Sulfur diagenesis and burial on the Amazon shelf: Major control by physical sedimentation processes

Abstract

Early diagenetic properties of Amazon shelf muds are dominated by nonsulfidic Fe and Mn cycling, resulting in relatively little S deposition compared to previously studied marine margin environments. Despite abundant potential reactants typical of sulfidic deposits, authigenic sulfides represent only ~ 10% of diagenetically reduced Fe, and DOP (degree of pyritization) is only ~0.02. The average C/S (wt wt−1) ratio of buried sediment below the zone of SO42- reduction is ~ 7.4, ~ 2.6 times more than the commonly assumed modern shelf average of ~ 2.8. The deltaic burial rate forΣS is ~ 0.65 × 106 tons yr−1. Relatively lowΣS deposition is promoted by terrestrial weathering that delivers reactive oxide debris, but apparently depends most strongly on reoxidation and rapid burial by intense physical reworking and fluid-mud formation. Diagenetic models of S distributions demonstrate rapid sediment reworking (~ 10–100 cm yr−1 as apparent advection), substantialΣS reoxidation (84–98%), and in one case, massive sediment deposition of up to ~ 5 m of sediment in ~ 1 year. Extremely low DOP coupled with dominance by nonsulfidic reduced-Fe minerals and lack of biogenic sedimentary structures may be an indicator in marine organic-rich muds of intense physical reworking under oxygenated waters.