Formation pathways of light hydrocarbons in deep sediments of the Danube deep-sea fan, Western Black Sea

Pape, T., Haeckel, Matthias, Riedel, Michael, Kölling, M., Schmidt, Mark, Wallmann, Klaus and Bohrmann, Gerhard (2020) Formation pathways of light hydrocarbons in deep sediments of the Danube deep-sea fan, Western Black Sea Marine and Petroleum Geology, 122 (Article number 104627). DOI 10.1016/j.marpetgeo.2020.104627.

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Supplementary data:

Abstract

Highlights

• MeBo drilling in Danube fan down to 147 m recovered limnic to marine deposits.
• Molecular and stable isotope characterization of light hydrocarbons, CO2, and H2O.
• H and O isotopic compositions of pore water reflect paleoclimate variations.
• Isotope relations prove microbial carbonate reduction as major methanogenic pathway.
• Control of δ2H–CH4 by δ2H–H2O may lead to misinterpretation of methanogenic paths.

Abstract

We report on the geochemistry of light hydrocarbons and pore water in sediments down to 147 m below seafloor (mbsf), at two sites within the gas hydrate stability field of the Danube deep-sea fan, Black Sea. Sediments were drilled with MARUM-MeBo200 and comprise the transition from limnic to the recent marine stage. Stable C/N ratios (mean 5.1 and 5.6) and δ13C-Corg values (mean −25.8‰ V-PDB) suggest relatively uniform bulk organic matter compositions. In contrast, pore water δ2H and δ18O values varied considerably from approx. −120‰ to −30‰ V-SMOW and from −15‰ to −3‰ V-SMOW, respectively. These data pairs plot close to the ‘Global Meteoric Water Line’ and indicate paleo temperature variations. Depletions of pore water in 2H and 18O below 40 mbsf indicate low temperatures and likely reflect conditions during (the) last glacial period(s).

Methane was much more abundant than the only other hydrocarbons found in notable concentrations, ethane and propane ((C1/(C2+C3) ≥20,000). Relatively constant δ13C–CH4 (~−70‰ V-PDB) and δ13C–C2H6 (~−52‰ V-PDB) values with depth indicate that methane and ethane are predominantly of microbial origin and that their formation was not limited by carbon availability. In contrast, δ2H–CH4 values varied in a large range (approx. −310 to −240‰ V-SMOW) with depth and positively correlated with trends observed for δ2H–H2O. Isotope separations (Δδ13C(CH4–CO2), Δδ2H(CH4–H2O)) substantiate that microbial carbonate reduction (CR) is the prevalent methanogenic pathway throughout the sediments irrespective of their geochemical history. Remarkably, in δ13C–CH4 – δ2H–CH4 diagrams widely used, samples characterized by δ2H–CH4 values more negative than approx. −250‰ plot out of the field assigned for pure CR. We conclude that assignments of microbial methanogenic pathways based on classical interpretations of δ13C–CH4 – δ2H–CH4 pairs can lead to misinterpretations, as severe 2H-depletions of methane formed through microbial CR can result from 2H-depletions of the pore water generated during low-temperature climatic periods.

Document Type: Article
Keywords: Black sea, Danube deep-sea fan, Light hydrocarbons, MARUM-MeBo200, Methane, Methane formation, Stable carbon isotopes, Stable hydrogen isotopes
Research affiliation: MARUM
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Refereed: Yes
DOI etc.: 10.1016/j.marpetgeo.2020.104627
ISSN: 0264-8172
Projects: SUGAR
Date Deposited: 14 Sep 2020 07:21
Last Modified: 14 Sep 2020 07:21
URI: http://eprints.uni-kiel.de/id/eprint/50473

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