A 370,000-yr history of vegetation and climate change around Lake Titicaca (Bolivia/Peru).

摘要

High Andean grasslands are sensitive to climate change due to their placement between the tree-line and snow-line. This study provided a 370 kyr history of climate change on the Altiplano (Bolivia/Peru), currently the longest terrestrial, lacustrine record in the Southern Hemisphere. Three full glacial cycles were identified (MIS 10/9-MIS 1), allowing a detailed analysis of interglacial characteristics using fossil pollen, spores, and charcoal. Fossil pollen concentration increased (>80,000 grains/cm3) during MIS 9, 7, and 5e, suggesting that these were periods of higher productivity on the Altiplano. Large dramatic lake-level fluctuations were evident when aquatic pollen and spore concentrations became more prevalent. Transitions into and out of interglacials were also a focus in this study. Polylepis pollen were common in the transitions into interglacials, suggesting warm and wet conditions caused by increased convection, followed by drier conditions during peak interglacial times.; In addition to this long Andean record, three other studies were employed in order to better interpret paleoecological data: a modern pollen study on the Altiplano to determine the use of analogs in paleoecological investigations, a detailed comparison of interglacials MIS 1 and MIS 5e, and an assessment of the preliminary Titicaca chronology in the light of other methods of tuning.; A modern pollen rain study on the Altiplano was investigated to address the confidence of using surface sediments as indicators of surrounding vegetation, as well as an assessment of the use of modern analogs for paleoecological analyses. After a vegetation survey and surface sediment extraction, a similarity coefficient confirmed the strong association between the pollen rain in the upper 20 cm of sediment and the surrounding Andean landscape.; The modern vegetation data were then compared to the long Andean records from Titicaca and Huinaimarca. The pollen data from the modern sites encompassed all data points from the 370 kyr records from Titicaca and Huinaimarca, suggesting that there has been no loss in species over time, just redistribution. The interpretation of the modern data was useful in understanding the dynamics of the landscape over time, however, anthropogenic activities on the Andean landscape currently affect the modern plant distribution on the Altiplano.; Previous research has found limited utility in using the Holocene as an analog for past interglacials. Using a detrended correspondence analysis of the fossil pollen percentage data from MIS 1 and MIS 5e from cores from Lake Titicaca and Huinaimarca, it was found that MIS 5e was more abrupt and extreme than the Holocene. This interpretation was based on the presence of dry taxa, such as Chenopodiaceae, as well as the representation of the pollen and spores of aquatic taxa. MIS 5e appeared to be warmer and drier than the present.; During the investigation, the assumptions of the chronology were questioned, and as a result, a new chronology was developed. The current chronology used 14C dates, and a mean value of the U-series dates for the past 122 kyr, and then tuned CaCO3 concentration to the Vostok deuterium record. Based on the updated EPICA record, which has been cited as being more accurate than Vostok, combined with the strength of the pollen concentration dataset, a new tuning method was employed. Using 36 tie points from a log pollen concentration record, the resulting chronology was accepted as more accurate and better suited for future publications.; This paleoecological investigation provided insight into relatively poorly known interglacial dynamics. These data form the first multiglacial record from the southern Neotropics and emphasize the unique character of the Holocene. Estimates of future climate change exceeds estimates of past climate ranges and, combined with anthropogenic impacts on the Altiplano, are probably going to lead to no-analog conditions and communities.