Organic matter accumulation and degradation in subsurface coastal sediments: a model-based comparison of rapid sedimentation and aquifer transport

摘要

The redox succession in shallow marine sediments generally exhibits apredictable pattern. Pore water profiles from a back barrier tidal flat inthe German Wadden Sea depart from the expected redox zoning. Instead, asulfate minimum zone associated with a sulfate-methane-sulfate doubleinterface and a distinct ammonium peak at 1.5 m below sea floor (mbsf) isdisplayed. Such evidence for significant degradation of organic matter (OM)in subsurface layers is challenging our understanding of tidal flatbiogeochemistry as little is known about processes that relocate reactive OMinto layers far distant from the sediment-water interface. The objectives ofour model study were to identify possible mechanisms for the rapid transportof organic matter to subsurface layers that cause the reversed redoxsuccession and to constrain several important biogeochemical controlparameters. We compared two scenarios for OM transfer: rapid sedimentationand burial of OM as well as lateral advection of suspended POM. Using adiagenetic model, uncertain process parameters, in particular those connectedto OM degradation and (vertical or lateral) transport, are systematicallycalibrated using field data.We found that both scenarios, advection and sedimentation, had solutionsconsistent with the observed pore water profiles. For this specific site,however, advective transport of particulate material had to be rejected sincethe reconstructed boundary conditions were rather improbable. In thealternative deposition set-up, model simulations suggested the deposition ofthe source OM about 60 yrs before cores were taken. A mean sedimentationrate of approximately 2 cm yr−1 indicates substantial changes in near coasttidal flat morphology, since sea level rise is at a much lower pace. Highsedimentation rates most probably reflect the progradation of flats withinthe study area. These or similar morphodynamic features also occur in othercoastal areas so that inverted redox succession by horizontal or verticaltransport may be more common than previously thought. Consequently, regionalvalues for OM remineralization rates may be higher than predicted fromsurface biogeochemistry.