REMINERALIZATION RATES, RECYCLING, AND STORAGE OF CARBON IN AMAZON SHELF SEDIMENTS

Diagenetic reactions and redox properties of Amazon shelf sediments ar e characterized by extensive vertical and lateral regions of Fe and Mn cycling. This is in contrast to many temperate estuarine and shelf de posits where S can dominate early diagenesis, but may be typical of we t-tropical regions draining highly weathered terrain with energetic co astlines. Although the major pathways of C-org remineralization in sur fical sediments apparently differ from previously studied areas, the a bsolute magnitude and relative importance of benthic decomposition on the Amazon shelf are comparable to many shallow water regions of equiv alent depth range (10-40 m). Net Sigma CO2 production over the upper s imilar to 1-2 m of deposits is >50 mmol m(-2) d(-1) and has a predomin antly planktonic isotopic composition (delta(13)C similar to-21 to -22 parts per thousand), indicating that marine organic matter largely dr ives diagenetic reactions and that greater than or similar to 20% of a verage water-column primary production is metabolized on the seafloor. The Sigma CO2 production rates in the upper 0-5 cm of sediment tend t o increase slightly alongshelf away from the turbid river mouth, but a re relatively uniform within cross-shelf transects at any given season and independent of net sedimentation rate. Near uniformity in surface decomposition rates, despite substantial offshore increases in water- column productivity and net accumulation at the delta front, implies r apid cross-shelf particle exchange by estuarine circulation and tidal currents. Build-up patterns of pore-water Sigma CO2 indicate in some c ases that the upper similar to 20 cm was deposited only a few days pri or to core collection. Benthic Sigma CO2 production is highest during periods of low or falling river flow, but no dramatic seasonality occu rs. O-2 penetrates similar to 2-4 mm into sediments and diffusive O-2 uptake averages similar to 13 mmol m(-2) d(-1) annually. Anaerobic met abolism accounts for >75% of sedimentary remineralization, but C/S bur ial ratios are usually >6 (average world shelf similar to 2.8). Season al patterns in sedimentary Fe oxidation states indicate that sediments can be partially reoxidized to depths similar to 0.6- 1 m during eros ion/redeposition and that subsequent Fe reduction can account for much of the anaerobic decomposition. Diffusive Sigma CO2 fluxes and pore-w ater inventories imply substantial loss of remineralized C to authigen ic sedimentary-carbonate formation or flux imbalances due to nonsteady -state ingrowth of disturbed pore-water profiles. Reoxidation of metab olites and nutrient release to the water column occur during massive p hysical remobilization of sediments. The total benthic N remineralizat ion flux (recycled) is comparable to external riverine and shelf-upwel ling fluxes. During stable seabed periods, however, little or no co-re mineralized N (NH4+, NO3-, NO2-) or P escapes diffusively into overlyi ng water, indicating the potential for loss of up to similar to 100% o f the benthic remineralized N (by denitrification) and P (during authi genic mineral precipitation and adsorption). Overall, the shelf appare ntly acts as an efficient, fluidized-bed denitrification processor (si milar to 50% recycled N) but an inefficient burial sink for riverine a nd upwelled N. In contrast to the atmospheric sink for N, rapidly rege nerated P is eventually lost to the open ocean during sediment resuspe nsion and desorption into the overlying water. Approximately 90% of th e remineralized Sigma CO2 production flux escapes the sediment and sim ilar to 10% is permanently hurled as authigenic carbonate. Despite reo xidation and carbonate dissolution during reworking, net burial of C i s similar to 5 x 10(12) g C-total per year, of which similar to 25-30% is carbonate from remineralized organics (similar to 70% of this frac tion is marine) similar to 20% is residual marine C-org, and the remai ning similar to 50% is residual terrestrial C-org. ''Refractory'' terr estrial POC is apparently subject to repetitive co-oxidation and redox cycling, resulting in remineralization of similar to 65-70% of input and leaving less than similar to 30-35% of the riverine POC flux store d on the shelf.