different orders of magnitude. An (Fig. 3): initial dataset of 28 variables per 3,909 samples was constructed. To test the robustness of the interpola- • a lower zone (from 4,500 mm of core depth to the tion, correlation coefficients among the different bottom of the core) characterised by the highest Si variable as well as Principal Component Analysis content (see Moreno et al. (2007) for further (PCA) were performed on a subdataset of values details). taken at the coarsest sampling resolution (i.e. • an intermediate zone (from 1,000 mm to 50 mm). The results were compared with those 4,500 mm of core depth) with the highest Ca obtained from the 2 mm interpolated dataset. The values, and strong coincidence between both results (not shown) • an upper volcanic-rich zone (which ranges from indicate that the statistical analyses provided in this the top of the core up to 1,000 mm of core depth) paper are robust. with maximum values in almost all elements.

Redundancy analyses Zone 1 defined in the mineralogical profile and the upper zone defined by XRF analyses (Figs. 2, 3 Redundancy analyses (RDA) were carried out given respectively) corresponds to the same core interval that many variables of the dataset represent similar (the uppermost meter of the core). On the contrary, aspects of the geochemical composition of the the limit of the mineralogical zones 2 and 3 (located sediments (Fig. 4). The mineralogical composition at 3,500 mm of core depth in the Fig. 2) and the limit of the sediments was used as a constraining matrix between the intermediate and lower XRF zones given that each mineralogical species represents a (situated at 4,500 mm of core depth in the Fig. 3) ‘‘compendium’’ of geochemical elements. Thus, do not correspond. This absence of correspondence RDA was used to identify the possible provenances could be attributed to the different methodologies of the light and heavy elements obtained by XRF, and used to characterize the presence of diatoms and the other geochemical parameters (TC, TOC, TBS, other amorphous material (amorphous material in the magnetic susceptibility and gray-colour curves). X-ray diffraction diagram and Si in the XRF RDA analysis allowed us to define five main diagram). The detailed characterization of the XRF ‘‘families’’ of variables in accordance with their zones and their geochemical implications are dis- possible origin (Fig. 4): cussed in detail in Moreno et al. (2007). • Family A: Most of the light and heavy elements (Fe, Ti, Rb, K, Ba, Zr, Al and Sr) had a volcanic Statistical analyses origin given that they were grouped with the plagioclase and the amphibole. The magnetic Construction of the dataset susceptibility data were also associated with the volcanic material. The identification and characterization of the forcings • Family B: Located opposite to Family A is that trigger the sedimentary infill of Lago Chungara´ Family B constituted by TBS and amorphous were carried out by applying statistical analyses to material. Variations in the diatom content could magnetic susceptibility, XRF, XRD, TC, TOC, TBS be the main factor responsible for this second and gray-colour curve data. Every proxy had a family. different sampling interval: the light XRF elements • Family C: The ‘‘carbonate family’’ is made up of were determined every 2 mm, the heavy XRF Ca and TC. According to the Fig. 2 almost all TC elements every 10 mm, the mineralogical composi- corresponds to TOC. This suggests that the tion, TC, TOC and TBS every 50 mm and the gray- organic matter (as a indicator of biological colour every 0.1 mm. Hence, all the variables were activity) plays an important role in the precipita- linearly interpolated with a regular spacing of 2 mm, tion of carbonates. The presence of quartz in this 123 aelmo 20)40:195–215 (2008) Paleolimnol J

Fig. 3 Analysed light and heavy elements (expressed as counts per second) in the sediments of Lago Chungara´. The zones correspond to those established in Moreno et al. (2007) 123 203 204 J Paleolimnol (2008) 40:195–215

