METALS AND MICROBES AND REDOX, OH MY: MOLYBDENUM ABUNDANCE AND ISOTOPES IN A FERRUGINOUS PRECAMBRIAN ANALOGUE

In order to improve and refine our interpretations of Mo concentrations and isotopes as redox proxies, it is imperative that we develop a framework for Mo in modern anoxic systems. The utility of Mo as a geochemical proxy is dependent on its known behavior under different redox regimes, which thus far has been limited to mostly oxic and sulfide-rich (i.e., euxinic) environments. Few studies consider Mo accumulation under other redox regimes (e.g., anoxic and iron-rich), complicating how we use Mo as a tool to reconstruct paleoenvironmental redox chemistry in general, but especially during times wherein anoxic and iron-rich oceans dominated the landscape (such as the Precambrian; 4.6–0.5 Ga). Furthermore, Mo isotope values in these kinds of systems are often ambiguous: while the Mo isotopic composition of seawater is directly recorded in euxinic sediments, significant isotope fractionations occur under suboxic, anoxic, and low-sulfide water columns.

We are exploring Mo and its isotopes in the waters of Brownie Lake, MN, a ferruginous meromictic lake considered an analogue for Earth’s early oceans. We find that dissolved Mo concentrations are greater in the oxic surface waters (9.6 ± 0.5 µmol L^-1) than the ferruginous deeper waters (1.9 ± 1.0 µmol L^-1). These results imply that much of the Mo in the surface waters remains dissolved as the molybdate anion (MoO₄^2-); this Mo could become adsorbed to Fe(III)-oxyhydroxides that develop in this part of the water column and shuttled to the sediments. Fluctuating sulfide availability in the deeper, iron-rich waters may lead to the formation of reactive thiomolybdates (MoO_xS_(4-x)^2-), which could subsequently be scavenged by Fe(II)-S phases or organic matter, both of which are present in Brownie Lake sediments. Future work will explore Mo abundances and isotopes in particulate matter and sediments, ultimately providing additional insight into Mo isotope systematics in ferruginous systems.