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

Approximately 10% of the sediment delivered by the Amazon River moves northwest along the coast of Amapá, Brazil, initiating the Guianas mobile mud belt. Amapá coastal muds generally have a two-layer transport structure and are characterized by highly non-steady-state sedimentation. Isotopic compositions of pore water ∑CO₂ and solid phase C_org demonstrate that remineralization in the surficial mobile zone (∼0.3-1 m thick) is dominated by terrestrial sources at sites in proximity to the mangrove fringe (∼1-2 m water depth), and marine (plus possible carbonate dissolution) sources further offshore (∼21 km, ∼7 m depth). The net δ¹³C of ∑CO₂ produced and C_org remineralized is ∼-26‰ and -25.9‰, respectively inshore, and ∼-14‰ and -18.6‰, respectively offshore (compared to average terrestrial and marine C_org end members of -28‰ and -20‰). Efficient remineralization in the suboxic mobile zone lowers particle surface loading of C_org from ∼0.35 mg C m^-2 in the Amazon delta topset to ∼0.13-0.16 along Amapá. 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 (∼50-200 μmol g^-1). The mass weighted δ¹³C of carbonates, ∼-15‰ to -19‰, is relatively restricted in range compared to pore water ∑CO₂, 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 δ¹³C values of pore water to heavier values than C_org reactant sources. At one offshore site (∼7 m), about 22% of pore water ∑CO₂ has undergone reduction during methanogenesis below the mobile surface zone, extracting ∑CO₂ with δ13C∼-90‰ and leaving a residual δ13C∼-0.37‰. Diagenetic processes in the suboxic mobile mud zone dominate C remineralization and storage along the coast of Amapá.