Stable carbon isotope cycling in mobile coastal muds of Amapa, Brazil

Approximately 10% of the sediment delivered by the Amazon River moves northwest along the coast of Amapa, Brazil, initiating the Guianas mobile: mud belt. Amapa coastal muds generally have a two-layer transport structure and are characterized by highly non-steady-state sedimentation. Isotopic compositions of pore water SigmaCO(2) and solid phase C-org demonstrate that remineralization in the surficial mobile zone (similar to 0.3-1 m thick) is dominated by terrestrial sources at sites in proximity to the mangrove fringe (similar to 1 2 m water depth), and marine (plus possible carbonate dissolution) sources further offshore (similar to 21 km, similar to 7 m depth). The net delta(13)C Of SigmaCO(2) produced and C-org remineralized is similar to -26parts per thousand and -25.9parts per thousand, respectively inshore, and similar to - 14parts per thousand and - 18.6parts per thousand, respectively offshore (compared to average terrestrial and marine C-org end members of -28parts per thousand and -20parts per thousand.). Efficient remineralization in the suboxic mobile zone lowers particle surface loading Of C-org from similar to0.35mg C m(-2) in the Amazon delta topset to similar to0.13-0.16 along Amapa. Sequential, temperature-dependent extractions were used to operationally fractionate inorganic C pools. Authigenic carbonates, mostly siderite and mixed Ca, Mg, Fe, Mn-carbonates, dominate sediment inorganic C (similar to50-200 mumol g(-1)). The mass weighted delta(13)C of carbonates, -15parts per thousand to -19parts per thousand, is relatively restricted in range compared to pore water SigmaCO(2), implying most precipitation in the reactive mobile surface sediments. Periodic mixing with bottom seawater and/or dissolution of biogenic carbonates in the surficial layer shift delta(13)C values of pore water to heavier values than Corg reactant sources. At one offshore site (similar to7m), about 22% of pore water SigmaCO(2) has undergone reduction during methanogenesis below the mobile surface zone, extracting SigmaCO(2) with delta(13)C similar to -90parts per thousand and leaving a residual delta(13)Csimilar to -0.37parts per thousand. Diagenetic processes in the suboxic mobile mud zone dominate C remineralization and storage along the coast of Amapa. (C) 2002 Elsevier Science Ltd. All rights reserved.