A 22 kyr BP sedimentological record of Lake Rukwa (8°S, SW Tanzania):: environmental, chronostratigraphic and climatic implications

The sedimentological study of a 12.8 in long core (R96-1) from Lake Rukwa (Tanzania, 8degreesS) provides a new record of past lake-level fluctuations that took place in response to changes of the regional climate since the last glacial period. From 21 to 15 cal kyr BP, nearshore and swamp/marsh environments are evidenced from clastic deposition, macrophytes debris and Ca-Mg carbonate enrichments. A transgressive sequence, starting around 15 cal kyr BP, lead to the humid lacustrine optimum between 15 and 7 cal kyr BP. From 12 to 10 cal kyr BP, anoxic lake bottom environments were favored by the concomitant effects of high primary productivity, increased subsidence and inputs of volcanic ash in the lake. The Middle Holocene (7-3 cal kyr BP) is characterized by high concentrations of silt, carbonates, and low organic content, which indicate the occurrence of relatively oxic, shallow and saline depositional environments, especially around 7 and 3.4 cal kyr BP. Although grainsize and TOC profiles suggest that shallow environments likely persisted in the Upper Holocene, low Mg concentration values for the last 3 kyr may indicate a trend toward relatively more dilute environments. The two main Lake Rukwa low-stand periods, at 21-15 and 7-3 cal kyr BP, correspond remarkably well with the insolation maximum at the Equator, which occurred at 17 kyr BP for the spring equinox, and at 6 kyr BP for the autumn equinox, respectively. We suggest that, during these periods of minimum inter-hemispheric insolation gradients, monsoon circulation weakened in south equatorial regions, and the ITCZ was located north of 8degreesS in east Africa. (C) 2002 Elsevier Science B.V. All rights reserved.