Late Quaternary paleolimnology in the southern hemisphere tropics.

摘要

Lake deposits are widespread throughout the Phanerozoic rock record and have long intrigued geologists and paleobiologists in search of natural resources or fossil biota. Low-energy lacustrine depositional environments, characterized by relatively rapid sediment influx rates and shallow zones of bioturbation, likewise produce highly-resolved archives of climate and ecosystems evolution. This dissertation describes four studies that use lake sediments for Quaternary environmental analysis.;In East Africa, many decades of prior study provided the critical framework necessary for in-depth paleoenvironmental research at Lake Tanganyika (3° – 9°S). Seismic stratigraphic analysis integrated with radiocarbon-dated sediment cores from the Kalya horst and platform document a dramatic lake level lowstand prior to ∼106 ka and a minor, short-lived regression during the Last Glacial Maximum (32 – 14 ka). Paleobathymetric maps reveal that Lake Tanganyika remains a large, connected water body even during episodes of extreme drought, which has implications for local and regional fauna. Over shorter timescales, geochronological, taphonomic and sedimentological analyses of shell beds around Kigoma (central Lake Tanganyika) document three distinct facies-types that are time-averaged over the latest Holocene. Lake level fluctuations associated with the termination of the Little Ice Age (∼ 16th century CE) and subsequent encrustation played a key role in shell bed formation and persistence along high-energy littoral platforms, which has implications for structuring specialized communities of benthic fauna.;In central South America (18° – 22°S), we studied the limnogeology of small lakes in the Puna and the Pantanal. Analyses of these sites were undertaken to: (1) ascertain how the lakes act as depositional basins; (2) assess sedimentation rates; and (3) construct limnogeological databases to guide future interpretations of ancient sediment cores. At Laguna de los Pozuelos (Argentine Puna), linear sedimentation rates approach 0.14 cm*y−1 in the playa-lake center, and litho- and organo-facies development are dominantly controlled by basin hydrology, climate and biological feedbacks (both nutrient cycling and bioturbation) from waterbirds. At Lagoas Gaíva, Mandioré and Vermelha (Brazilian Pantanal), short-lived radioisotopes indicate uninterrupted depositional rates of 0.11 – 0.24 cm*y−1, and hydrochemical and depositional patterns respond sensitively to changes in the seasonal flooding cycle of the Upper Paraguay River.