The relationship between sedimentary organic carbon isotopic composition and organic biomarker compound concentration

Abstract

A study of organic biomarker compounds which could serve as tracers of terrigenous and marine sedimentary organic matter sources was performed on samples from a 208.7 m hydraulic piston core hole (DSDP Hole 619) from the hemipelagic Pigmy Basin in the northern Gulf of Mexico. Organic carbon-normalized concentrations of total long chain (C 37–C 39) alkenones and some individual C 27–C 29 desmethyl sterols were determined to be useful proportional indicators (tracers) of preserved marine and terrigenous organic carbon, respectively. The alkenones, whose only known source is marine phytoplankton of the class Prymnesiophyceae, generally occurred in higher concentrations in interglacial isotope stages 1 and 5a-b than in the intervening glacial stages. Sterols (C 27-C 29), apparently of a dominantly terrigenous origin, occurred in lower concentrations during interglacial stages than in glacial stages. Tracers of both terrigenous and marine organic matter appear to be affected by the differential diagenetic alteration of the biomarker/C org ratios, as indicated by a simple, first-order kinetic model. The lack of any desmethylor 4a-methylsterol which is linearly related to the proportion of marine sedimentary organic matter (as scaled by δ 13C org) indicates that either 1. (1) sedimentary diagenesis has obscured the biomarker/C org vs. δ 13C org record 2. (2) phytoplanktonic assemblage changes caused variations in the biomarker/C org ratio of the primary input. Preferential preservation of terrigenous sterols may result in a biased sedimentary record of sterol input which could be misinterpreted as indicating solely terrigenous sterol sources. A simple model which characterizes the effects of sedimentary diagenesis on the relationship between C org-normalized biomarker ratios and δ 13C org demonstrates the potential problems of long-term, differential-diagenetic skewing on those tracer records.