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

Abstract

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 (C25) that partly originate from n-Alkanes from Jurassic black clay shales in the upper catchment. The remaining petrogenic contributions on the longer chains (C25) were corrected for by using a constant correction factor that was based on the n-Alkane concentrations in a black clay shale sample from the upper catchment. Due to different degrees of pre-Aging and reworking, the corrected leaf wax n-Alkane ages still indicate relatively large age offsets between n-Alkane formation and deposition: while intensively developed (paleo)soils showed no age offsets due to a dominance of leaf wax n-Alkanes produced onsite, less intensively developed paleosols showed much larger age offsets due to larger proportions of inherited leaf wax n-Alkanes from the fluvial parent material. Accordingly, age offsets in nonpedogenic fluvial sediments were largest and strongly increased after 4 ka cal BP. The leaf wax n-Alkane homolog distribution from intensively developed (paleo)soils indicates a local dominance of grasses and herbs throughout the Holocene, which was most likely caused by anthropogenic activity. The leaf wax n-Alkanes from fluvial sediments show a dominance of deciduous trees and shrubs as well as grasses and herbs in different parts of the catchment between 8 and 5:6 ka cal BP. Since no older deciduous tree-or shrub-derived n-Alkanes were dated, this seems to confirm a delayed regional postglacial reforestation of parts of the catchment compared with western and central Europe.