Optical-proxies of Particulate Iron Formation Kinetics in Hydrothermal Plumes: A Proof-of-concept Study for Future In-situ Measurements
*Funded through the Ocean Ridge Initiative and the DOEI
Iron fluxes to the ocean through hydrothermal vents may be similar in size to river inputs. The biogeochemical fate of this hydrothermal iron depends, in part, on how far currents transport it before particulate forms precipitate and grow to sizes where they settle out of the water column. Particle formation and evolution under rapidly changing conditions near plumes occur rapidly such that high spatiotemporal-resolution observations are required to determine the reaction timescales. Currently available discrete sampling technologies cannot achieve this resolution, although the samples returned can be analyzed in great detail to determine mineral particle composition. However, the light-absorption and -scattering properties of these particles can be measured rapidly and directly at depth using commercially available sensors. Such optical properties are used routinely as proxy measurements for particle concentration and composition in surface ocean environments. Here, we propose a series of shipboard laboratory experiments that will (I) measure the kinetics of hydrothermal particle formation and (II) test the utility of these sensors in plume environments. At a vent site characterized by high iron concentration, we will collect and return filtered plume water to the shipboard lab. Spectral beam attenuation, absorption and angular scattering, as well as particle composition and mineral phase, will be monitored simultaneously as the plume water oxidizes and particles form. Optical and compositional properties from the experimental timeseries will be analyzed to quantify characteristic optical properties of different forms of particulate iron. This study will provide new insight into hydrothermal plume chemistry and demonstrate the feasibility of rapid, in situ proxy measurements of particles as they form in vent plumes. Final results from this lab study will also include estimates of in situ detection limits, which can guide future field deployments, as well as modification of sensors for future hydrothermal plume applications, if necessary.