Fig. 4 Redundacy Analysis (RDA) carried out on the dataset. The mineralogical species dataset was used as constraining matrix (arrows) whereas the rest of the dataset were considered as data matrix (crosses). The five red circles mark the five main ‘families’, suggesting their possible origin (for further details see text)

family indicates that a percentage of these Stratigraphically-unconstrained cluster analysis carbonates could have precipitated in the shallow littoral areas of the lake and, subsequently, This analysis groups the samples in accordance with remobilized and transported to deep offshore their affinity. The samples that have similar geo- areas. TOC seems to play an intermediate role chemical and mineralogical features are plotted close in families B and C. This seems reasonable given to each other, whereas those samples that present a that the organic matter is one of the main very distinctive geochemical and mineralogical sig- components in both families. nature are plotted at the very ends of the tree. Hence, • Family D: Opposite to Family C there is a group the stratigraphically unconstrained cluster analysis of variables composed of Si, S and Sn, forming allows us to identify the main geochemical and Family D. The two main sources of silicium and mineralogical families. According to this statistical sulphur are related to the volcanic material and analysis, three main geochemical and mineralogical diatoms. Hence, this family occupies intermediate groups of samples were identified (Fig. 5) (1) the positions between Families B and A. The origin ‘‘volcanic’’ group (samples dominated by volcanic of Sn is unclear. material), (2) the ‘‘non-volcanic’’ group (samples • Family E: Family E is composed of pyrite and mainly composed of organic matter and diatoms) and Mn. SEM observations highlighted the presence (3) the ‘‘outsiders’’. Furthermore, two kinds of of framboids of pyrite between the frustules of the ‘‘outsider’’ samples were identified: diatoms and their relation to the Mn content as • Isolated samples that present an ‘‘anomalous’’ minor element. Thus, this family would represent value in one variable (commonly one variable some early diagenetic processes related to Eh/pH from the XRF analyses). These samples are changes within the sediment. distributed throughout the sequence. The visual According to the RDA analysis, the dataset was inspection of the lithological section of the core simplified to 17 variables (Amorphous, TBS, mag- corresponding to these ‘‘anomalous’’ samples did netic susceptibility, gray-colour curve, S, Si, Al, Ba, not reveal any remarkable features. Hence, K, Zr, Rb, Ti, Fe, Sr, Ca, TC and TOC). these anomalies were interpreted as anomalous 123 J Paleolimnol (2008) 40:195–215 205

Fig. 5 Stratigraphically unconstrained cluster dendrogram performed in the database, and identification of the three families of samples (see text for further details)

acquisitions of the XRF scanner and they were flows of the Ajata cones is different (greater eliminated from the dataset. abundance of mafic minerals and absence of • Groups of contiguous samples showing abrupt plagioclases) from that of the Parinacota volcano values changes in almost all variables compared (Wo¨rner et al. 2000). The groups of samples to those located immediately upwards and/or isolated by the stratigraphically unconstrained downwards. These samples are located in the cluster analyses could represent the tephra depos- uppermost 3,500 mm of the core, except two its proceeding from the eruptive activity of the samples located at 6,806 and 6,808 mm of core Ajata satellite cones. Such groups of samples depth. The number of samples that constitutes were also removed from the dataset. these groups range between 2 (e.g. 2,047– 2,049 mm) and 41 (2,305–2,461 mm). In all cases, these groups correspond to the uppermost Principal component analyses (PCA) part of the volcanic tephra layers. This geochem- ical and mineralogical signature was interpreted Finally, a dataset of 17 variables and 3,770 samples as tephra deposits that differed from the volcanic was created. PCA was carried out using this final material present throughout the sedimentary infill. dataset. This analysis was used to highlight the main According to Wo¨rner et al. (1988), the main underlying processes that trigger the input, distribu- source of volcanic material present in Lago tion and sedimentation of particles in the Lago Chungara´ was related to the emplacement of Chungara´ offshore deposits. Parinacota volcano. At ca. 6,000 cal years BP, The first two eigenvectors accounted for 65.81% two Parinacota satellite cones started their erup- of the total variance. The first eigenvector represented tive activity. The geochemical composition of the 50.22% of the total variance, and this eigenvector

