Early Holocene Changes in Lake Level and Productivity in Lake Malawi as Interpreted from Oxygen and Carbon Isotopic Measurements of Authigenic Carbonates

Abstract

Analyses of stable carbon and oxygen isotopes from authigenic lacustrine carbonates were undertaken in order to determine fluctuations in lake level and epilimnetic productivity during the early Holocene. The carbonates form a horizon in three cores taken from the bottom of Lake Malawi. The horizon has been interpreted as having been deposited during a lake lowstand dated to have existed between 5 and 10 kyr B.P. by 14C dating. Modeling of the response of the oxygen isotopic composition of the lake waters to climate forcing indicates that a direct comparison between oxygen isotopic composition and lake level is difficult. Carbon isotope data from authigenic carbonates in two of the cores may indicate that primary productivity in the lake fluctuated widely from 7.5 to 9 kyr B.P. Between 5 and 7.5 kyr B.P. carbon isotopic values are less variable and probably indicate that primary productivity was relatively low during that time. Carbon isotopic data from the third core indicate that a portion of its carbonate formed diagenetically in a methanogenic environment.