Hydrothermal Fe flux analysis of Loihi Seamount using size partitioning and Fe isotopes

Hydrothermal vents have gained attention as a source of dissolved iron (dFe; <0.2μm) to the ocean. Using δ³He as a conservative hydrothermal tracer, any non-conservative hydrothermal dFe losses can be assessed as a vent plume migrates. The relatively shallow submarine hydrothermal system (~1300m max) of Loihi Seamount was studied as part of the U.S. GEOTRACES GP15 Pacific Meridional Transect. In the water column at Loihi, dFe reached 150nmol/kg, with a dFe:³He molar ratio of 9.3±0.3 x 10⁶ at the 1100m depth where the vent plume escapes local bathymetry. However, while Loihi enriched dFe as far as 1500km away, at the stations within 500km of Loihi the dFe:³He molar ratio had already dropped by half to 4.1±0.8 x 10⁶, suggesting a non-conservative sink of dFe during distal transport. Here, we use size partitioning and Fe isotopes to investigate which Fe transformations may have caused this dFe loss in the distal Loihi plume. At Loihi, ultrafiltration-based separation of the truly soluble (<0.003μm) dFe from the colloidal (0.003-0.2μm) species revealed a dominant soluble-sized speciation at depth, unique for near-field hydrothermal plumes. Combined with an isotopically lighter δ⁵⁶Fe signature at depth, we suggest that these results are consistent with the presence of soluble-sized Fe²⁺. Distal dFe size partitioning and Fe isotope results will be used to discriminate two potential mechanisms for distal dFe loss: 1) abiotic Fe²⁺ oxidation and subsequent precipitation and 2) microbial dFe utilization. Finally, by applying the distal dFe:³He ratio to an advection-diffusion model of He inventory, we estimated that Loihi could be responsible for 3±1.5% of the global hydrothermal Fe flux and that Loihi plume waters should upwell to surface waters of coastal California and the western subarctic Pacific. We will test whether Loihi-derived dFe could persist during this transport by adding our measured Loihi ³He flux and distal dFe:³He ratio to a global Fe biogeochemical model.