The impact of hydrothermal vent geochemistry on the addition of iron to the deep ocean

Abstract

Abstract. Supply of iron (Fe) to the surface ocean supports primary productivity and while hydrothermal input of Fe to the deep ocean is known to be extensive, it remains poorly constrained. Global estimates of hydrothermal Fe supply rely on using the dissolved Fe (dFe) to excess He (xs3He) ratios to upscale fluxes, but observational constraints on dFe / xs3He may be sensitive to assumptions linked to sampling and interpolation. We examined the variability in dFe / xs3He using two methods of estimation, for four vent sites with different geochemistry along the Mid-Atlantic Ridge. At both Rainbow and TAG, the plume was sampled repeatedly and the range of dFe / xs3He was 4 to 63 and 4 to 87 nmol/fmol, respectively, primarily due to differences in plume age. To account for background xs3He and shifting plume position, we calibrated He values using contemporaneous dissolved Mn (dMn). Applying this approach more widely, we found dFe / xs3He ratios of 12, 4–8, 4–44, 4–86 nmol/fmol for the Menez Gwen, Lucky Strike, Rainbow and TAG hydrothermal vent sites, respectively. Differences in plume dFe / xs3He across sites were not simply related to the vent end member Fe and He fluxes. Within 40 km of the vents, the dFe / xs3He ratios decreased to 3-38 nmol/fmol, due to the precipitation and subsequent settling of particulates. The ratio of colloidal Fe to dFe was consistently higher (0.67–0.97) than the deep N. Atlantic (0.5) throughout both the TAG and Rainbow plumes, indicative of Fe exchange between dissolved and particulate phases. Our comparison of TAG and Rainbow shows there is a limit to the amount of hydrothermal Fe released from vents that can form colloids in the rising plume. Higher particle loading will enhance the longevity of the Rainbow hydrothermal plume within the deep ocean assuming particles undergo continual dissolution/disaggregation. Future studies examining the length of plume pathways required to escape the ridge valley will be important in determining Fe supply from slow spreading mid-ocean ridges to the deep ocean, along with the frequency of ultramafic sites such as Rainbow. Resolving the ridge valley bathymetry and accounting for variability in vent sources in global biogeochemical models will be key to further constraining the hydrothermal Fe flux.

Microbial strong organic ligand production is tightly coupled to iron in hydrothermal plumes

“…1d ), similar to previous studies in other neutrally buoyant plumes ( Fig. 1e ) (Lough et al, 2022; Buck et al, 2018, 2015). The strong coupling between dFe and ligands was only observed at sites where L 1 ligands were detected.…”

“…One explanation is that both the dFe and L 1 ligands originate from the vent fluids themselves, yielding a tightly coupled hydrothermal endmember. However, the concentration of L 1 ligands did not correlate with excess mantle Helium-3 ( 3 He xs , Fig S2, Table S2 ) (Lough et al, 2022), a nearly conservative tracer of the mixing of hydrothermal fluids with seawater (Buck et al, 2018). These results suggest the L 1 ligands were not sourced from the vent fluids along with dFe.…”

“…However, the strong coupling between dFe and ligands was only observed at sites where L1 ligands were detected. 2) 18 , a nearly conservative tracer of the mixing of hydrothermal fluids with seawater 2 . These results suggest that L1 ligands along the MAR are likely not sourced from the vent fluids themselves.…”