Biogeochemical speciation of cryospheric trace metals at the seawater-surface interface of the Arctic Ocean

Aerosol inputs are a significant source of many trace elements to the surface ocean. However, the physicochemical speciation of aerosol-sourced metals can vary, with important consequences for the lifetime and bioavailability of the metal fluxes. In the Arctic Ocean, dry (dust) and wet (snow) deposition supply trace metals to the cryosphere (sea ice, melt ponds, seawater), which then interact with each other and with surface seawater over variable timescales that can affect their speciation and fate. In the simplest case, dust and snow are deposited onto sea ice, which melts to form ponds that experience temporal changes in salinity, temperature, and pH, creating an “incubator” in which biogeochemical transformations can occur. These melt ponds then mix with underlying seawater on variable timescales. Here, we present size-fractionated dissolved (<0.2μm) and truly soluble (<0.02μm, <10kDa) concentration data for Fe, Mn, Cu, Cd, Zn, Ni, and Pb from sea ice, melt ponds, and snow samples, alongside measurements from the underlying surface seawater, at five discrete locations. Our motivation was to understand how metal transformations within the Arctic cryosphere might influence overall dissolved metal fluxes to seawater through changes in speciation and subsequent aggregative losses. Based on prior work in sea ice cores, we expected that any observed sea ice melt would convey high concentrations of dissolved metals with significant colloidal contributions. Our results showed that dissolved metals fall largely in the colloidal size fraction in snow, while in seawater many metals are preferentially found in the smaller soluble size fraction, with sea ice and melt pond speciation falling in between. We will discuss concentration and size partitioning trends in the context of the processes influencing Arctic cryospheric samples, including exposure to UV light, variable biogeochemical conditions, reactions with particle surfaces, and biological production.