On the origin of highly active biogeochemistry in deeper coastal sediments – inverse model studies

Abstract

Abstract. In coastal sediments, zones with highly active diagenesis may be situated below the usually studied first decimeters. Pore water profiles from a backbarrier tidal flat in the German Wadden Sea display a SO4 minimum zone associated with a distinct NH4 peak at a sediment depth of around 1.5 m. Such evidence for significant degradation of organic matter (OM) is challenging our understanding of tidal flat biogeochemistry as little is known about processes that rapidly transfer reactive OM into layers far distant from the sediment-water interface. We here test and compare two different scenarios for OM transfer: scenario A assumes rapid sedimentation and burial of OM, scenario B assumes lateral advection of suspended POM. A diagenetic model is adapted to describe both hypotheses. Uncertain process parameters, in particular those connected to OM degradation and (vertical or lateral) transport are systematically calibrated using existing data. We found that both scenarios, advection and sedimentation, have solutions consistent with the observed pore water profiles. Constrained process parameters are within the range of reported values. Solutions to scenario B describing advective transport of particulate material are, however, rather improbable due to highly specific assumptions on the OM source and flow geometry. In the alternative deposition set-up, model simulations suggest that the source OM was deposited about 60 yrs earlier (1945). A mean sedimentation rate of approximately 2 cm yr−1 indicates substantial changes in near coast sediment morphology, since sea level rise is at a much lower pace. High sedimentation rates most probably reflect the progradation of flats within the study area. These or similar morphodynamic features also occur in other coastal areas so that regional values for OM remineralization rates may often be much higher than predicted from surface biogeochemistry.