Lake highstands on the Altiplano (Tropical Andes) contemporaneous with Heinrich 1 and the Younger Dryas: new insights from 14C, U–Th dating and δ18O of carbonates
Highlights
► New U–Th (n = 6) and 14C dates (n = 10) of Altiplano lacustrine deposits. ► Data indicate 3 lake highstands episodes: 24–19 ka, 18–14.5 ka and 12.5–11.9 ka. ► Two highest highstands synchronous with Heinrich 1 and Younger Dryas. ► δ18O of lake paleowater indicates limited local recycling of precipitation (<50%).
Introduction
Many recent observations have led to a reconsideration of the role of the tropics in paleoclimate. The tropics are now recognized to be the driver of high frequency climatic variability, including El Niño-Southern Oscillation (ENSO) (Chiang, 2009). It has also been proposed that the tropics may be an amplifier of the abrupt millennial changes recorded in the Greenland ice and in the sediments of the Northern Atlantic (e.g. Leduc et al., 2007). In particular, several paleoprecipitation records (Cruz et al., 2005) have led to the suggestion that the north–south oscillation of the tropical rainfall belt might be a key mechanism for a tropical “butterfly effect” (e.g. Peterson et al., 2000; Leduc et al., 2007). According to this hypothesis, millennial scale fluctuations in moisture transport across the Isthmus of Panama may modulate the North Atlantic freshwater budget and therefore serve as a positive feedback into abrupt climate changes. Accurate and well-dated records of tropical precipitation are thus of critical importance to improve our understanding of the role the tropical hydrologic cycle may have as a driver or amplifier of climate change. Records of past lake levels reflect temporal and spatial changes in precipitation patterns. Paleoshorelines provide direct benchmarks of water depth and can thus be used as quantitative records of regional hydrological budget.
In the closed Titicaca–Altiplano watershed (Central Andes), numerous very well-preserved paleoshorelines and carbonate deposits represent direct and spectacular archives of significant changes in net moisture over the southern tropical Andes. Several studies conducted over the two last decades have allowed chronological constraints to be placed on the timing and causes of these large fluctuations in lake level and demonstrated their abruptness (e.g. Sylvestre et al., 1999; Baker et al., 2001; Placzek et al., 2006a, 2006b). In particular, comprehensive studies based on a large number of U–Th and radiocarbon dates from paleolake carbonates permitted identification of two large oscillations in lake level synchronous with the abrupt millennial cooling events recorded in the North Atlantic, namely Lake Tauca (coincident with the Heinrich 1 event, 17–15 ka) and Lake Coipasa (coincident with the Younger Dryas, 13–12 ka) (Sylvestre et al., 1999; Placzek et al., 2006a, 2006b). Sr and U isotopes have also recently been used to characterize the spatial pattern of these pluvial events (Placzek et al., 2011). However, several questions remain unanswered:
- i)
What is the timing of other oscillations in lake level? The majority of existing U–Th and 14C data belongs to the Lake Tauca cycle, but the chronologies of the other oscillations in lake level are not as well established. This is notably the case for the lowstand episode between the so-called Sajsi episode (25–18 ka) and the Lake Tauca cycle (17.5–15 ka), as both the amplitude and duration of the Sajsi–Tauca lowstand is still unclear (Placzek et al., 2006b). Similarly, the chronology and amplitude of the Coipasa cycle needs to be better characterized.
- ii)
What triggered the lake highstands in this region, and how do oscillations in lake level reflect changes in atmospheric circulation (Sylvestre et al., 1999; Placzek et al., 2006b)? Various isotopic systems have been used to propose several models for precipitation patterns over the Altiplano (Coudrain et al., 2002; Placzek et al., 2011), but many questions remain open about these highstands, particularly over the amount of local water that was recycled, thus impacting the hydrological budget of these lakes.
In this contribution we present a new set of 14C and U–Th ages of the Altiplano lake cycles that occurred during the late Pleistocene (Sajsi, Tauca and Coipasa cycles). We combine these new ages with published datasets (Sylvestre et al., 1999; Placzek et al., 2006b). These new data are crucial to refine the chronology of these episodes and to establish coherent regional scenarios for net precipitation changes. We also provide new stable isotope data (δ18O and δ13C) from well-dated lake carbonates. These data allow calculation of the δ18O of past lake waters and are used to constrain the contribution of local recycling to the hydrologic budget of the lake.
