The palaeolimnology of northern Lake Malawi over the last 25 ka based upon the elemental and stable isotopic composition of sedimentary organic matter

Abstract

Sediments accumulating in offshore areas of northern Lake Malawi typically contain 1–4% organic carbon. This organic matter (OM) is of mainly phytoplankton origin with varying mixtures of terrestrial and degraded, reworked material. High-resolution, stable carbon and nitrogen isotopic, elemental and Rock-Eval pyrolysis analyses of the bulk OM contained in three cores have been used to characterise the OM and provide detailed insights into the palaeolimnology and palaeoclimatology of northern Lake Malawi over the last 25 ka. Preservation of the OM as reflected by its Rock-Eval hydrogen index (HI) is highly variable. Periods characterised by notably low HI (<250) indicate a lowered lake level when oxic conditions episodically reached the deep lake floor. Particularly well-defined lowstands occurred between ca 11.8 and 10 cal ka and around 700 years ago. Isotopic and other evidence also suggest lowstand conditions before and during the Last Glacial Maximum (LGM) lasting until ca 17 ka. However, the OM that accumulated at that time has relatively high HI. Generally high HI values persisted until ca 11.8 ka, when there was a rapid decline leading in to the terminal Pleistocene–early Holocene lowstand. This abrupt change divides the OM record into two distinct phases, one preceding and the other following the decrease in HI. Higher HI values in the earlier period, even during times of low lake level, reflect high rates of primary production stimulated by vigorous vertical mixing of the water column that was driven by a greater incidence of northerly winds. After ca 11.8 ka the wind regime changed to one similar to today's, with mixing and nutrient cycling largely controlled by the southerly trade wind system, and the locus of high primary production at the south end of the lake. Periods of low δ 15 N during the mid- and late Holocene suggest intervals of stable stratification due to significantly reduced wind-driven mixing in the northern part of the lake, when N-fixing cyanophytes formed a major component of the phytoplankton. The geochemical data support recent suggestions of lowstand conditions in Lake Malawi at the time of the LGM, and provide additional evidence of periods of intense but localised aridity in the terminal Pleistocene–earliest Holocene and at the time of the Little Ice Age. The change in mixing regime at ca 11.8 ka seems to have been the result of a radical reorganisation of the dominant surface wind flow over south-eastern Africa. The transition was, within the resolution of currently available chronologies, apparently contemporaneous with marked changes in the circulation patterns of adjacent oceans, and the climate of more distant parts of the Northern Hemisphere.