Description: The impact of oxygen on the preservation of organic matter in marine surface sediments is still controversial. We revisited this long-standing debate by determining the burial efficiency of sedimentary organic matter in the Black Sea, the largest anoxic and euxinic basin in the modern ocean. Surface sediments were sampled in the Danube paleodelta on the northwestern margin of the Black Sea at 420–1550 m water depth. Steady-state modeling of solid species (particulate organic carbon and nitrogen) and solutes (ammonium, sulfate, and total alkalinity) in sediments was performed to quantify rates of mass accumulation, particulate organic matter (POM) degradation, and POM burial. We develop a novel analytical model to quantify these rates applying an inverse modelling approach to down core data accounting for molecular diffusion, sediment burial and compaction. Our model results indicate that 56.7 ± 6.6 % of the particulate organic matter deposited in the study area is not degraded in surface sediments but accumulates below 10 cm sediment depth. This burial efficiency is substantially higher than those previously derived for seafloor areas underlying oxygenated bottom waters. Hence, our study confirms previous studies showing that euxinic bottom water conditions promote the preservation of particulate organic matter in marine sediments.

Description: Highlights • This study simulates the sedimentation-driven development of multiple stacked BSRs in the Danube paleo-delta, Black Sea. • Formation of multiple BSRs in the Black Sea is controlled by the sequence of sedimentation events of the levees induced by sea-level changes. • Kinetics of phase transitions plays a key role in the coexistence, location, and timing of the multiple BSRs. • Development of multiple stacked BSRs is possible only under a narrow range of parameters, unique for the Danube delta setting. Abstract The gas hydrate stability zone (GHSZ) is defined by pressure-temperature-salinity (pTS) constraints of natural gas hydrate (GH) system. It refers to a depth interval which usually extends several hundred meters into the sediment column at sufficient water depths. The lower boundary of the GHSZ often coincides in seismic reflection data with a bottom simulating reflector (BSR), which indicates the transition between the underlying free gas and the overlying no-free gas zone at the thermodynamic stability boundary. The GHSZ in geological systems is dynamic and can shift in response to sedimentation processes and/or changes in environmental conditions such as bottom water temperatures, hydrostatic pressure, and water salinity. The appearance of multiple BSRs has been interpreted as remnants of former GHSZ shifts which have persisted over geological timescales. In this study, we numerically simulate the sedimentation-driven development of multiple stacked BSRs in the Danube deep-sea fan in the Black Sea. We show that in this dynamic sediment depositional regime sufficient amounts of residual gas remain trapped in the former GHSZ, given sufficiently high initial gas hydrate saturations, so that paleo-BSRs could persist over long time scales (similar to 300 kyr). In particular, the formation and persistence of multiple BSRs in the Danube Delta is controlled by the sequence of sedimentation events of the levees induced by sea-level change. The kinetics of methane phase transitions between gas hydrate, dissolved methane, and free gas plays a key role in the coexistence, location and timing of the multiple BSRs. Thus, For a given permeability, distinct multiple BSRs appear only for a narrow range of GH formation (10(-14) 〈 k(f) [mol/m(2) Pa s] 〈= 10(-12)) and dissociation rates (10(-16) 〈 k(d) [mol/m(2) Pa s] 〈 10(-14)).