Anomalously low radiocarbon content of modern n-alkanes

Abstract

Compound-specific radiocarbon (14C) ages of lipid biomarkers commonly pre-date co-occurring macrofossils or sediment matrices in lacustrine and marine sedimentary sequences. Hypothetically, this age offset is the result of long residence and transport times of organic matter in the terrestrial realm and is referred to as ‘pre-aging’. Here we measure the 14C content of n-alkanes extracted from living leaf tissue of six different plant species. In all samples, the modern n-alkanes contained anomalously low 14C content resulting in apparent radiocarbon ages ranging from ∼ 530 to 1580 cal yr B.P. while bulk tissue samples yielded the expected modern 14C ages. These results indicate a potentially strong fractionation against 14C during lipid biosynthesis also documented for 13C and 2H stable isotope systems. This fractionation could, at least partially, explain observed age offsets between lipid biomarkers and co-occurring sediments.