The influence of water column and porewater trace metal bioavailability on sediment geochemistry in a Proterozoic ocean analogue

Redox sensitive trace metals such as Fe and Mo are widely used proxies for reconstructing oxygenation dynamics of ancient aquatic systems. However, while traditional interpretations of sediment geochemistry are based upon euxinic marine systems with an abundance of free HS^- present within the water column (i.e., the Black Sea), fewer studies consider metal burial in environments without abundant O₂ or HS^- in the water column. In addition, for ancient systems, sediment geochemistry is typically considered to be directly connected to water column chemistry, and the influence of early diagenetic processes is often neglected. This work aims to address these knowledge gaps by considering how these redox proxies behave within a ferruginous system: the Middle Island Sinkhole (MIS), a karst feature located 23 m beneath the surface of Lake Huron (USA). The sinkhole water chemistry (low O₂, high SO₄^2-) supports the growth of microbial mats at the sediment-water interface, which include metabolically flexible cyanobacteria and sulfur-oxidizing and reducing bacteria. With water chemistry and microbiology similar to that inferred for the Proterozoic, MIS is considered to be an analogue for Proterozoic oceans, which allows us to interpret ancient biogeochemistry in the context of modern geochemical results. Without appreciable sulfide present in the water column, the degree of pyritization of MIS sediments must instead be due to significant porewater sulfide, a mechanism that is largely not considered in ancient systems. Sediment Fe geochemistry indicates that MIS is oxygen-poor and Fe-rich, which is consistent with known water chemistry. However, these signals are influenced by the strong redoxcline of the MIS site (<3 m of water column), and not the ~20 m of overlying oxygenated water column. These results indicate that we should take caution when interpreting sediment geochemical results to represent water column processes in the rock record. Furthermore, trace metals such as Mo and Zn feature strong depletions or enrichments across the redoxcline. However, whether these enrichments or depletions translate to the sediment geochemistry is variable, challenging our use of sediment trace element contents to gauge micronutrient availability in ferruginous paleoenvironments.