Origins of lipid biomarkers in Santa Monica Basin surface sediment: a case study using compound-specific Δ14C analysis

Abstract

Compound-specific Δ14C values are reported for 31 different lipid biomarker molecules obtained from Santa Monica Basin and Santa Barbara Basin surface sediments. These organic compounds represent phytoplanktonic, zooplanktonic, bacterial, archaeal, terrestrial higher plant, and fossil carbon sources. The lipid classes include the following: long-chain n-alkanes, fatty acids (as methyl esters; FAMEs), n-alcohols, C30 midchain ketols and diols, sterols, hopanols, and C40 isoprenoid side chains from the ether-linked glycerols of Archaea. The data show that the carbon source for the majority of the biomarkers is marine euphotic zone primary production or subsequent heterotrophic consumption of this biomass. A small amount of benthic incorporation of 14C-depleted dissolved inorganic carbon was identified for the bacterial hopanols and C15 linear and branched-chain fatty acids. However, there is no apparent uptake of 14C-depleted dissolved inorganic carbon in Santa Monica Basin by the bacteria, including filamentous Beggiatoa spp., that produce C18:1ω7 fatty acid. Two of the lipid classes did not reflect carbon originally fixed by marine photoautotrophs. These were the n-alkanes, for which the Δ14C data are consistent with mixed fossil carbon and contemporary terrestrial higher plant sources, and the archaeal isoprenoids, for which the Δ14C data are consistent with chemoautotrophic growth below the euphotic zone.