Section snippets
The Altiplano endorheic watershed
The Titicaca–Altiplano watershed (15–23°S, 66–70°W) is the largest endorheic basin of the Tropical Andes, with a total area of ∼196,000 km2 (Fig. 1). This large plateau, located at an average elevation of ∼3800 m, is flanked by two north–south mountain ranges rising up to 6500 m: the Oriental and the Occidental Cordilleras. The Altiplano hydrological system may be divided in two main sub-catchments: the Titicaca basin (58,000 km2) in the north and the Uyuni–Coipasa basin (138,000 km2) in the
Samples
Because the two main goals of this study are (i) to refine the water level chronology and (ii) to provide new constraints on the δ18O of the paleolake water, the new samples studied here were selected according to specific criteria. A detailed list of the processed samples along with the type of analyses (U–Th, 14C, δ13C, δ18O) is provided in Table 2.
Carbonate samples deposited in shallow waters (<10 m water depth) are not only the best indicators of the lake surface paleoelevation, and hence
Methods
Tufa sub-samples of few mg were collected by micro-drilling. Microscope observations indicate that some of these algal carbonates are finely laminated (Fig. 2C), a structure that can possibly be interpreted to result from seasonality. However, the performed drilling was large enough (holes between 1 and 2 mm diameter) to incorporate several layers and to minimize a seasonal sampling bias. Some tufas were sampled at several spots separated by several centimeters as a test for spatial variability
14C results
For clarity, all new 14C data (Table 3) and previously published 14C ages (Sylvestre et al., 1999; Placzek et al., 2006b) were recalibrated using the most recent calibration curve IntCal09 (Reimer et al., 2009). The IntCal04 calibration (Reimer et al., 2004) is also shown for comparison (Table 3). However, all the 14C ages given in the text are reported as calibrated ages (cal BP) using IntCal09. We did not apply any reservoir age correction. Given the good agreement between the 14C and the
Results
δ18O and δ13C data are displayed in Table 6. For the paleocarbonate from Lakes Coipasa and Tauca, δ13C compositions range from 1.768 ± 0.002 to 5.438 ± 0.002 (V-PDB) ‰ and the δ18O from −2.186 ± 0.005 to 0.357 ± 0.005 (V-PDB) ‰ (Table 6; Fig. 9). Replicates of sub-samples all agree within a range of 1‰ or less, which indicates that the isotopic compositions of these samples are quite homogeneous. The absence of correlation between the δ13C and the δ18O values of the Coipasa and Tauca carbonates
Implications for regional climate and possible teleconnections
This updated lake level chronology for the Altiplano provides strong evidence that the wet–dry fluctuations in the tropical Andes during the Late Pleistocene are temporally correlated with the cold–warm events recorded by the Northern Atlantic SST (Martrat et al., 2004, 2007) and the Greenland temperatures records (Andersen et al., 2004; Rasmussen et al., 2006; Svensson et al., 2006; Vinther et al., 2006). The Lake Tauca and the subsequent Lake Coipasa highstands are in phase with the Heinrich
Conclusion
New 14C and U–Th data obtained from Altiplano lake carbonates were combined with published data (Sylvestre et al., 1999; Placzek et al., 2006b) to refine the lake level chronology within the Altiplano watershed over the last 25 ka:
- (i)
the Lake Sajsi cycle lasted between 25 and 19 ka and culminated at 3670 m at about 22 ka almost synchronously with the global LGM;
- (ii)
the Lake Tauca cycle lasted from 18 to 14.5 ka and is characterized by the highest water level, that reached at least 3770 m from 16.5 to
Acknowledgments
Jean-Marie Rouchy (Museum National d'Histoire Naturelle) is kindly thanked for providing us very well characterized samples. This work was funded by the French Programs INSU “Relief de la Terre” and “EVE-LEFE” and by NSF. Radiocarbon dating was performed at the Laboratoire de Mesure du Carbone 14 (LMC14) – UMS 2572 (CEA/DSM – CNRS – IRD – IRSN – Ministère de la Culture et de la Communication) at Gif sur Yvette, France, on the 14C AMS facility ARTEMIS. John Eiler is thanked for providing us
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