Abstract
Hydrothermal iron supply contributes to the Southern Ocean carbon cycle via the regulation of regional export production. However, as hydrothermal iron input estimates are coupled to helium, which are uncertain depending on whether helium inputs are based on ridge spreading rates or inverse modelling, questions remain regarding the magnitude of the export production impacts. A particular challenge is the limited observations of dissolved iron (dFe) supply from the abyssal Southern Ocean ridge system to directly assess different hydrothermal iron supply scenarios. We combine ocean biogeochemical modelling with new observations of dFe from the abyssal Southern Ocean to assess the impact of hydrothermal iron supply estimated from either ridge spreading rate or inverse helium modelling on Southern Ocean export production. The hydrothermal contribution to dFe in the upper 250 m reduces 4–5 fold when supply is based on inverse modelling, relative to those based on spreading rate, translating into a 36–73% reduction in the impact of hydrothermal iron on export production. However, only the spreading rate input scheme reproduces observed dFe anomalies >1 nM around the circum-Antarctic ridge. The model correlation with observations drops 3 fold under the inverse modelling input scheme. The best dFe scenario has a residence time for hydrothermal iron that is between 21 and 34 years, highlighting the importance of rapid physical mixing to surface waters. Overall, because of its short residence time, hydrothermal Fe supplied locally by circum-Antarctic ridges is most important to the Southern Ocean carbon cycle and our results highlight decoupling between hydrothermal iron and helium supply.
Highlights
The important role of hydrothermal iron (Fe) supply in shaping iron biogeochemical cycling has emerged over the last two decades (German et al, 2016; Tagliabue et al, 2017)
Calculated in this way, full depth integrated hydrothermal Fe signals largely follow the differences in hydrothermal Fe input between the two scenarios (Figures 2A,B), with the inverse experiment exhibiting a strong increase in dFe adjacent to the South East Pacific Rise (SEPR) and locally south of the Azores along the northern mid Atlantic ridge
The residence times derived are at the lower end of those previously proposed, yet we find hydrothermal Fe maintains a strong contribution to the export production that drives the Southern Ocean biological carbon pump
Summary
The important role of hydrothermal iron (Fe) supply in shaping iron biogeochemical cycling has emerged over the last two decades (German et al, 2016; Tagliabue et al, 2017). Alternative estimates of 3He input are based on inverse modelling and do not make any a priori link between 3He supply and spreading rate, but instead optimise the distribution of He input along ridges using water column 3He measurements in a data-constrained ocean circulation inverse model (OCIM; DeVries and Holzer, 2019) This approach is referred to here as “inverse” boundary forcing. Simple hydrothermal Fe model experiments using the inverse 3He input scenario suggest that hydrothermal Fe would have a short residence time and be largely trapped in the ocean interior with very little reaching the surface ocean (Roshan et al, 2020) This challenges the role of hydrothermal Fe in supporting export production and the regional biological carbon pump
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