The transport of metal-binding humic substances and dissolved iron from a hydrothermal vent site (TAG, North Atlantic)

Hannah Whitby1, David Gonzalez Santana2, Marie Cheize3, Arthur Gourain4, Thomas Holmes5, Yoan Germain6, Cecile Cathalot6, Ewan Pelleter7, Yves Fouquet8, Geraldine Sarthou9 and Hélène Planquette9, (1)IUEM Institut Universitaire Européen de la Mer, LEMAR, Plouzané, France, (2)Institut Universitaire Européen de la Mer, Université de Bretagne-Occidentale, Plouzané, France, (3)LEMAR, Institut Universitaire Européen de la Mer, Brest, France, (4)University of Liverpool, Earth Ocean & Ecological Sciences, Liverpool, United Kingdom, (5)University of Washington Seattle Campus, School of Oceanography, Seattle, WA, United States, (6)IFREMER, Department of Marine Geosciences, Plouzané, France, (7)Ifremer, centre de Brest, LGM, Plouzané, France, (8)IFREMER, Plouzané, France, (9)IUEM Institut Universitaire Européen de la Mer, LEMAR, CNRS, Plouzané, France
Originating from the decomposition of cell-derived products, humic substances (HS) compose around half of DOC in seawater, with a small fraction (around 5%) able to bind metals such as iron [1]. The stability of iron-humic complexes allows dissolved iron (DFe) to be transported long distances by ocean currents, while recent work suggests humic complexation may buffer the deep ocean iron inventory [2]. Hydrothermal vents along mid-ocean ridges are considered a significant source of trace elements to this deep ocean inventory [3]. Basin scale DFe transport has been observed from the fast-spreading East Pacific Rise, with the far-field hydrothermal plume consisting of background DFe plus an unidentified organically-bound DFe fraction of hydrothermal origin [4]. Although humic-like fluorescence is often elevated around hydrothermal systems, there is very little quantitative data on metal-binding humics specifically.

To investigate the organic ligand pool supporting hydrothermal-derived DFe at a slow-spreading ridge, samples were collected during the HERMINE (GEOTRACES GPrA07) cruise around the TAG vent site in March 2017. Here we present the concentrations of dissolved metal-binding HS and compare results to DFe transport along the detailed transect. High-resolution sampling around the vent consisted of 5 stations in the first 2 km, followed by a 75 km transect of 4 additional stations following the plume using L-ADCP data. In the upper 2800 m at the vent site, HS concentrations represented typical open ocean values. In contrast, within the rising and non-buoyant plume, HS concentrations were much more variable and included the highest and lowest concentrations of the water column, but all at iron-binding equivalents well below (~1-20%) DFe concentrations. However, by 10 km away, DFe concentrations fell to within the range of iron-binding equivalents of the HS, suggesting a sizeable proportion of transported DFe (~40-100%) could be accounted for by complexation with HS.

[1] Laglera et al. (2009) Limnol. Oceanogr. 54, 610-619

[2] Whitby et al. (under review) Nat. Commun.

[3] Tagliabue et al. (2010) Nat. Geosci. 3, 252–256.

[4] Fitzsimmons et al. (2017) Nat. Geosci. 10, 195–20.