In:
Biogeosciences, Copernicus GmbH, Vol. 20, No. 2 ( 2023-01-25), p. 405-420
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 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, and 4–86 nmol fmol−1 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 endmember Fe and He fluxes. Within 40 km of the vents, the dFe/xs3He ratios decreased to
3–38 nmol fmol−1, 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.
Type of Medium:
Online Resource
ISSN:
1726-4189
DOI:
10.5194/bg-20-405-2023
DOI:
10.5194/bg-20-405-2023-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2023
detail.hit.zdb_id:
2158181-2
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