Preprints
https://doi.org/10.5194/bg-2022-73
https://doi.org/10.5194/bg-2022-73
 
04 Apr 2022
04 Apr 2022
Status: this preprint is currently under review for the journal BG.

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

Alastair Jason Mackenzie Lough1, Alessandro Tagliabue2, Clement Demasy1, Joseph A. Resing3, Travis Mellett4, Neil J. Wyatt1, and Maeve C. Lohan1 Alastair Jason Mackenzie Lough et al.
  • 1Ocean & Earth Science, University of Southampton, Southampton, SO14 3ZH, UK
  • 2Earth, Ocean & Ecological Sciences, Liverpool, L69 3BX, UK
  • 3Cooperative Institute for Climate, Oceans, and Ecosystem Studies, University of Washington and NOAA-PMEL, Seattle, USA
  • 4University of South Florida, St. Petersburg, FL, FL 33701, United States

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.

Alastair Jason Mackenzie Lough et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-73', Anonymous Referee #1, 10 Apr 2022
    • AC1: 'Reply on RC1', Alastair Lough, 26 May 2022
  • RC2: 'Comment on bg-2022-73', Anonymous Referee #2, 27 Apr 2022
    • AC2: 'Reply on RC2', Alastair Lough, 26 May 2022
  • RC3: 'Comment on bg-2022-73', Anonymous Referee #3, 05 May 2022
    • AC3: 'Reply on RC3', Alastair Lough, 26 May 2022

Alastair Jason Mackenzie Lough et al.

Alastair Jason Mackenzie Lough et al.

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Short summary
Iron is a key nutrient for ocean primary productivity. Hydrothermal vents are a source of iron to the oceans, but the size of this source is poorly understood. This study examines the variability in iron inputs between hydrothermal vents in different geological settings. The vents studied release different amounts of Fe resulting in plumes with similar dissolved iron concentrations but different particulate concentrations. This will help to refine modelling of iron limited ocean productivity.
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