The controls on sedimentary pyrite formation in Southern Ocean deep-sea sediments

The formation of pyrite in deep-marine sediments is typically limited by organic carbon or/and reactive iron availability. The Southern Ocean generally exhibits an iron deficiency, limiting bioproductivity and thus the sequestration of CO2 from the atmosphere. Based on iron deficiency in the photic zone, it could be assumed that the formation of pyrite is iron-limited, too.

To evaluate the controls on sedimentary pyrite formation, sediments from ca. 4 Ma to recent, recovered during IODP Expedition 382 (Iceberg Alley) in the Scotia Sea, were examined for their carbon, nitrogen, sulfur, major and minor element contents.

The organic carbon and sulfur contents in the sediments have a mean value of c. 0.4 and 0.3 wt.%, respectively, whilst iron has a mean value of c. 4 wt.%. Organic carbon and sulfur demonstrate a good correlation and a significant increase from c. 1.25 Ma toward the stratigraphically younger. Further, the relationship between iron, sulfur, and organic carbon suggests that sulfur is fixed in pyrite.

The good correlation between sulfur and organic carbon points to a organic carbon limitation for pyrite formation. Also, the excess of iron speaks against an iron limitation for pyrite formation. Therefore, the increase in sulfur and organic carbon contents from the onset of the mid-Pleistocene transition might demonstrate higher export of organic carbon from the photic zone to the ocean floor, and an enhanced formation of pyrite in the sediments.