Constraining the Contribution of Hydrothermal Iron to Southern Ocean Export Production Using Deep Ocean Iron Observations

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.

Mechanisms Driving the Dispersal of Hydrothermal Iron From the Northern Mid Atlantic Ridge

“…The first was the standard global configuration using the ORCA2 configuration at a horizontal resolution of 2°x cos(latitude) curvilinear grid, with an enhancement to 0.5° around the equator and 31 irregularly spaced vertical levels. This NEMO‐ORCA2 configuration is typical of those models coupled to biogeochemical models to address questions regarding biogeochemical cycling (e.g., Tagliabue et al., 2022). We applied the default settings and boundary conditions of the reference configuration ORCA2_ICE_PISCES (NEMO‐Consortium, 2019).…”

“…Elevated iron signals have been observed in plumes above most mid ocean ridge systems (Baker et al., 2002; Gamo et al., 1996; Hahm et al., 2015; Massoth et al., 1994; Rudnicki & Elderfield, 1993). More recently, as part of the GEOTRACES program, iron has been shown to persist as DFe above and beyond the global ridge crest system (Hatta et al., 2015; Klunder et al., 2011; Nishioka et al., 2013; Resing et al., 2015; Tagliabue et al., 2022). Crucial in this growing role for hydrothermalism in shaping basin scale distributions is the question of how DFe is transported away from hydrothermal vent sites at the >100 km scale (Tagliabue & Resing, 2016).…”

Mechanisms driving the dispersal of hydrothermal iron from the northern Mid Atlantic Ridge

“…The first was the standard global configuration using the ORCA2 configuration at a horizontal resolution of 2°x cos(latitude) curvilinear grid, with an enhancement to 0.5° around the equator and 31 irregularly spaced vertical levels. This NEMO-ORCA2 configuration is typical of those models coupled to biogeochemical models to address questions regarding biogeochemical cycling (e.g., Tagliabue et al, 2022). We applied the default settings and boundary conditions of the reference configuration ORCA2_ICE_PISCES (NEMO-Consortium, 2019).…”

“…The NEMO-AGRIF configuration has a 1/8° (or 12.48 km) resolution regional nested grid around TAG (Figure S3 in Supporting Information S1), but the vertical resolution of NEMO-AGRIF and NEMO-ORCA2 are around three-fold lower than the particle tracking model. This set of model configurations were designed to link the very high-resolution regional modeling with the types of models used for larger scale biogeochemical modeling that tend to have horizontal resolutions of between 1 and 5° (Roshan et al, 2020;Somes et al, 2021;Tagliabue et al, 2022).…”

“…Adding further complexity is the fact that the slow spreading ridges like the mid-Atlantic ridge are typified by large axial valleys with topographic relief varying by kilometers across relatively small spatial scales. It is not known how resolving these scales of variability affects the transport of DFe into the wider basin and the implications for coarse resolution ocean biogeochemical models that are typically used to test hypothesis about hydrothermal DFe supply and cycling (Roshan et al, 2020;Somes et al, 2021;Tagliabue et al, 2022). For instance, improved understanding of the critical processes and scales regulating the dispersal from ridges would aid our understanding of the necessary process resolution for ocean models.…”