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  • Center for Marine Environmental Sciences; MARUM  (2)
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  • 1
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    PANGAEA
    In:  Supplement to: Blumenberg, Martin; Pape, Thomas; Seifert, Richard; Bohrmann, Gerhard; Schlömer, Stefan (2018): Can hydrocarbons entrapped in seep carbonates serve as gas geochemistry recorder? Geo-Marine Letters, 38(2), 121-129, https://doi.org/10.1007/s00367-017-0522-6
    Publication Date: 2023-03-03
    Description: The geochemistry of seep gases is useful for an understanding of the local petroleum system. Here it was tested whether individual light hydrocarbons in seep gases are representatively entrapped in authigenic carbonates that formed near active seep sites. If applicable, it would be possible to extract geochemical information not only on the origin but also on the thermal maturity of the hydrocarbon source rocks from the gases entrapped in carbonates in the past. Respective data could be used for a better understanding of paleoenvironments and might directly serve as calibration point for, amongst others, petroleum system modeling. For this approach, (sub)-recent seep carbonates from the Black Sea (Paleodnjepr region and Batumi seep area), two sites of the Campeche Knoll region in the Gulf of Mexico, and the Venere mud volcano (Mediterranean Sea) were selected. These seep carbonates derive from sites for which geochemical data on the currently seeping gases exist. During treatment with phosphoric acid, methane and higher hydrocarbons were released from all carbonates, but in low concentrations. Compositional studies demonstrate that the ratio of methane to the sum of higher hydrocarbons (C1/(C2+C3)) is (partly strongly) positively biased in the entrapped gas fraction. δ13C values of C1 were determined for all samples and, for the samples from the Gulf of Mexico and the Mediterranean Sea, also of C2 and C3. The present dataset from six seep sites indicates that information on the seeped methane can be—although with a scatter of several permil—recorded in seep carbonate matrices, but other valuable information like the composition and δ13C of ethane and propane appears to be modified or lost during, for example, enclosure or at an early stage of diagenesis.
    Keywords: Center for Marine Environmental Sciences; MARUM
    Type: Dataset
    Format: application/zip, 2 datasets
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  • 2
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    Unknown
    PANGAEA
    In:  Supplement to: Kasten, Sabine; Nöthen, Kerstin; Hensen, Christian; Spieß, Volkhard; Blumenberg, Martin; Schneider, Ralph R (2012): Gas hydrate decomposition recorded by authigenic barite at pockmark sites of the northern Congo Fan. Geo-Marine Letters, 32, 515-524, https://doi.org/10.1007/s00367-012-0288-9
    Publication Date: 2023-05-12
    Description: The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates "from above". As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.
    Keywords: Center for Marine Environmental Sciences; MARUM
    Type: Dataset
    Format: application/zip, 7 datasets
    Location Call Number Expected Availability
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