Distributions of branched GDGTs in soils and lake sediments from western Uganda: Implications for a lacustrine paleothermometer

Bacterially produced branched glycerol dialkyl glycerol tetraethers (GDGTs) are ubiquitous in soils and lakesediments and can potentially be used to reconstruct past temperatures. In lakes, however, it is still unclear if these compounds are derived from eroded soils or if they are produced in situ. To better understand environmental controls on the distributions of these compounds and the sources of branchedGDGTs to lakesediments, we compare branched GDGT distributions and concentrations in lakesediments and catchment soils within a 3600 m altitudinal transect in western Uganda. Reconstructed mean annual air temperature (MAAT), determined from the degree of methylation (MBT) and cyclisation (CBT) of branchedGDGTs in soils, decreases with increasing altitude, as is expected from the air temperature gradient in our transect. However, we observe significant offsets between observed and reconstructed temperatures in soils from wet, high elevation soils but not in most dry, low elevation soils. Branched GDGT distributions differ significantly between lakesediments and soils at all elevations, with greater differences at low elevations than at high elevations. These data support previous hypotheses that branchedGDGTs are produced in situ in lakes and suggest that the abundance of water in soil environments may play a role in controlling the distribution of branchedGDGTs. While branchedGDGTs in lacustrinesediments can be used to reconstruct temporal temperature variations in lakes, we urge caution in utilizing this proxy in lacustrine systems with high soil loadings, especially if there is evidence of changing clastic fluxes through time