14 C analysis of aliphatic hydrocarbon fractions from the hypersaline Lake Tyrrell, southeast Australia

Abstract

Conventional biomarker studies typically interpret the distribution, structure and stable isotopic (e.g. 13C, D) composition of sedimentary hydrocarbons and polar compounds. However, compound and compound class specific 14C analysis (CSRA) is becoming increasingly relevant for characterising millennial scale residence and mobilisation of sedimentary organic carbon (OC). Here, the 14C content of the aliphatic and bulk fractions from shallow cores from the hypersaline playa, Lake Tyrrell, southeast Australia were compared. The aliphatic hydrocarbon fractions (predominantly n-alkanes) were substantially older than the corresponding bulk fractions, indicating the presence of active reservoirs of ancient carbon, likely derived from aeolian reworking of sediments. The 14C ages of the aliphatic hydrocarbons in the core revealed two noticeable shifts in age and source of ancient OC that were not apparent using biomarker composition and sedimentology alone. The study shows that aliphatic hydrocarbons are relatively simple to isolate, even from organically lean (ca. 0.05% TOC) terrestrial sediments, and their 14C ages yield information about carbon mobilisation and preservation not amenable to conventional analysis.