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

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 SO₄^2- reduction is ∼7.4, ∼2.6 times more than the commonly assumed modern shelf average of ∼2.8. The deltaic burial rate for ∑S is ∼0.65 × 10⁶ tons yr^-1. Relatively low ∑S 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), substantial ∑S 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.