Preprints
https://doi.org/10.22541/essoar.175399495.59610535/v1
https://doi.org/10.22541/essoar.175399495.59610535/v1
07 Oct 2025
 | 07 Oct 2025
Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Imprint of minute hydrocarbon seepage on solid phase and pore water geochemistry in organic-poor subseafloor sediment

Ellen Schnabel, Jessica A. Stammeier, Stefanie Poetz, Kai Mangelsdorf, Aurèle Vuillemin, Rolando di Primio, Jens Kallmeyer, and the PROSPECTOMICS Consortium

Abstract. In marine environments, small hydrocarbon (HC) fluxes through organic-poor sediments are often fully degraded by microorganisms before reaching the seabed. Yet, these fluxes influence sediment geochemistry by stimulating microbial activity. We analyzed 50 gravity cores from the southwestern Barents Sea, covering zones affected by inconspicuous HC seepage and unaffected reference zones. Using various organic and inorganic geochemical analyses of the sediment along with pore water geochemistry, we assess the effects of low-intensity seepage and identify potential geochemical signatures.

While analysis of the organic geochemical analyses provided limited insights, inorganic geochemical analyses revealed formation of minerals such as carbonates and sulfides, linked to microbial reductive processes. Element concentrations suggested that HC degradation leaves distinct signatures, particularly in redox-sensitive minerals. Pore water profiles in HC-affected zones showed significant variation, indicating carbonate precipitation. In contrast, sediments not affected by HC seepage displayed more uniform pore water profiles. Estimated fluxes of sulfate, calcium, and alkalinity varied notably between cores, particularly in HC-affected zones, suggesting local and potentially transient differences in seepage intensity.

While microbial HC degradation likely occurs deeper than our sampling interval, high-resolution geochemical analysis of both sediment fractions and pore water revealed a clear imprint of HC seepage. This imprint, reflected in authigenic minerals and pore water gradients, allows identification of past and present seepage activity, with authigenic minerals providing evidence for past seepage, and pore water profiles informing about ongoing seepage.

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Ellen Schnabel, Jessica A. Stammeier, Stefanie Poetz, Kai Mangelsdorf, Aurèle Vuillemin, Rolando di Primio, Jens Kallmeyer, and the PROSPECTOMICS Consortium

Status: open (until 18 Nov 2025)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
Ellen Schnabel, Jessica A. Stammeier, Stefanie Poetz, Kai Mangelsdorf, Aurèle Vuillemin, Rolando di Primio, Jens Kallmeyer, and the PROSPECTOMICS Consortium

Data sets

Pore water solutes, total cell counts, dissolved gases and sulfate reduction rates of the 50 gravity cores of the PROSPECTOMICS project Ellen Schnabel et al. https://doi.org/10.1594/PANGAEA.974341

XRF measurements of major oxides and trace elements on bulk sediment of the 50 gravity cores of the PROSPECTOMICS project Ellen Schnabel et al. https://doi.org/10.1594/PANGAEA.974346

Ellen Schnabel, Jessica A. Stammeier, Stefanie Poetz, Kai Mangelsdorf, Aurèle Vuillemin, Rolando di Primio, Jens Kallmeyer, and the PROSPECTOMICS Consortium
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Latest update: 07 Oct 2025
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Short summary
Even minor hydrocarbon seepage from reservoirs into overlying seafloor can affect marine sediment biogeochemistry. Our analysis of 50 sediment cores from the Barents Sea revealed that subtle seepage alters chemical balances and stimulates carbonate and sulfide mineral formation. The cores also showed strong local and spatial variation, highlighting patchy migration pathways. Minerals and pore water chemistry together provide clues to identify both past and ongoing seepage activity.
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