Paleoenvironmental Variability in the Southeast African Tropics Since the Last Glacial Maximum: Molecular and Isotopic Records from Lake Malawi

Abstract

This study utilizes molecular and isotopic techniques to examine past variability in terrestrial and aquatic tropical ecosystems from southeast Africa. Two different timescales are investigated, the past 23 cal ka to examine glacial-interglacial climate variability, and the past 730 years to examine decadal to centennial scale climate variability. Carbon isotope measurements of plant leaf waxes provide a sensitive indicator of aridity and document dry conditions in southeast Africa during the Last Glacial Maximum, the Younger Dryas cold period, and during the Little Ice Age. Peak wet conditions are observed at 13.6 and 5 cal ka, and a shift to wetter conditions is also noted from 1800 AD to the present. Arid conditions in southeast Africa are associated with southward migrations of the mean latitudinal position of the Intertropical Convergence Zone (ITCZ) during Northern Hemisphere cold periods. In contrast to studies that have suggested Holocene climates were relatively stable, the Holocene in southeast Africa was characterized by extreme and abrupt changes in moisture availability, which likely affected human and faunal migrations as well as the development and collapse of human civilizations. In addition to affecting aridity in southeast Africa, southward migrations of the ITCZ also influenced algal productivity in Lake Malawi. Lipids of aquatic algae indicate a major increase in the primary productivity at the Pleistocene/Holocene boundary, which is likely related to a northward migration of the ITCZ over Africa at this time. The Younger Dryas stands out as a major feature in the records of nearly all algal lipids and is marked by an abrupt increase in algal productivity, which can be attributed to increased northerly winds over Lake Malawi. During the past 730 years there is also evidence for changes in algal productivity with decreasing abundances of diatom lipids and increasing abundances of dinoflagellate lipids noted over the past few centuries.