123 206 J Paleolimnol (2008) 40:195–215 was controlled mainly by the light and heavy elements organic rests or pure calcite crystals. Seventeen at the positive end, and partially by amorphous AMS 14C dates were obtained from (1) bulk organic material and TBS at the negative end (Fig. 6). On the matter from the central plain cores and (2) aquatic other hand, the second eigenvector accounted for organic macrorests picked from littoral cores 15.59% of the total variance, and was controlled (Table 1). Other 17 samples of calcite were employed by the presence of TOC, TC, gray-colour curve for 238U/230Th dating. and Ca at the positive end and by Si and S and, in Most of the 238U/230Th dates were discarded due to minor proportion, by TBS and amorphous material their high 232Th content, indicative of high terrige- at its negative end (Fig. 6). In fact, family B nous particles (Table not shown). Only one occupies an intermediate position contributing to 238U/230Th date located at 3,440 mm of core depth both eigenvectors. (Table 2) was coherent with the reservoir-corrected The other eigenvectors defined by the PCA 14C model (Moreno et al. 2007;Sa´ez et al. 2007). On analysis were not taken into account given that they the other hand, two radiocarbon dates (AA56905 and explain progressively lower percentages of the total Poz-8725) were also discarded because they were variance (the other fifteen eigenvectors only clearly reversed, probably due to sedimentary accounted for 12% of the total variance). reworking processes. Both dates are located in the upper part of the Unit 1a. The three other samples (Poz-8723, AA56903 and Poz-8724) from the volca- Construction of the chronological framework nic-rich Unit 2a are consistent with one and other. for the Lago Chungara´ sequence They are older than the radiocarbon dates of the Units 2b and 1b, show no evidence of depositional Obtaining radiometric dates to construct a robust and reworking, and are older than the only reliable reliable chronological framework for the sediments of 238U/230Th radiometric date. It is worth noting that Lago Chungara´ was very complicated owing to the the activity of the Parinacota volcano started during scarcity of suitable material, whether terrestrial the deposition of this sedimentary unit. Hence, one

Fig. 6 Plot of the samples (numbers) and the variables (arrows) in the plane defined by the first two eigenvectors of the Principal Component Analysis

123 J Paleolimnol (2008) 40:195–215 207 possibility is that the volcanic activity could have been described in Laguna Lejı´a (Geyh et al. 1998), in altered the CO2 balance among the different sources, Laguna Miscanti (Geyh et al. 1999; Grosjean et al. modifying the 14C value of these three samples 2001), and specifically for Lago Chungara´ (Geyh and (Moreno et al. 2007). A similar effect has been Grosjean 2000). documented in other lakes from the Altiplano Therefore, the approach followed in Moreno et al. (Valero-Garce´s et al. 1999) and it has been consid- (2007) and in this study to correct the dates for the ered as a source of anomalous 14C dates in some variable reservoir effect was based on two assump- volcanic lakes from the Azores Islands (Bjo¨rck et al. tions: (1) the Lago Chungara´ ecosystem had an 2006). These three radiocarbon dates were initially environmental status during deposition of Unit 2b retained in order to construct the final chronological similar to that currently found and (2) the present-day framework for the Unit 2a (Table 1). lake level is at its highest. Accordingly, a different Other problems that hamper the construction of a correction of the reservoir effect was applied to Units reliable chronological framework