Effects of temperature, pH and nutrient concentration on branched GDGT distributions in East African lakes: Implications for paleoenvironmental reconstruction

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids found in soils and sediments and their relative abundance correlates with temperature and pH, enabling them to be used as proxies in reconstructing past climatic and environmental conditions. However, the potential for other environmental variables, such as nutrient concentration, to affect brGDGT distributions remains largely unexplored. We have examined the influence of a suite of environmental factors, including temperature, lake water and sediment chemistry, and lake morphometry on brGDGT concentration and distributions in the surface sediments of 111 lakes in East Africa. We found that temperature was the major control on the distributions, while the influence of pH was relatively minor. Water depth also had a minor but statistically significant influence, perhaps due to the relationship between lake depth and deep water anoxia. Water column nutrient concentration did not have a significant effect on the distributions or concentration. We further explored the potential for these variables to affect brGDGT temperature reconstruction by examining the correlation between them and the residuals of our brGDGT temperature calibration. We found that, while the distribution of some cyclized brGDGTs may be influenced by pH and other environmental variables, they are necessary in brGDGT calibration equations in order to accurately reconstruct temperature, likely due to covariation between temperature and other environmental variables. While surface water pH correlated with the relative abundance of certain brGDGTs, caution should be exhibited when using brGDGTs as a pH proxy because of systematic calibration errors.