Lithological and geochemical differentiations in an urbanized, shallow marine cold-seep environment. The case of Patras Gulf active pockmark field

Abstract

The Patras Gulf Pockmark Field (PGPF), located in a shallow, tectonically active marine basin in western Greece, offers a unique setting for studying lithological and geochemical variability in an urban-influenced cold-seep environment. Here we examine nine short sediment cores (∼30 cm) collected from both within pockmarks and the surrounding seafloor to assess sediment texture, elemental composition, and anthropogenic impacts. We find that pockmark sediments are finer-grained, better-sorted, less bioclastic, more organic-rich, and higher in trace metals loads compared to the surrounding seafloor. The sedimentary, geochemical, and hydrodynamic processes within the pockmarks follow the pockmark morphology; the larger the pockmark is, the more terrigenous, fine-grained, and metal-enriched the sediment is, and less oxygenation characterizes the bottom waters. Multivariate factor analysis identified five dominant geochemical processes. Factors 1 and 2, prevailing outside the pockmarks, reflect clay–metal sorption processes and the anticorrelation between bioclastic material and fine-grained terrigenous inputs. Within pockmarks, Factor 3 indicates anthropogenic sources of Ag, Hg, and Zn through organic carbon–metal sequestration, especially in the southern sector of PGPF, influenced by Glafkos River discharges. Factor 4 reflects localized contamination by Zr, Ta, and Nb, while Factor 5 captures sulfidic, low-oxygen conditions linked to redox dynamics. Despite documented fluid escape activity, no clear imprints of active seepage were observed in the studied pockmark sediments. These findings emphasize the dual control of geomorphology and anthropogenic forcing on geochemical processes in shallow marine cold-seep environments, with implications for trace metal mobility, sediment trapping efficiency, and environmental monitoring in similar settings.