Nutrient and iron cycling in a modern analogue for the redoxcline of a Proterozoic ocean shelf

Abstract

Proterozoic oceans are characterized by a thin veneer of oxygenated surface waters underlain by anoxic or euxinic deeper waters, and it has been hypothesized that metabolically flexible microbes such as cyanobacteria would have thrived under these conditions. However, relatively little is known about the relationship between water chemistry, microbial mats, and the biogeochemical cycling of macronutrients (C, N, P, and S) and Fe in these redox transition zones. We investigated the burial of these elements in a modern analogue for the Proterozoic to constrain these relationships better. The low-oxygen and high sulfate water chemistry of Middle Island Sinkhole (MIS; a sinkhole within Lake Huron) allows for the proliferation of a microbial mat community with diverse metabolisms within the photic zone at the sediment-water interface similar to what has been hypothesized following the Great Oxidation Event (~2.35 Ga). Sediments from MIS were compared to those of a fully oxygenated Lake Huron control site in order to determine how water chemistry and microbial mat presence impact biogeochemical cycling. MIS sediments have a greater accumulation of macronutrients than Lake Huron, with stable C isotopes and C:N ratios demonstrating that the C buried within MIS and Lake Huron sediments comes from the same phytoplankton source. Given that both sites bury C from the same source, the increased organic matter burial in MIS is not dependent on microbial mat presence and instead must be controlled by differences in preservation. Sequential extraction of iron indicates that MIS is ferruginous, but not euxinic, and that Lake Huron is oxic; both results are consistent with measured water chemistry. At the same time, the presence of ferruginous iron speciation in sediments under a partly oxygenated water column suggests that the sediment is recording either rapid transformations across the redoxcline rather than the whole water column, some in situ re-speciation of iron, or both. This has important implications for our interpretations of Fe speciation results in the geologic record. Taken together, nutrient and Fe burial in the analogue MIS site demonstrate that Proterozoic biogeochemical cycling in surficial sediments may be predominately driven by a combination of water column and porewater redox chemistry, and not microbial activity.