Removal of dissolved arsenic from deep seawater around hydrothermal vents and seamounts

Abstract

Oceanic cycling of arsenic (As) is closely linked to that of nutrient and trace metal elements such as phosphorus (P) and iron (Fe), primarily due to similar particle-reactivity of As and P and their association with particulate carriers including Fe (oxyhydr)oxides. However, this particle scavenging effect is rarely reflected in seawater depth profiles of total dissolved inorganic As (DAs), which typically resemble those of nutrients, generally showing increasing concentrations with depth. Departing from conventional views, we observed distinct decreases in DAs concentration in deep waters around independent deep-sea systems in the subtropical western North Pacific: hydrothermal vents, seamounts, and island sediments. DAs removal corresponds to elevated dissolved and total dissolvable particulate Fe concentrations, indicating a major control of particle adsorption on As behavior in specific deep-ocean regions. Particle scavenging effect varied among the three deep-sea regions, mainly ascribed to varying particulate elemental compositions, and influence of temperature, pH, and dissolved oxygen. Our findings highlight a previously overlooked sink term for DAs removal from seawater by particles of various deep-sea origins. Newly estimated output fluxes around hydrothermal or seamount systems are comparable to the individual input flux from rivers, atmosphere, and hydrothermal vents, thus helping to balance the global oceanic As budget.