What causes iron survival in a distal hydrothermal plume?: Reversible exchange between dissolved and

Until recently, the contribution of hydrothermal iron to the ocean budget was not considered to be an important factor because many scientists thought that hydrothermal irons were principally removed from hydrothermal plume by precipitating Fe-bearing sulfide and/or oxide minerals near the vent site. However, Chu et al.1 reported that up to 50% of deep-ocean dissolved iron (dFe; < 0.2 ㎛) may have come from hydrothermal sources. In recent years with the advent of high-resolution sampling and analysis of trace metals, other studies have demonstrated that both chemical mechanism and physical processes stabilize dFe released from hydrothermal vent, preventing its oxidation and/or precipitation as insoluble minerals2,3, and thus facilitating its long-range dispersion into the ocean4. However, a study on influence of particulate metals (pFe; > 0.2 ㎛) is notably absent. Feely et al.5 only reported pFe distribution in 15°S, but it is inadequate to evaluate the long-range fate of pFe in distal hydrothermal plume due to limited sampling.

In this regard, Fitzsimmons et al.6 first found that both dFe and pFe deepened progressively by ~ 350 m relative to other chemical tracer of hydrothermal plume such as Mn and 3He over the plume length (~ 4,300 km; Fig.1). What is the cause of this phenomenon? To explain these findings, Fitzsimmons et al. report an unprecedented mechanism for the sustaining hydrothermal iron during long-range transportation called “reversible exchange”. They suggests that balance between the dissolved and particulate phases may significantly affect iron distribution in the ocean.

Fitzsimmons et al. used X-ray microscopy to verify what constituted hydrothermal iron particles. The results showed that the Fe-bearing particles nearer to hydrothermal vent site consist of Fe(Ⅲ) oxyhydroxides largely associated with organic matter about 5 ㎛ in size, whereas those of distant plume were smaller and embedded within low-density organic carbon. Hence, Fitzsimmons et al. argue that the high specific gravity of any embedded Fe minerals is offset by the low specific gravity of associated organic matter, resulting in the persistence of particulate iron in the plume. However, X-ray microscopy analysis only provide information about the pFe phase. Thus, Fitzsimmons et al. also conducted isotopic analysis and to examine the physicochemical speciation of dFe. The result indicates that dFe is stabilized by organic ligands and comprised mostly colloidal species. Isotope mixing model also attribute this result to fraction of iron held in solution by organic ligands throughout the plume. Although the chemical speciation of iron is not yet known, in view of conserved total dFe concentration and heavier dissolved δ56Fe, Fitzsimmons et al. propose reversible exchange on a rapid timescale relative to pFe sinking, of which possible mechanism could be ‘hydrophobic attraction’ and/or ‘ligand-exchange’ between organic complexes. Such a reversible dFe scavenging with the organic association of both dFe and pFe would be analogous to formation of marine gels7. However, this process would not apply to dMn because there were negligible Mn colloids6. Although both most Fe and Mn of hydrothermal plume are exponentially removed by aggregation with sinking particles, this decoupling implies that Fe- and Mn-bearing particles have different behavior properties. Thus, Fitzsimmons’s findings have important implication for other dissolved elements fluxes in hydrothermal plumes and the scavenging removal of Fe from the entire ocean regime.

Fig.1. Dispersion sketch of hydrothermal iron from SEPR hydrothermal plume, compared with Mn and 3He (Fitzsimmons et al., 2017).

Reference

1. Chu, N. C., Johnson, C. M., Beard, B. L., German, C. R., Nesbitt, R. W., Frank, M., ... & Graham, I. (2006). Evidence for hydrothermal venting in Fe isotope compositions of the deep Pacific Ocean through time. Earth and Planetary Science Letters, 245(1), 202-217.

2. Bennett, S. A., Achterberg, E. P., Connelly, D. P., Statham, P. J., Fones, G. R., & German, C. R. (2008). The distribution and stabilisation of dissolved Fe in deep-sea hydrothermal plumes. Earth and Planetary Science Letters, 270(3), 157-167.

3. Hawkes, J. A., Connelly, D. P., Gledhill, M., & Achterberg, E. P. (2013). The stabilisation and transportation of dissolved iron from high temperature hydrothermal vent systems. Earth and Planetary Science Letters, 375, 280-290.

4. Resing, J. A., Sedwick, P. N., German, C. R., Jenkins, W. J., Moffett, J. W., Sohst, B. M., & Tagliabue, A. (2015). Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean. Nature, 523(7559), 200-203.

5. Feely, R. A., Baker, E. T., Marumo, K., Urabe, T., Ishibashi, J., Gendron, J., ... & Okamura, K. (1996). Hydrothermal plume particles and dissolved phosphate over the superfast-spreading southern East Pacific Rise. Geochimica et Cosmochimica Acta, 60(13), 2297-2323.

6. Fitzsimmons, J. N., John, S. G., Marsay, C. M., Hoffman, C. L., Nicholas, S. L., Toner, B. M., ... & Sherrell, R. M. (2017). Iron persistence in a distal hydrothermal plume supported by dissolved-particulate exchange. Nature Geoscience, 10(3), 195-201.

7. Verdugo, P., Alldredge, A. L., Azam, F., Kirchman, D. L., Passow, U., & Santschi, P. H. (2004). The oceanic gel phase: a bridge in the DOM–POM continuum. Marine Chemistry, 92(1), 67-85.