VARIATIONS IN PYRITE TEXTURE, SULFUR ISOTOPE COMPOSITION, AND IRON SYSTEMATICS IN THE BLACK SEA: EVIDENCE FOR LATE PLEISTOCENE TO HOLOCENE EXCURSIONS OF THE O2-H2S 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 O2-H2S boundary. Sediments from the basin margin collected below the impingement of the O2-H2S 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 O2-H2S transition retreated to depths below 200 m. However, during these same intervals, the δ34S 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 O2-H2S boundary relative to the sediment-water interface, but rather is likely related to biogeochemical processes.