text: The Cariaco Basin in Venezuela has been used to study the cycle of organic material in euxinic marine environments. An increase in productivity coincident with post glacial nutrient loading probably caused a transition from oxic to anoxic and subsequently euxinic conditions around 14.5 thousand years ago. High productivity at the surface produces a rain of particulate organic matter to the sub surface where anoxic, sulfidic conditions persist. The organic matter in this region is oxidized with sulfate, producing reduced sulfur (HS) as a waste product. Free sulfur exists deep in the water column and up to 6m in depth in the sediment.