Variations in pyrite texture, sulfur isotope composition, and iron systematics in the Black Sea: evidence for Late Pleistocene to Holocene excursions of the o 2-h 2s redox transition

Abstract

—Time-dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the Black Sea. In the laminated, deep-basin sediments of Unit I and Unit II, >86% of pyrite particles are present as fine-grained framboidal aggregates. In these sediments, the dominance of pyrite framboids, with a narrow size distribution and maximum size < 18 μm, is evidence of syngenetic (water column) pyrite formation subjacent to the O 2-H 2S boundary. Sediments from the basin margin collected below the impingement of the O 2-H 2S boundary contain an increased proportion of fine-grained euhedral grains of pyrite relative to framboidal aggregates, suggesting increased additions of diagenetic pyrite below the sediment-water interface. The more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins, and is tied to increasing %pyrite as framboids and greater degrees of pyritization (DOP) in the basin center relative to the basin margins. An evaluation of Fe-S-C relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of Unit III, C-limited for upper Unit III due to the input of sulfate-rich Mediterranean seawater, and Fe-limited in the laminated, organic carbon-rich sediments of Units I and II. These changes in pyrite-limiting factors occurred within 4 m of compacted burial representing <15 ka. Sediment fabric and framboid size distributions at a shallow, basin-margin site suggest that during two extended periods of Unit II deposition, the O 2-H 2S transition retreated to depths below 200 m. However, during these same intervals, the δ 34S of pyrite remains uniform compared to overlying and underlying laminated sediments, implying that this chemical signature may not be uniquely tied to the position of the O 2-H 2S boundary relative to the sediment-water interface, but rather is likely related to biogeochemical processes.