Paleoclimate interpretation of stable isotope records from inorganic carbonates, Lakes Malawi and Turkana, East Africa.

摘要

The stable isotopic composition of inorganic carbonates precipitated in closed-basin lakes records the temperature at which the carbonate precipitated and the composition of the lake water from which it precipitated. The composition of lake water is a reflection of several climatic parameters including the evaporation and input flux of the basin. Thus, the isotopic composition of carbonates can be a good proxy of the paleo-environment in a basin. Many workers have interpreted the {dollar}deltasp{lcub}18{rcub}{dollar}O record from inorganic carbonates to directly record lake level in closed-basin lakes. This is not always correct. Quantitative modeling of the mass balance of oxygen isotopes and water in a lake give an indication of what parameters, including evaporation and input, can affect the {dollar}deltasp{lcub}18{rcub}{dollar}O composition of lake waters and lacustrine carbonates.; This work interprets several stable isotope records from Lakes Malawi and Turkana in the East African Rift system. Six piston cores and nine freeze cores from the lakes were analyzed and interpreted. A quantitative analysis of the {dollar}deltasp{lcub}18{rcub}{dollar}O records from these cores allowed for a more accurate interpretation of the paleoclimate in the two basins.; There is good agreement between a historical record of lake level for Lake Turkana and lake level generated using stacked {dollar}deltasp{lcub}18{rcub}{dollar}O records from several freeze cores from Lake Turkana. On a longer time scale, the lake went from high lake level conditions to low level conditions beginning at 4 ka. The shift in {dollar}deltasp{lcub}18{rcub}{dollar}O values seen in a Lake Turkana piston core record at 4 ka cannot be described solely by the decrease in lake level but must involve a 2000 year shift in the climatic system. In Lake Malawi, the {dollar}deltasp{lcub}18{rcub}{dollar}O composition of carbonates delineate four "dry" events in the early Holocene. Carbonate {dollar}deltasp{lcub}18{rcub}{dollar}O composition from the late Pleistocene in Lake Malawi either denotes greater variability in the composition of Lake Malawi waters or a change in climate between 35-37ka and 31-34ka. This shift in isotopic composition during the late Pleistocene may be explained by a drop in temperature or changes in other climatic parameters.