SULFUR DIAGENESIS AND BURIAL AN THE AMAZON SHELF - MAJOR CONTROL BY PHYSICAL SEDIMENTATION PROCESSES

Early diagenetic properties of Amazon shelf muds are dominated by nons ulfidic Fe and Mn cycling, resulting in relatively little S deposition compared to previously studied marine margin environments. Despite ab undant potential reactants typical of sulfidic deposits, authigenic su lfides represent only similar to 10% of diagenetically reduced Fe, and DOP (degree of pyritization) is only similar to 0.02. The average C/S (wt wt(-1)) ratio of buried sediment below the zone of SO42- reductio n is similar to 7.4, similar to 2.6 times more than the commonly assum ed modern shelf average of similar to 2.8. The deltaic burial rate for Sigma S is similar to 0.65 x 10(6) tons yr(-1). Relatively low Sigma S deposition is promoted by terrestrial weathering that delivers react ive 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 rewo rking (similar to 10-100 cm yr(-1) as apparent advection), substantial Sigma S reoxidation (84-98%), and in one case, massive sediment depos ition of up to similar to 5m of sediment in similar to 1 year. Extreme ly low DOP coupled with , dominance by nonsulfidic reduced-Fe minerals and lack of biogenic sedimentary structures may be an indicator in ma rine organic-rich muds of intense physical reworking under oxygenated waters.