A 2000 Year Sub-annual Record of Climate Change from Lake Malawi

Abstract

Lake Malawi, in East Africa, provides a rare opportunity to look at a sub-annual record of climate change from a region were there is a lack of good climate records. This is possible because the upper most sediments in the north basin Lake Malawi are varved. The varved record extends from back 1200-2000 years depending on dating differences and is well dated back 800 years. In order to construct an overall sub-annual record of climate change, overlapping sections of two cores from the north basin of Lake Malawi taken during the recent Malawi drilling project were examined on a scanning XRF with a step size of .2 mm. This provided an average of 6 measurements per annual varve couplet. Because of the chemical differences between light and dark varves it was possible to count these varves using a high resolution photograph and x-radiography images in conjunction with the chemical data. Four major chemical elements or ratios of chemical element were used to represent the four major components in the sediments of the core: Fe is a proxy for the clay, Si/Ti is a proxy for the Biogenic Silica (BSi, inc/coh scattering ratio is a proxy for %Total Organic Carbon (TOC), and K/Ti is a proxy for volcanic sediments. Comparing Fe to recent records of lake level fall and rise suggests that Fe be when the planktonic community is dominated by diatoms it reflects changes in year to year fluvial input. However at times of change in the planktonic community, the Fefluvial imput relationship breaks down. Comparisons of both the lake level records and to coral records from the Indian Ocean show that both lake level rise and fall are enhanced by increased ocean temperatures over periods of 5-10 years. This suggests that changes in Indian Ocean temperatures have an affect on Lake Malawi. The lake records show no connection to Zimbabwe rainfall but an opposite relationship with Indian rainfall. The inc/coh shows a strong relationship over the last 150 years with IOD and proportion of diatoms in the planktonic community, which tends to change during times with great IOD variability. These relationships suggest that IOD causes changes in the winds that stimulate production of planktonic during the dry season. Looking further back in the record shows that similar shifts to non-diatom dominated planktonic community occurred twice before in the record and there is some evidence in the record that changes in winds might be responsible for these events as well. 11 year and 3-7 year cycles were also found in the record but their meaning is unclear. The times when the 11 year cycle is strong in the lake are not assonated with the record of sunspots and the 3 to 7 year cycle does not seem to be related to changes in ESNO as would be expected. Therefore the meaning of the cycles is still not understood. The longer record suggests dry periods from the bottom of the varved section to around 1250 AD and from 1550 to 1650 AD, with an interceding wetter period from 1250 to 1650 AD. The record from 1650 AD to current day shows some rises and falls and an overall steady record. Comparing this to other records of African lakes and caves shows that this record has similarities to the records of East African lakes but is in opposite of records from Cold Air Cave in Southern Africa. Lastly it also shows similarities to records of Indian monsoon wind strength which is controlled by Indian Ocean temperatures and Cariaco basin records which are a proxy for the ITCZ. Both of these records show that the migration of the ITCZ and changes in Indian Ocean temperatures are important drivers for Lake Malawi and the findings of this paper may be useful in looking at changes both at longer time scales and in the future as well.