Coupling between sedimentary dynamics, early diagenetic processes, and biogeochemical cycling in the Amazon–Guianas mobile mud belt: coastal French Guiana

Abstract

Coastal French Guiana lies within the vast mobile mudbelt that characterizes much of the S. American inner shelf downdrift of the Amazon River. Approximately ∼15–20% of the Amazon sediment outfall moves NW along the Guianas, in part as a series of 20–25 coastal mud waves. Seasonal studies of a representative migrating mudwave located between Kourou and Sinnamary, French Guiana demonstrate that diagenetic properties and biogeochemical cycling are closely coupled to sedimentary dynamics and the refluxing of reduced components between the seabed, coastal facies, and well-oxygenated overlying water. An unsteady two-zone diagenetic regime is typically present in the shallow subtidal and low intertidal regions. A surficial layer, usually ∼0.5–1 m thick, is nonsulfidic, highly reactive, and dominated by Fe, Mn cycling. This suboxic layer unconformably overlies more consolidated sediment in which net SO 4 2− reduction is often observed, and contracts shoreward toward the mangrove fringe as sediments stabilize and become bioturbated. The relatively stable intertidal flats are also sites of significant benthic primary production. Despite suboxic conditions over extensive intervals, anaerobic remineralization rates are high (ΣCO 2∼0.2–0.6 mM day −1), and often show little evidence of attenuation with depth. Integrated ΣCO 2 fluxes are ∼20–235 mmol m −2 day −1 (wet/dry season average ∼120), far exceeding diffusive O 2 fluxes into the bottom. High reactivity (∼0.08 day −1) and low C/N remineralization release ratios (∼5.3) of decomposing material imply fresh substrate of marine origin. Most reduced solid phase Fe is present as nonpyritic authigenic Fe minerals (<20% Fe(II) is in pyrite), and low sedimentary C/S ratios (∼6–8) reflect the generally nonsulfidic conditions. A broad range of tracers (e.g. 234Th ( t 1/2=24 days), 210Pb ( t 1/2=22 years), seasonal Cl − profiles) and nonsteady state diagenetic models of pore water concentrations and oxidant–reductant relationships demonstrate that the upper 0.1–1 m of deposits are reworked and exchanged with overlying water on timescales of <10 days to seasonally. Thus, the seafloor acts as a massive suboxic batch reactor, entraining and processing reactive marine plankton, regenerating Fe, Mn oxides, exchanging metabolites and nutrients with the oxygenated water column, and generating suites of nonsulfidic authigenic minerals. The apparent paradoxical properties of high remineralization rate, low reductant content (C org∼630 μmol g −1), and dominance of suboxic metal cycling are a direct consequence of frequent entrainment of highly reactive organic substrate and simultaneous regeneration of the major Fe oxidants in oxygenated water. The Amazon–Guianas mobile mudbelt is a zone of extraordinarily intense sedimentary and biogeochemical recycling, greatly exceeding stable coastal systems, such as salt marshes, in material exchange with the sea.