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

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 delta C-13-C-org values (mean -25.8 parts per thousand V-PDB) suggest relatively uniform bulk organic matter compositions. In contrast, pore water delta H-2 and delta O-18 values varied considerably from approx. -120 parts per thousand to -30 parts per thousand V-SMOW and from -15 parts per thousand to -3 parts per thousand V-SMOW, respectively. These data pairs plot close to the 'Global Meteoric Water Line' and indicate paleo temperature variations. Depletions of pore water in H-2 and O-18 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 ((C-1/(C-2 + C-3) >= 20,000). Relatively constant delta C-13-CH4 (similar to -70 parts per thousand V-PDB) and delta C-13-C2H6 (similar to -52 parts per thousand 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, delta H-2-CH4 values varied in a large range (approx. -310 to -240 parts per thousand V-SMOW) with depth and positively correlated with trends observed for delta H-2-H2O. Isotope separations (Delta delta C-13(CH4-CO2), Delta delta H-2(CH4-H2O)) substantiate that microbial carbonate reduction (CR) is the prevalent methanogenic pathway throughout the sediments irrespective of their geochemical history. Remarkably, in delta C-13-CH4 - delta H-2-CH4 diagrams widely used, samples characterized by delta H-2-CH4 values more negative than approx. -250 parts per thousand plot out of the field assigned for pure CR. We conclude that assignments of microbial methanogenic pathways based on classical interpretations of delta C-13-CH4 - delta H-2-CH4 pairs can lead to misinterpretations, as severe H-2-depletions of methane formed through microbial CR can result from H-2-depletions of the pore water generated during low-temperature climatic periods.