are (1) the assess- 1 and 2. ment of the large and variable radiocarbon reservoir Unit 2 is made up of dark gray massive diatoma- effect that influences most lake deposits in the ceous oozes with abundant volcanic layers, whereas Andean Altiplano (Geyh et al. 1999; Geyh and Unit 1 is mainly characterized by laminated sedi- Grosjean 2000; Grosjean et al. 2001), and (2) the ments without volcanic input. This different pattern temporal evolution of this radiocarbon reservoir allowed us to consider a constant reservoir effect for effect. The radiocarbon dating of the modern DIC Unit 2 since the average lake characteristics most resulted in 2,320 ± 40 14C years BP (Table 1), but likely did not vary much over time (Moreno et al. this value must be corrected due to the atmospheric 2007). A constant reservoir effect of 3,260 years was thermonuclear bomb tests carried out during the late subtracted from the radiocarbon dates present in this 1950s–early 1960s. These nuclear tests doubled the unit. It is only possible to hypothesize about the amount of 14C in the atmosphere (Reimer et al. variations over time of the reservoir effect in Unit 1. 2004b). The modern reservoir effect in lakes must In accordance with the hypothesis advanced by Geyh be calculated as the difference between the 14C age of et al. (1998), the reservoir effect was probably lower 14 the water and of the atmospheric CO2 measured in Unit 1 than in Unit 2 since the lake was, on at the same time (Goslar comm. pers.). The effects of average, shallower than during the deposition of the these thermonuclear bomb tests in the modern carbon upper unit (Sa´ez et al. 2007). Since it was not cycle of a lake depends on (1) the year that the possible to estimate the reservoir effect by applying sample was taken and (2) the residence time of the the methodology established by Geyh et al. (1998), lake. The Lago Chungara´ water residence time is the ages of the two extreme reservoir values (a about 15 years (Herrera et al. 2006) and the DIC minimum of 0 and a maximum of 3,260 years) were sample was obtained and dated in 2004, therefore, the calculated (Table 1). All the corrected radiocarbon real present-day reservoir effect of this lake is dates were calibrated using the CALIB 5.02 software 3,260 years BP: 2,320 years BP (radiocarbon date package (Reimer et al. 2004a) selecting the mid-point of the DIC) minus –920 years (apparent radiocarbon of 95.4% of the distribution. 14 age of the atmospheric CO2 for the period 1988– The upper and lower limits of the chronological 2002) (Hua and Barbetti 2004; Goslar et al. 2005). model were calculated employing the Cagedepth The reservoir effect of 3,260 years is a value that software (Heegaard et al. 2005). Therefore, the obtained by Geyh et al. (1999). The reservoir effect column ‘‘Calibrated agea (calendar years BP)’’ of in the Altiplano lakes proved to be highly variable Table 1 refers to the corrected and calibrated radio- over time. One of the factors that could influence carbon dates considering a constant reservoir effect of the reservoir effect is the change in the volume/ 3,260 years (dotted line in Fig. 7) whereas the surface ratio of the lake, which is a function of the column ‘‘Calibrated ageb (calendar years BP)’’ shows water depth (Geyh et al. 1998; Grosjean et al. 2001). the non-corrected reservoir effect and calibrated According to these authors the reservoir effect radiocarbon dates (Table 1 and dashed line in decreases when the lake level diminishes, and vice Fig. 7). A theoretical mid point between the two versa. This lake level-reservoir effect correlation has extreme reservoir effect points were calculated for 123 208 J Paleolimnol (2008) 40:195–215

