Grain Size Associations of Branched Tetraether Lipids in Soils and Riverbank Sediments: Influence of Hydrodynamic Sorting Processes

We analyzed the abundance and distribution of branched glycerol dialkyl glyceroltetraethers (brGDGTs) in grain size fractions of 7 globally distributed river flank sedimentsand catchment soils in order to determine if and how the initial soil-brGDGT signatureis influenced by hydrodynamic sorting upon entering a river and during subsequenttransport from land to sea. BrGDGTs are hypothesized to form associations withhigh-surface-area fine-grained minerals in soils. Such associations, if maintained duringtransport, may impart resistance to degradation and promote downstream transport,reducing potential interferences by aquatic brGDGTs. We find that brGDGTs a re indeedprimarily associated with organic carbon (OC) bound t o the clay-silt fraction (<63 µm)in both soils and river sediments, and that these associations appear to be maintainedduring river transport. However, the relative distribution of individual brGDGTs among sizefractions is relatively uniform, suggesting that brGDGTs are well mixed in river sedimentsand that OC-mineral associations are continuously renewed. Consequently, the brGDGTsignature finally stored in continental margin sediments appears insensitive to differentialparticle transport processes. Nevertheless, the lower (upstream) temperature signalgenerally reflected by brGDGTs in river sediments may also be explained by a contributionof in situ produced brGDGTs leading to an underestimation of reconstructed airtemperatures.