Organic carbon as a palaeoenvironmental indicator in the marine realm

Abstract

Recent developments in biogeochemistry have made accessible all information currently known to be present in sedimentary organic compounds, i. e. the carbon skeleton structure, the positions of functional groups and stable carbon isotope ratios. This combined information has strengthened the applications of sedimentary organic compounds as biomarkers and palaeoenvironmental indicators. This review summarizes the most important biomarkers and their applications in reconstructing marine palaeoenvironments and processes. The marine environment is characterized by the combined presence of specific compounds, such as organic sulphur compounds, dinosteroids, 24-n-propylsteroids (even in trace amounts), high amounts of C₂₅ HBI hydrocarbons and the absence of long chain n-alkanes with odd over even predominance, bicadinanes and specific higher-plant triterpenoids (e. g. oleanane). Furthermore, palaeoenvironmental conditions, such as photic zone anoxia, hypersalinity, microbial sulphate-reducing activity, algal blooming, palaeo-upwelling and palaeo-surface seawater temperature are indicated by molecular and/or isotopic characteristics. Biomarkers from terrestrial sources are included because land-derived organic matter is an important contributor to coastal and shallow-marine environments. A short case study, within a Holocene terrestrial/marine setting (Florida Bay, USA), is also presented to illustrate the reconstruction of transgressive-regressive trends using selected organic compounds.