Fig. 7 Chronological framework of Lago Chungara´ con- effect of 3,260 years for the upper sedimentary unit and of structed using 15 14C AMS dates and after applying the 0 years for the lower one. The intermediate continuous line Heegaard’s method (Heegaard et al. 2005) in calendar years represents the average model of the two earlier ones and is BP (modified from Moreno et al. (2007)). The dotted line applied to Lago Chungara´. Horizontal bars are the confidence represents the chronological model constructed considering a error of the radiometric dates after applying the method of constant reservoir-effect of 3,260 years. The dashed line Heegaard. See text for further details symbolizes the chronological model considering a reservoir-

Unit 1 and this theoretical value was employed to changes in the volcanic content in the sediments. construct the final chronological model. The final High values of the first eigenvector will point to the age-depth model can be seen in Fig. 7 (intermediate presence of tephra layers whereas low values will continuous line). The corrected 14C dates of the indicate the presence of diatomite-rich sediments. Subunit 2a using the proposed method by Heegaard The interpretation of the second eigenvector is less et al. (2005) are consistent with the only 238U/230Th clear. Total silica has been widely used as proxy for retained date, validating this correction. siliceous microfossils such as diatoms, chrysophy- cean cysts, sponges and phytoliths (Conley and Schelske 2001) as well as for inorganic components Discussion such as siliceous minerals. The total silica content in the Lago Chungara´ sediments was determined by a Sedimentological significance of the eigenvectors wet chemical digestion (TBS), X-ray diffraction (amorphous material) and an XRF scanner (Si). The X-ray diffractions showed that the main minerals TBS has been widely employed as a good proxy for are related to the volcaniclastic deposits. Furthermore, siliceous microfossil content (Conley and Schelske the RDA analysis also revealed that (1) most of the 2001) whereas Si is an indicator of the presence of analysed geochemical elements have the same origin, silica particles of both organic and inorganic origin. and (2) are related to the Ca-plagioclase. Hence, the In fact, the presence of two subgroups of variables (Si first eigenvector of the PCA analysis indicated and S on the one hand, and TBS and amorphous 123 J Paleolimnol (2008) 40:195–215 209 material in a minor proportion on the other hand) at of the x- and y-coordinates of every sample with the negative end of the second eigenvector could be respect to their core depth will allow us to qualita- interpreted as indicative of the presence of particles tively reconstruct the evolution of the input of the from two different sources, organic (diatoms) and volcanic material (Fig. 8) and that of water avail- inorganic (mainly clay and pyrite). TC and TOC are ability (Fig. 9). indicative of the presence of organic matter remains, According to the chronological framework, it has and Ca could be associated with the existence of been possible to reconstruct the input of the volcanic calcium carbonates (as suggested by the RDA material and the water availability variations for the analysis). Hence, the second eigenvector would last 12,300 cal years BP. reflect changes in the organic matter, carbonates, and diatoms. This alternance is clearly visible in the lower laminated Unit 1 (Sa´ez et al. 2007). Eruptions history of the Parinacota volcano Variations along this second eigenvector could be in the Lago Chungara´ sedimentary sequence interpreted as changes in water availability. These changes in water availability would be a direct The plot of the x-coordinates of the samples with consecuence of effective regional precipitation– respect to their age allowed us to reconstruct the evaporation variations. More rainfall in the lake volcaniclastic input in Lago Chungara´ (Fig. 8). All catchment would imply more runoff (more erosion of this volcaniclastic material originated from the the soils), and then, a higher nutrient input into the different eruptions of the Parinacota volcano and its lake. This higher nutrient input would allow satellite cones on the northern shore of the lake. enhanced diatom blooms. By contrast, low rainfall Parinacota and its satellite cones have been the only values would imply low runoff coefficients in the lake active volcanoes in the area in the last 14,500 years catchment, and hence, diminished diatom blooms. (DeSilva and Francis 1991). Furthermore, low precipitation values would also Two main periods can be established in accor- lead to falls in the water level, resulting in an increase dance with the evolution of the volcaniclastic input to in water salinity and in carbonate precipitation. the lake: In summary, the PCA analysis showed that the 1. From ca. 12,300 cal years BP to ca. 7,800 cal main processes that control the input and infill of years BP: During this period the volcaniclastic Lago Chungara´ are closely related to the volcanic input to the lake was low, with a slight tendency to activity of the surrounding volcanoes (first eigenvec- decrease towards the latest stages of the Early tor). On the other hand, the processes related to Holocene. Only a thin tephra layer, dated at ca. changes in water availability are highlighted in the 11,000 cal years BP and identified as M1 in Sa´ez second eigenvector. et al. (2007), was deposited within the lake. SEM observations carried out in the lower laminated Temporal reconstruction of the volcanic input sediments showed that volcaniclastic particles and of water availability in Lago Chungara´ were distributed fairly homogeneously within the sediments, which suggests that this volcanic The location of every sample with respect to the new material was constantly incorporated into the vectorial spaces defined by PCA (in other words, lake. This continuous input to the lake can be their xy coordinates) indicates a specific position with interpreted (1) as erosion of the earlier deposited respect to the volcanic input and the water availabil- volcanic material in the catchment area, and (2) as ity processes, respectively. All samples located at the a direct input of minor Parinacota volcano erup- positive end of the first eigenvector (high values of tions which did not form tephra layers. The the x-coordinate and low values of the y-coordinate) Chungara´ river as well as the other minor effluents are composed of volcanic material, and vice versa. could be the responsible for this. In any case, this The same is true for those samples located at the riverine supply should not be interpreted as large positive or negative end of the second eigenvector, fluvial deposits but as plume suspended material. but with respect to water availability. Hence, the plot The presence of shards and of terrigenous

