Glacial-interglacial changes in moisture balance and the impact on vegetation in the southern hemisphere tropical Andes (Bolivia/Peru)
Introduction
The tropical Andes play a fundamental role in global climate systems today (Zhou and Lau, 1998, Lenters and Cook, 1999, Garreaud et al., 2003), yet the contribution of different mechanisms affecting these systems in the past remains controversial. Debate has focused primarily on the nature of the moisture balance in the Andes during the Last Glacial Maximum (LGM, c. 26,000–21,000 cal yr BP/22,000–18,000 14C yr BP). Geomorphological, sedimentological, biological and geochemical data have been presented from terrestrial and aquatic records in support of both wetter and drier conditions (e.g. Clapperton, 1993, Thompson et al., 1998, Heine, 2000, Smith et al., 2005a, Smith et al., 2005b). In the southern hemisphere tropical Andes, records from: i) Lake Siberia (17° 50′S, 64° 43′W; Mourguiart and Ledru, 2003) and ii) Huiñaimarca (16° 20′S, 68° 57′W; Argollo and Mourguiart, 2000) have been interpreted as indicative of lowered LGM precipitation, although alternative mechanisms and interpretation have been postulated by Baker et al. (2003). Conversely, LGM sediments from the Lago Grande basin of Lake Titicaca (Baker et al., 2001b) and fluvial sediments from its southerly outflow, the Río Desaguadero (Rigsby et al., 2005), suggest deep fresh water. These data imply that there must have been a positive moisture balance at this time to allow the overtopping of Lake Titicaca, i.e. wet conditions persisted.
While controversy surrounds the interpretation of LGM records, little is known regarding longer-term fluctuations in moisture balance and its impact on the vegetation of the region. Radiocarbon and U–Th dating of palaeoshorelines in the Altiplano have suggested that six lake cycles occurred during the last glacial-interglacial cycle (Placzek et al., 2006a, Placzek et al., 2006b): the Ouki (120,000–98,000 cal yr BP), the Salinas (95,000–80,000 cal yr BP), the Inca Huasi (c. 46,000 cal yr BP), the Sajsi (c. 24,000–20,500 cal yr BP), the Tauca (18,100–14,100 cal yr BP) and the Coipasa (13,000–11,000 cal yr BP). Placzek et al. (2006b) suggest that the Ouki and the Tauca cycles created the deepest palaeolakes, reaching c. 80 m and c. 140 m, respectively. Cores recovered from the Salar de Uyuni (20°S, 68°W) provide the only published palaeoecological records from the southern hemisphere tropical Andes that cover the last glacial-interglacial cycle (Sylvestre et al., 1999, Baker et al., 2001a, Fritz et al., 2004, Chepstow-Lusty et al., 2005). However, the palynological record from Salar de Uyuni is not continuous because during arid phases the evaporitic deposits did not preserve micro-fossils. The intermittent presence of lake sediments beneath this modern salt pan indicates that the precipitation:evaporation (P:E) ratio was greater than today at various points during the last glacial-interglacial cycle (Baker et al., 2001a). Palynological data from the layers of lake sediment found beneath the Salar de Uyuni suggest that during these episodes of high P:E ratios the glacial landscape was dominated by grasses with elements of high Andean woodlands continually present (Chepstow-Lusty et al., 2005).
In this paper, we present a more continuous palynological record from the last glacial-interglacial cycle (c. 151,000–14,200 cal yr BP) obtained from a 119 m sediment core from the Huiñaimarca sub-basin of Lake Titicaca. Data are used to assess moisture balance changes and their impact on the vegetation through this period. Comparison and correlation with other records from the Altiplano place these data within a regional context.
Section snippets
Climate and vegetation
The precipitation of the Altiplano is governed by the strength and duration of the South American Summer Monsoon (Vuille et al., 2000, Garreaud et al., 2003). Pronounced precipitation minima occur across the central Andes between June and August, although precipitation is possible throughout the year due to the orographic effect of the high cordillera (Johnson, 1976). Precipitation is concentrated between December and March when prevailing wind direction reverses and Amazonian moisture flows
Chronology
The Huiñaimarca chronology was established by two methods. Firstly, the age-depth curve between 40 and 0 m below lake floor (mblf) was constructed from 6 AMS 14C dates from core LT01-3B (Table 1) and 24 14C AMS dates from the sister core LT01-3A (Table 2). Cross correlation of dates between these two cores was justified as they were recovered only 50 m apart and have near identical stratigraphic and magnetic susceptibility profiles (Fig. 3a and b); the consistency in sedimentation patterns was
Chronology
Prior to 151,000 cal yr BP (> 93.77 mblf) no age vs. depth correlation has been attempted because of the absence of radiometric dating and the lack of obvious tie points (Fig. 4).
Between 151,000 and 40,000 cal yr BP (93.77–40 mblf) the chronology has been established by the correlation of a tephra layer at 151,000 cal yr BP with the equivalent layer in Lago Grande. Through this period the age vs. depth curve has been fine tuned by linking aridity indicators with lows in insolation (Table 3). The
Environmental reconstruction
The major palynological changes in the Huiñaimarca record, both compositionally and in terms of abundance, occur on the 100,000 year glacial-interglacial (eccentricity) time scale. Interglacial sediments are dominated by Cheno/Am pollen while the majority of the glacial is dominated by Poaceae (Fig. 6a). The dominance of salt-tolerant Cheno/Ams and a c. 8000 year sedimentary hiatus during the last interglacial are interpreted as indicative of a P:E lower than present, i.e. evaporation exceeded
Conclusions
The pollen productivity of the vegetation in the southern hemisphere tropical Andes of Bolivia and Peru covaries with the eccentricity (100,000 year) glacial-interglacial cycle. We have linked shorter fluctuations in the moisture balance to variations in insolation on precessional (21,000 year) timescales. Links between lake level fluctuations and palaeolakes Tauca and Minchin, as identified by previous authors, remain tentative. Differences in the timing of events highlight the variation in
Acknowledgements
This work was funded by NSF grant ATM 9906107 and ATM 0317539 (M.B. Bush) and core recovery, logging and radiocarbon dates were funded by ICDP and NSF (Earth Systems History program) (S.A. Fritz, P. Baker and G. Seltzer). We acknowledge the assistance of the staff of LacCore in core sampling (http://lrc.geo.umn.edu/LacCore/laccore.html). The authors thank reviewers Henry Hooghiemstra (University of Amsterdam) and Robert Marchant (University of York) and acknowledge the assistance of Sheri C.
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