Variations of calcium carbonate, organic carbon and iron sulfides in anoxic sediment from the Orca Basin, Gulf of Mexico

Sediment from the anoxic Orca Basin is highly laminated and enriched in iron sulfides and organic carbon, unlike sediment from the surrounding northwest Gulf of Mexico continental slope. Black laminae are enriched in calcium carbonate (19 vs. 15%), organic carbon (1.8 vs. 0.7%) and iron sulfides (0.7 vs. 0.4%) compared to light gray laminae. However, more of the FeS is converted to FeS 2 in the light gray laminae. Dark gray laminae of intermediate composition are also common. The laminae probably reflect annual and longer-term variations in organic matter supply to the basin and variations in mixing at the brine—seawater interface.