Environmental evolution of the East China Sea inner shelf and its constraints on pyrite sulfur contents and isotopes since the last deglaciation

To better understand the depositional constraints on the sulfate reduction process, we present a set of sulfur isotope data for hand-picked pyrites (δ³⁴S_pyr) from the coarse fraction (63–250 μm) of bulk sediments, coupled with the contents of carbonate, total organic carbon (TOC) and nitrogen (TN), as well as δ¹³C values of the TOC, from sediments of core EC2005 on the inner shelf of the East China Sea (ECS) since 16.4 ka. Our results show that core sediments were deposited in a terrestrial environment before 13.1 ka (stage I) when pyrite rarely occurred. During stages II (13.1–12.3 ka) and III (12.3–12.1 ka) seawater began to influence the core location, forming a tidal flat environment where abundant pyrites are well preserved. Then the inner shelf of the ECS was fully flooded, and two intervals with high (stage IV:12.1–6.0 ka; stage VI: 5.2–1.5 ka) and low sedimentation rate (stage V: 6.0–5.2; stage VII: 1.5 ka-present) are identified respectively. When compared with previous δ³⁴S values of chromium-reducible sulfur (δ³⁴S_CRS) derived from the bulk sediments, δ³⁴S_pyr values are usually higher, indicating the macroscopic pyrites form in the later stages of sedimentary diagenesis when ongoing sulfate reduction has distilled porewater sulfate to a high degree. Considering the wide occurrence of biogenic shallow gas (dominated by CH₄) in these sediments, sulfate-driven anaerobic oxidation of methane (SO₄-AOM) and organoclastic sulfate reduction (OSR) are expected to be two competitive pathways for sulfate reduction. We propose that the changes in sedimentation rate controls the pace of migration of the sulfate methane transition zone (SMTZ). When sedimentation rate is low, the SMTZ may be fixed in a certain depth (e.g., sediments deposited in stage III and V), where H₂S is produced by SO₄-AOM. In contrast, when sedimentation rate is very high (> 1 cm/yr), the SMTZ would be deep, and H₂S is mostly produced by OSR. Further, our results imply that sulfur diagenesis in recent (since 1.5 ka) ECS mud sediments might be influenced by physical reworking (e.g., typhoon) and/or biological activity (bioturbation), resulting in partial reoxidation of pyrite crystals in fluid mud. Our findings therefore demonstrate that pyrite sulfur isotope compositions (bulk δ³⁴S_CRS & hand-picked δ³⁴S_pyr) are sensitive to local environmental evolution and the combined use of these two indicators can provide new insights into pathway of sulfate reduction in unsteady environments.