the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Oct 2025
Imprint of minute hydrocarbon seepage on solid phase and pore water geochemistry in organic-poor subseafloor sediment
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.
Status: open (until 14 Dec 2025)
- CC1: 'Comment on egusphere-2025-4851', Andreas, P. Teske, 30 Oct 2025 reply
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
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- 1
The current version of this manuscript is much improved compared to previous versions from one or two years ago. I noticed that the current version of the manuscript includes a detailed bathymetric map with clearly defined pockmarks, a much-needed improvement over the previous version that contained no bathymetry whatsoever. Also, the concentration profiles are now shown in a high-resolution figure where data points are color-coded by sampling region – a marked improvement over the previous version that showed only linearized functions (in greyscale) and no datapoints at all (and it was all hidden in the supplements, if I recall this correctly). So, I am happy to see these improvements that really make a difference.
Lines 179ff: I guess that these sediment samples (3 cm3) were used for methane and CO2 concentrations, as indicated by figure 1C? It would be helpful to mention this explicitly.
Table 1. In terms of fluxes, there seems to be a gap of two orders of magnitude (1 mmol x m2 x d-1 to 100 mmol x m2 x d-1) between low flux seeps and moderate seeps. Is this a “grey zone” where current data are not sufficient to draw a clearer distinction?
In seep cores, do you observe covariance between iron and sulfur or sulfide concentrations in the seep cores, something to be expected since sulfide traps reactive iron and keeps it out of porewater? The manuscript text is specifically outlining the scenario in lines 550 ff, and discusses sulfide mineral formation in deeper sediments and at the SMTZ (below the range of these cores) in several paragraphs that follow; however I did not see any plots examining the Iron-sulfide relationship in any detail. Even the manuscript does not contain solid-phase data on sulfides, the porewater data are available (Fe concentrations in Figure 2h; H2S concentrations in Supplementary Figure S3), a clear case of low-hanging fruit. Other studies have examined the links between Fe and S concentrations in seep sediments (for subsurface cores, see Kars, M., Pastor, L., Burin, C., Koornneef, L.M.T., 2025. doi:10.14379/iodp.proc.385.207.2025; for push cores, see Rochelle-Bates et al. 2024, Doi:10.3389/fmicb.2025.1523696, Fig. S3). To summarize, frequently discussing metals and sulfides in the text calls for checking [and plotting] the data.