Glacial-Interglacial changes in moisture balance and the impact on vegetation in the southern hemisphere tropical Andes (Bolivia/Peru)

Gosling, William D.; Bush, Mark B.; Hanselman, Jennifer A. and Chepstow-Lusty, Alex (2008). Glacial-Interglacial changes in moisture balance and the impact on vegetation in the southern hemisphere tropical Andes (Bolivia/Peru). Palaeogeography, Palaeoclimatology, Palaeoecology, 259(1) pp. 35–50.



A palynological investigation of the last glacial-interglacial cycle in the southern hemisphere tropical Andes reveals changes in the moisture balance as the main driver in vegetation change. Thirty accelerator mass spectrometry radiocarbon dates, biostratigraphy and tephra correlation reveal that a 119 m sediment core recovered from the Huiñaimarca sub-basin of Lake Titicaca (16.0° to 17.5° S, 68.5° to 70° W; 3810 masl) contains sediments covering > 151,000 years. Correlation of aridity indicators with precessional variations in insolation is used to fine tune the structure of the age-depth curve within this period.

Variations in Isoëtes concentration (above/below 10,000 grains/cm3) identify the extent of shallow water environments. Examination of another palaeolimnological indicator (Pediastrum) and consideration of the bathymetry of the Huiñaimarca sub-basin allow the reconstruction of lake-level fluctuations. These data indicate five wet/dry cycles between c. 151,000 and 14,200 cal yr BP. High stands are suggested during the transition into (c. 134,000 cal yr BP), and out of (c. 114,000 and 92,000 cal yr BP), the last interglacial, and during full glacial conditions (c. 70,000 and 45,000 cal yr BP). These cycles are superimposed on a general trend of deepening lake levels through the glacial period.

This interpretation is supported by correlation with sediments from Salar de Uyuni (20°S, 68°W; 3653 masl). The youngest wet episode is concurrent with palaeolake Minchin (c. 45,000 cal yr BP), with further evidence for an additional wet period commencing c. 28,000 cal yr BP, concomitant with palaeolake Tauca. The timing of lake level fluctuations is also supported by palaeoshoreline reconstructions from the Uyuni-Poopó region. However, our data do not suggest a major peak in lake level in Huiñaimarca during the Ouki lake cycle (c. 120,000–98,000 cal yr BP) as inferred from U–Th ages obtained from palaeoshorelines around Lago Poopó. The most extreme dry event occurs during the last interglacial period and resulted in a sedimentary hiatus tentatively dated to c. 121,000–129,000 cal yr BP.

The observed wet/dry cycles are shown to have a marked and rapid impact on the vegetation. The aridity of the last interglacial promoted a community dominated by Chenopodiaceae/Amaranthacae, with no modern Andean analogue. Polylepis/Acaena pollen is also shown to fluctuate markedly (0–20%), particularly during the transitions into, and out of, the last interglacial. It is probable that this pollen taxon is primarily representative of the high altitude arboreal genus Polylepis, which is a key component of highly biodiverse Andean woodlands today. Rapid fluctuations indicate the sensitivity of this ecosystem to natural environmental pressure and potential vulnerability to future human impact and climate change.

The 100,000 year (eccentricity) solar cycle is shown to be the major controlling factor in moisture balance and vegetation over the last glacial-interglacial cycle. However, significant fluctuations in moisture balance are also evident on timescales considerably shorter than the full glacial-interglacial cycle. We have linked these to precessional (21,000 year) forcing. Nevertheless, precise independent dating during the full glacial cycle is required to confirm the importance of this forcing mechanism.

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.

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