Abstract
Lake Challa is a permanently stratified crater lake located in Equatorial East Africa. The high biomarker preservation potential and the fact that it lies east of the Congo Air Boundary, thereby only recording climate variations linked to the Indian Ocean, highlights its potential for paleoclimatic reconstruction. However, previous biomarker studies
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conducted at Lake Challa have mainly concentrated on biomarker variations over the past 25 kyr, with little emphasis on modern variations and interactions between biomarkers, the climate and lake water column processes. Thus far, studies have focused on specific biomarkers such as the glycerol dialkyl glycerol tetraethers (GDGTs), without expanding on the distributions and temporal variations of other various biomarkers that may prove useful in gaining a deeper understanding of past and present changes in vegetation and climate. The relationship between biomarkers, biota and climate has been explored in a previous study assessing the temporal distributions of biomarkers in the suspended particulate matter (SPM). While this provided some useful insights into the potential sources of various biomarkers and how their distributions varied over depth and time, uncertainties still remain with regards to their sources and transport mechanisms to the sediment. The aim of this study is to gain a deeper understanding of the sources of terrestrial, aquatic and bacterial biomarkers and to examine their temporal distributions in the sinking particles over time. Biomarker distributions in sinking particles were analysed from a sediment trap deployed at a depth of 35m that was collected between September 2010 and January 2015. The results indicate that major biomarker production occurred between July and November 2014, which coincides with the breakdown of the thermocline and an upwelling of nutrients. In general, aquatic, plant and bacterial biomarkers were observed both during stratification and mixing events, with certain similarities in temporal variations. Seasonality most likely explains these variations, with possible links to ENSO events. A potential algal or mixed source is implied for the C14, C20, C22, C24 and C26 fatty acids. Peaks in the concentrations of the C14, C20, C22, C24 and C26 fatty acids during the ‘long’ mixing in 2013 and 2014 imply that diatoms may be their source. However a mixed source is most likely, particularly for the C20 and C22 fatty acids. A possible algal source for the C23:1, C25:1 and C27:1 n-alkenes was also suggested based on the similarity in their temporal distributions and the δ13C values for the C27:1 n-alkene. The C28 fatty acid, previously assumed to originate from the plant leaf waxes of terrestrial origin, was detected in the sinking particles and is most likely produced by an aquatic organism. The temporal profiles of the biomarkers are similar for the SPM and sinking particles. However an absence in terrestrial biomarkers in the SPM was previously observed, which is not the case for the sinking particles and implies that aggregation may be responsible for this. The source organisms of many compounds could not be confirmed, thus emphasizing the need for further biomarker studies.
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