123 210 J Paleolimnol (2008) 40:195–215

Fig. 8 Reconstruction of the evolution of the volcanic input in Lago Chungara´ for the last 12,300 cal years BP (right) and comparison of this reconstruction with the dust particle ([63 lm) content of the Nevado Sajama record (left) (modified from Thompson et al. (1998)). Asterisks denote the location of the main ash layers in the Lago Chungara´ sediments (Sa´ez et al. 2007). See text for further details

volcanic particles in the sediments support both comparison of the reconstruction of the volcaniclastic these interpretations. input in Lago Chungara´ with the record of dust particles 2. From ca. 7,800 cal years BP to ca. 1,000 cal exceeding 63 lm from the Nevado Sajama ice core years BP: This period was characterised by a (Thompson et al. 1998) shows a similar evolution marked increase in the input of volcanic material (Fig. 8). The dust particle record from the Nevado to the lake when several large eruptions of the Sajama shows a marked increase at ca. 7,500 cal years Parinacota volcano deposited, at least, 10 ash- BP, coinciding with the onset of the large explosive layers (labelled from M2 to M11 according to period of the Parinacota volcano recorded in the Lago Sa´ez et al. (2007)) in about 6,800 years. Chungara´. This suggests that these particles do not 39Ar/40Ar dates performed in the of the correspond to dust particles eroded during the dry Parinacota volcano indicate that this cone was periods, as reported previously (Thompson et al. 1998), built within the last 5,000 years (Hora et al. but to volcaniclastic particles deposited in the Nevado 2007). The volcanic input reconstruction sug- Sajama during the major explosive eruptions of the gests that the construction of the present day Parinacota volcano. Furthermore, it should be pointed cone started earlier than 5,000 cal years BP. The out that the volcaniclastic record of the Lago Chungara´ maximum activity of the Parinacota volcano indicates a much more complex volcanic history in this ended at ca. 2,100 cal years BP with the depo- area than has been suggested (Wo¨rner et al. 1988;Hora sition of the last identified tephra layer (M11 et al. 2007). according to Sa´ez et al. (2007)). After this maximum, the volcanic activity decreased Regional integration of the reconstruction of Lago slightly, and from ca. 2,100 cal years BP to ca. Chungara´ water availability 1,000 cal years BP it remained constant. The eruptions of the Paricanota volcano affected It has not been possible to date to precisely constrain both Lago Chungara´ and its surroundings. A the chronology of the lower half of the Lago

123 J Paleolimnol (2008) 40:195–215 211

Fig. 9 Latitudinal comparison of the reconstructed evolution Baker et al. (2001b)) and the natural gamma radiation of the of the water availability in the Lago Chungara´, with the d18O Salar Uyuni (modified from Thompson et al. (1998)) for the record of Paco Cocha (modified from Abbott et al. (2003)), the last 12,300 cal years BP percentage of benthic diatoms of Lake Titicaca (modified from

