MOBILE DELTAIC AND CONTINENTAL-SHELF MUDS AS SUBOXIC, FLUIDIZED-BED REACTORS

The oft-cited general correlation between net sediment accumulation an d preservation of organic matter, while revealing in many ways, can be a misleading indicator of general elemental cycling processes and con trols on storage of biogenic material at the continental-ocean boundar y. Deltaic environments are characterized by the highest rates of net sedimentation and are the single most important class of depocenters o n Earth. Available data indicate that sedimentary organic C (C-org) of both terrestrial and marine origin is efficiently decomposed in delta ic areas, with decomposition percentages reaching greater than or equa l to 70% and greater than or equal to 90%, respectively, the latter pe rcentage (marine) being quite comparable to deep-sea, low sedimentatio n environments. Despite high primary productivity associated with most deltas and evidence of substantial deposition of fresh planktonic deb ris, patterns of SO4= reduction indicate that the reactivity of organi c material being buried is low, and that a larger proportion of C-org is often degraded compared to other marine deposits of similar net acc umulation rate. As indicated by properties of the surficial Amazon del ta and downdrift coastal region of northeast South America(similar to 1600-km extent), the primary reasons for efficient remineralization ar e related to intense and massive physical reworking of sediment associ ated with estuarine fronts, upwelling, tidal oscillation, and wind-dri ven waves. Fluid muds and mobile surface material cause the seafloor a nd continental boundary to act as a massive, suboxic, fluidized bed re actor dominated in some cases by bacterial rather than macrofaunal bio mass. Reoxidation, repetitive redox successions, metabolite exchange, and continual mixing-in of fresh planktonic debris with refractory ter restrial components, result in an efficient decomposition system large ly decoupled from net accumulation. Similar processes occur on smaller scales in most estuarine-shelf systems, but appear to be most dramati cally expressed off the major rivers forming deltas. (C) 1998 Elsevier Science B.V. All rights reserved.