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

Abstract

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 ∑CO 2 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 δ 13C of ∑CO 2 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 δ 13C of carbonates, ∼−15‰ to −19‰, is relatively restricted in range compared to pore water ∑CO 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 δ 13C values of pore water to heavier values than C org reactant sources. At one offshore site (∼7 m), about 22% of pore water ∑CO 2 has undergone reduction during methanogenesis below the mobile surface zone, extracting ∑CO 2 with δ 13C∼−90‰ and leaving a residual δ 13C∼−0.37‰. Diagenetic processes in the suboxic mobile mud zone dominate C remineralization and storage along the coast of Amapá.