Biogenic and anthropogenic sources of sedimentary organic matter in marine coastal areas: A multi-proxy approach based on bulk and molecular markers

Abstract

The investigation of organic matter sources is an especially challenging issue for marine coastal areas influenced by both natural and anthropogenic inputs. This study focuses on the provenance of sedimentary organic matter deposited in the Gulf of Gdańsk (southern Baltic) and Oslofjord/Drammensfjord (southern Norway) based on the analysis of multiple specific proxies in recent sediments (0–5 cm layer) sampled at 12 sites in 2014. Bulk markers (total organic carbon/total nitrogen [TOC/TN] and stable organic carbon isotopes [δ13CTOC]) and various molecular geochemical markers of organic matter origin (n-alkanes, branched alkanes, unresolved complex mixture [UCM], polycyclic aromatic hydrocarbons [PAHs], hopanes, steranes, and polar steroids including sterols and stanols) were used to comprehensively assess the origin of sedimentary organic matter in the studied basins. The results of this study indicate that the combined use of TOC/TN and δ13CTOC may lead to ambiguous outcomes, and anthropogenic input may affect the sedimentary TOC/TN ratio. The analysis of molecular proxies allowed the identification of locations most impacted by anthropogenic organic matter derived from petrogenic sources and/or sewage effluents and the classification of the studied sites based on the contribution of marine/terrestrial biogenic matter. This study demonstrates that key issues, such as the forms of sedimentary chemical markers, post-depositional changes, and specific diagenetic products, although neglected by some researchers, must be considered for the reliable assessment of organic matter sources. To the best of our knowledge, this is the first such extensive study on surface sediments from the Gulf of Gdańsk and Oslofjord/Drammensfjord in terms of the use of both bulk and molecular markers to investigate organic matter sources.