Indicators of inconspicuous hydrocarbon seepage in marine sediments of the southwestern Barents Sea

Seepage is the migration of gas or liquids from reservoirs into overlying sediments. Hydrocarbon seeps influence the biogeochemical cycling of sulfur, nitrogen, and carbon, as well as the composition and activity of microbial communities in sediments, by supplying additional nutrients. In addition to the classic, easily visible seeps with surface expressions like gas flares or carbonate crusts, less noticeable seep forms, so-called inconspicuous seeps, exist, which are often difficult to detect, especially since they do not form obvious surface manifestations. However, inconspicuous seeps play a significant role, as they make up the majority of seep occurrences and thus have an important role in the global element cycles. The investigation of such subtle seepage signals is thus highly relevant. It expands our understanding of coupled sulfur, nitrogen, and carbon cycling, provides insights into microbial adaptation strategies, and may, through systematic analysis of geochemical, mineralogical, and microbial indicators, serve as a proxy for underlying hydrocarbon reservoirs. Based on these considerations, the study addresses three central research questions: (1) What are the effects of low-intensity, diffuse seepage on redox zonation and element cycling in sediments? (2) Can persistent biogeochemical and microbial signatures be identified that remain measurable under weak seepage conditions? (3) What is the diagnostic potential of such signatures for detecting active or fossil seepage, and are there potential applications in hydrocarbon exploration? To answer these questions, I analyzed 56 gravity cores that were collected in two field campaigns in the southwestern Barents Sea (SV21 and GS23). All cores were investigated using a multi-proxy approach, combining pore water chemistry, microbial activity measurements, including e.g., sulfate turnover rates determination, solid-phase analyses (XRF, FT-ICR-MS), gas analyses, cell counts, and molecular analyses of microbial communities (DNA, RNA). This integrative approach links classical geochemical markers with microbial processes and genetic potential to capture a comprehensive picture of subtle hydrocarbon seepage. The investigations revealed that even subtle hydrocarbon seepage leaves measurable geochemical, mineralogical, and microbial signatures in subsurface sediment, but not all parameters are equally affected. Parameters such as cell abundance and gas content showed no differences between seepage-impacted and reference sites, while porewater parameters, sediment compositions, and microbial signatures were found to be potential indicators. The data also revealed pronounced spatial variability both between different seeps and within individual seeps sites. This local heterogeneity underscores that the expression of geochemical and microbial signatures is strongly dependent on the specific sediment microenvironment, highlighting the need for sufficient spatial coverage and replication in exploratory applications to reliably detect seepage signals. Reference samples were found to be significantly more uniform. Although only a single time point was measured, the sedimentary geochemical signatures, e.g., elevated sulfur concentrations, suggest that even minor, continuous seepage inputs can leave cumulative traces in the sediment over extended periods. This study thus demonstrates that diffuse hydrocarbon seeps, despite their low intensity, leave measurable geochemical and microbial imprints. This provides valuable insights for studying biogeochemical cycles in oligotrophic sediments and highlights potential for exploratory applications, where robust, multi-proxy-based signatures can be used to reliably identify even weakly expressed seeps. Local variability and possible cumulative effects over extended periods should also be taken into account.