Depositional Environment and Biological Activity Implications of the Jining BIF, Western Shandong Province, China: Evidence from Elements and C-O Isotopic Compositions

In the early Paleoproterozoic, the Earth’s atmosphere–ocean system shifted from a reducing to an oxidizing state, triggering the extensive deposition of banded iron formations (BIFs) in the Siderian period (2.5–2.3 Ga). As a key sedimentary formed during the hydrospheric oxidation stage, BIFs are expected to preserve abundant microbial fossils or organic carbon. However, evidence for contemporaneous widespread biological activity remains limited. This paper focuses on C-O isotopes and the trace element geochemistry of the 2.5 Ga Jining BIF to constrain the redox state of paleo-oceans and associated biogeochemical cycling during BIF deposition. The δ13Ccarb values of the BIF samples range from −18.6‰ to −9.6‰, with an average of −12.7‰, exhibiting a notable negative value, and TOC contents (0.04–0.19 wt.%) are extremely low. This suggests the incorporation of oxidized organic carbon to pore water via ferrihydrite reduction during early diagenesis process. The globally negative δ13Ccarb value of BIFs and iron-rich carbonates reflect enhanced biological activity at ~2.5 Ga. REE patterns reveal negative Ce/Ce*(SN) and Eu/Eu*(CN) anomalies, and the presence of primary hematite mesobands together indicate that the Jining BIF records a redox transition in seawater from reducing to oxidizing conditions.