Chungara´ record owing to the difficulty of establish- spatial and temporal pattern. This complexity has also ing the precise reservoir-effect correction for the been pointed out by other authors (Abbott et al. 2003; radiocarbon dates of Unit 1. Given these chronolog- Grosjean et al. 2003; Latorre et al. 2003; Placzek ical limitations, the reconstruction of the water et al. 2006). This comparison has been focussed in availability of Lago Chungara´ was compared with three key temporal intervals: Late Glacial–Early some of the closest paleoenvironmental records of Holocene, Mid Holocene and Late Holocene. Paco Cocha (Abbott et al. 2003), Lake Titicaca (Tapia et al. 2003; Baker et al. 2001b), and Salar de Uyuni (Baker et al. 2001a) in order to obtain a Late Glacial–Early Holocene general regional picture of the evolution of the water availability during the Late Glacial and the Holocene The final stage of the Late Glacial period is consti- (Fig. 9). A comparison of these four paleoenviron- tuted by the Coipasa (13,000–11,000 cal years BP) mental records shows that the water availability in the humid phase (Argollo and Mourguiart 2000; Placzek northern Chilean Altiplano followed a heterogeneous et al. 2006). Although the Coipasa phase has been 123 212 J Paleolimnol (2008) 40:195–215 described as ending at 11,000 cal years BP (Placzek occurred between ca. 7,400 cal years BP and et al. 2006), this humid phase ended much earlier (ca. 6,600 cal years BP, lasting for approx. 800 years. 11,600 cal years BP) in Lake Titicaca and Salar Salar Uyuni displayed the driest conditions between Uyuni. On the other hand, the paleohydrological ca. 6,500 cal years BP and 4,200 cal years BP (Baker reconstruction of Lago Chungara´ shows that this et al. 2001a). These variations could be attributed to period was characterized by high lake level stands, the size, lake morphology, catchment size and with some minor dry oscillations, and it lasted up to lacustrine ecosystem responses to these abrupt epi- ca. 10,900 cal years BP, like in the Rı´o Salado basin sodes. The differential response to these abrupt water (9,600 cal years BP) (Latorre et al. 2006), whereas availability changes hampers the identification of the Coipasa phase in Paco Cocha ended much more clear spatial and temporal moisture patterns. earlier (at about 12,300 cal years BP). Therefore, the end of this humid phase followed a heteroge- neous spatio-temporal pattern (Fig. 9). This hetero- Late Holocene geneous pattern has also been highlighted by other authors (Latorre et al. 2003). The last 4,000 cal years BP were characterized by a The end of the Late Glacial and the onset of the general return to moist conditions, probably related to Holocene, characterized by the end of the Coipasa enhanced precipitation, decreased evaporation, a humid phase, triggered a fall in the water availability shorter dry season or a combination of these factors in the region, and hence, the sharp fall in the water (Marchant and Hooghiemstra 2004). This resulted in levels of Lake Titicaca (Baker et al. 2001b). This dry (a) an overall rise in lake water levels due to the period affected the ecosystems differently. The water prevalence of the summer precipitation as a result of level of Lake Titicaca fell sharply at least 85 m below the southwards displacement of the Inter Tropical the modern lake level and this dry spell lasted approx. Convergence Zone (ITCZ) (Argollo and Mourguiart 1,000 years (Tapia et al. 2003) whereas the lake level 2000; Marchant and Hooghiemstra 2004), and (b) in a of Paco Cocha remained low from approx. 12,000 cal reduction in the intensity of dry spells. Lake Titicaca years BP until 8,000 years BP (Abbott et al. 2003). showed a rise in its lake level and a decline in the On the other hand, the water availability in the Lago intensity of arid crises (Rowe et al. 2003; Tapia et al. Chungara´ area followed a progressive decrease, quite 2003) whereas Lago Chungara´ and Paco Cocha similar to that followed by the Salar Uyuni (Baker followed a high oscillating lake level pattern with et al. 2001a) (Fig. 9). recurrent lake level falls. The evolution of Salar de Uyuni showed an overall trend of lake level increase punctuated with minor lake level decreases (Baker Mid Holocene et al. 2001a) (Fig. 9). The correlation of some of the falls in lake level in the four lacustrine ecosystems The four paleoclimate records show that the arid suggests that these falls could be related to the dry period of the Mid Holocene, between the ca. spells that affected this region. 8,000 cal years BP and the 4,000 cal years BP, which has been described in many other works (Valero-Garce´s et al. 1996; Schwalb et al. 1999; Conclusions Grosjean et al. 2001, 2003; Latorre et al. 2006), is in fact a succession of dry and humid abrupt episodes as Statistical analyses allowed us to characterize the many authors have previously stated (Fig. 9). The Parinacota volcano explosive activity and the regio- timing and intensity of such episodes are reflected nal water availability oscillations as the two main differently by each lake. The driest period in Paco forcings that triggered the environmental evolution of Cocha occurred between ca. 7,000 and 4,800 cal Lago Chungara´ in the last 12,300 years. years BP and lasted for almost 2,000 years, whereas The stratigraphically unconstrained cluster analy- in Lake Titicaca it took place earlier, between ses allowed us to identify the ‘‘outsiders’’ of the 6,600 cal years BP and 5,000 cal years BP with a database. These ‘‘outsiders’’ were regarded as (1) duration of ca. 1,600 years, and in Lago Chungara´ punctual ‘‘anomalous’’ values due to erroneous XRF 123 J Paleolimnol (2008) 40:195–215 213 acquisitions and (2) as ash layers with a different and to George von Knorring and an anonymous referee for geochemical composition, and hence a different improving the final version of the paper. origin. The Principal Component Analyses (PCA) enabled us to distinguish the two main forcings References controlling the sedimentary infill of Lago Chungara´. 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