Dissolved iron and iron isotopes in the southeastern Pacific Ocean

Abstract

The Southeast Pacific Ocean is a severely understudied yet dynamic region for trace metals such as iron, since it experiences steep redox and productivity gradients in upper waters and strong hydrothermal iron inputs to deep waters. In this study, we report the dissolved iron (dFe) distribution from seven stations and Fe isotope ratios (δ 56 Fe) from three of these stations across a near-zonal transect from 20 to 27°S. We found elevated dFe concentrations associated with the oxygen-deficient zone (ODZ), with light δ 56 Fe implicating porewater fluxes of reduced Fe. However, temporal dFe variability and rapid δ 56 Fe shifts with depth suggest gradients in ODZ Fe source and/or redox processes vary over short-depth/spatial scales. The dFe concentrations decreased rapidly offshore, and in the upper ocean dFe was controlled by biological processes, resulting in an Fe:C ratio of 4.2 μmol/mol. Calculated vertical diffusive Fe fluxes were greater than published dust inputs to surface waters, but both were orders of magnitude lower than horizontal diffusive fluxes, which dominate dFe delivery to the gyre. The δ 56Fe data in the deep sea showed evidence for a À0.2‰ Antarctic Intermediate Water end-member and a heavy δ

Geochemical and iron isotopic insights into hydrothermal iron oxyhydroxide deposit formation at Loihi Seamount

“…Higher G 56 Fe values may also reflect mixing with open deep seawater around Loihi considering that deep Pacific seawater G 56 Fe values are enriched in heavy isotopes, as shown by previous studies in the subtropical deep Pacific Ocean where G 56 Fe values ranged from -0.4 ‰ to +0.6 ‰(Radic et al, 2011;Fitzsimmons et al, 2016). However, we do not favor this hypothesis considering that open seawater Fe concentrations in this region of the Pacific Ocean (e.g.…”

“…These values are however within the range of G 56 Fe DFe values measured in a buoyant plume in the East Scotia Sea which yielded distinctly lower G 56 Fe DFe (as low as -1.19 ‰) than the initial hydrothermal fluids (-0.29 ‰) (Lough et al, 2017). These values are also lighter than the value (G 56 Fe DFe = +0.02 ± 0.03 ‰) measured downstream of the Loihi Seamount hydrothermal system at Station ALOHA on July 2012 (Fitzsimmons et al, 2016).…”

“…By non-buoyant plume height, a large amount of Fe-sulfide precipitates will have been lost from the plume with a concomitant removal of isotopically light Fe. A similar model has been also proposed to explain the relatively heavy Fe isotope signatures of the water column Fe enrichment attributed to the EPR at 25°S; 105-90°W (Fitzsimmons et al, 2016;Fitzsimmons et al, 2017) and Scotia Sea (Klar et al, 2017;Lough et al, 2017).…”

“…(Boyle et al, 2005;Fitzsimmons et al, 2015b), which is much lower than the DFe concentrations of 13 to 70 nM measured in bottom seawater at Pele's Pit. Surprisingly, the far-field hydrothermal G 56 Fe signature of Loihi Seamount measured at Station ALOHA, determined at +0.02 ± 0.03 ‰ (Fitzsimmons et al, 2016) is significantly heavier than bottom seawater G 56 Fe values at Loihi. Reconciling both observations would require a more comprehensive study of Fe isotope fractionation in the nearfield plume environment, which is beyond the scope of this study.…”

Iron persistence in a distal hydrothermal plume supported by dissolved–particulate exchange

“…Such an exchange-sinking mechanism would not apply to dMn because dMn is minimally complexed by organic matter in natural waters 2 , and negligible Mn was observed in the colloidal fraction ( Figure S4). 45 . However, our model-derived hydrothermal ligand-bound dFe concentrations (~0.3 nM) are much lower than those currently parameterized in modeling efforts focused on near-field vent geochemistry 8 .…”