Tracing differences in iron supply to the Mid-Atlantic Ridge valley between hydrothermal vent sites: implications for the addition of iron to the deep ocean

[摘要] Supply of iron ( Fe ) to the surface ocean supports primary productivity, and while hydrothermal input of Fe to the deep ocean is knownto be extensive it remains poorly constrained. Global estimates of hydrothermal  Fe  supply rely on using dissolved Fe ( dFe ) toexcess He ( xs 3 He ) ratios to upscale fluxes, but observational constraints on dFe / xs 3 He may be sensitive toassumptions linked to sampling and interpolation. We examined the variability in dFe / xs 3 He using two methods of estimation, forfour 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 / xs 3 He was 4 to 63 and 4 to 87 nmol:fmol, respectively, primarily due to differences in plume age. To account for background xs 3 He and shifting plume position, we calibrated He values using contemporaneous dissolved Mn ( dMn ). Applying thisapproach more widely, we found dFe / xs 3 He ratios of 12, 4–8, 4–44, and 4–86  nmol fmol −1 for the Menez Gwen, LuckyStrike, Rainbow, and TAG hydrothermal vent sites, respectively. Differences in plume dFe / xs 3 He across sites were not simplyrelated to the vent endmember Fe and He  fluxes. Within 40  km of the vents, the dFe / xs 3 He ratios decreased to3–38  nmol fmol −1 , due to the precipitation and subsequent settling of particulates. The ratio of colloidal Fe to dFe wasconsistently higher (0.67–0.97) than the deep N. Atlantic (0.5) throughout both the TAG and Rainbow plumes, indicative of Fe  exchangebetween dissolved and particulate phases. Our comparison of TAG and Rainbow shows there is a limit to the amount of hydrothermal  Fe releasedfrom vents that can form colloids in the rising plume. Higher particle loading will enhance the longevity of the Rainbow hydrothermal plume withinthe deep ocean assuming particles undergo continual dissolution/disaggregation. Future studies examining the length of plume pathways required toescape the ridge valley will be important in determining Fe  supply from slow spreading mid-ocean ridges to the deep ocean, along with thefrequency of ultramafic sites such as Rainbow. Resolving the ridge valley bathymetry and accounting for variability in vent sources in globalbiogeochemical models will be key to further constraining the hydrothermal  Fe  flux.

[学科分类] 大气科学