GCA: The characteristics of Fe speciation and Fe-binding ligands in the Mariana back-arc hydrothermal plumes

Hu Wang ^a, Joseph A. Resing ^{b, c},Qiaoyang Yan ^a, Nathaniel J. Buck ^{b, c}, Susanna M. Michael ^{b, c}, Haiyang Zhou ^a, Meitong Liu ^a, Sharon L Walker ^c, Qunhui Yang ^a, Fuwu Ji ^a

a State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, P R China

b Joint Institute for the Study of the Atmosphere and the Ocean, University of Washington, Seattle, Washington 98105, USA

c  Pacific Marine Environmental Laboratory-NOAA, Seattle, Washington 98115, USA

Abstract：

We investigated the speciation of Fe and distributions of Fe-binding ligands in the hydrothermal plumes over high- and low-temperature vents and over diffuse venting fields above the Mariana back-arc spreading center. The concentrations of ligands ([L]) and conditional stability constants (K’_FeL) of the natural Fe ligand pool were measured by a reverse titration-competitive ligand exchange-adsorption cathodic stripping voltammetry (RT-CLE-ACSV). The results showed that the buoyant and non-buoyant plume samples over the low-temperature Burke had the highest dissolved Fe/total Fe (DFe/TFe) of 82.2 ± 8.8% and labile Fe/total Fe (Fe_Lab/TFe) of 43.3 ± 5.6%. In contrast, in the plume samples above the high-temperature Perseverance field, TFe had the lowest proportions of DFe (48.8 ± 12.2%) and Fe_Lab (19.4 ± 8.8%). The linear relationships between Fe species and total Mn (TMn) in the buoyant plume over Burke suggest a simple conservative mixing with ambient seawater, which resulted in constant values of DFe/TFe. However, in the non-buoyant plume over Burke, DFe/TFe decreased with plume dilution. The plume samples sourced from the diffuse flow over newly erupted lava had the highest proportion of DFe present as Fe_Lab (74.6 ± 4.0%). The [L] in the buoyant plumes over low-temperature Burke vent were up to 113.6 nM, and the logK’_FeL decreased with increasing DFe and [L], suggesting the importance of weaker ligands in stabilizing Fe. For all the plume samples, the organically complexed Fe (FeL) constituted significant proportions of the DFe (29.0 ± 9.3%) and Fe_Lab (57.5 ± 15.6%) fractions. These Fe-binding ligands were likely sourced from diffuse venting fluids adjacent to the venting sites, and may be also produced by microbes within the hydrothermal plumes.

Full article：https://doi.org/10.1016/j.gca.2020.09.016