Age and origin of leaf wax n-alkanes in fluvial sediment–paleosol sequences and implications for paleoenvironmental reconstructions

Leaf wax n-alkanes are increasingly used for quantitative paleoenvironmental reconstructions. However, this is complicated in sediment archives with associated hydrological catchments since the stored n-alkanes can have different ages and origins. 14C dating of the n-alkanes yields independent age information for these proxies, allowing their correct paleoenvironmental interpretation. This also holds true for fluvial sediment–paleosol sequences (FSPSs) that integrate two different n-alkane signals: (i) a catchment signal in fluvial sediments and (ii) an on-site signal from local biomass that increasingly dominates (paleo)soils with time. Therefore, the age and origin of n-alkanes in FSPSs are complex: in fluvial sediment layers they can be pre-aged and reworked when originating from eroded catchment soils or from organic-rich sediment rocks in the catchment. In (paleo)soils, besides an inherited contribution from the catchment, they were formed on-site by local biomass during pedogenesis. Depending on the different relative contributions from these sources, the n-alkane signal from an FSPS shows variable age offsets between its formation and final deposition. During this study, we applied compound-class 14C dating to n-alkanes from an FSPS along the upper Alazani in eastern Georgia. Our results show that preheating the n-alkanes with 120 ∘C for 8 h before 14C dating effectively removed the shorter chains (