1065 / 2024-09-20 09:55:13

Multiple controls on dissolved Ba isotope cycle in the Peruvian upwelling zone

Ba isotopes,Oxygen Minimum Zone,Fe shuttle,Redox,Adsorption,Water mass mixing

Recent barium isotope studies have shown contrasting Ba biogeochemical behaviors at oxic open ocean vs. ocean boundaries (e.g., hydrothermal vents and estuaries). Particulate Ba is strongly coupled to POC fluxes, yet how Ba isotopes fractionate at high surface productivity, low subsurface dissolved oxygen regions remains unclear. In this study, we present shelf and offshore water column dissolved Ba and Ba isotopes from the Peruvian oxygen minimum zone (OMZ) where dissolved oxygen (DO) concentrations generally drop below detection limits at depths > 50 - 100 m. Distributions of Ba and Ba isotopes at intermediate and deep waters in our study are in good agreement with those from the central and North Pacific, and are mainly governed by water mass mixing. At the surface and subsurface (< 300 m), we observed some of the highest Ba isotopic compositions (δ¹³⁸Ba) (0.65-0.80‰) reported to date. Correspondingly, particulate Fe (pFe) and Mn (pMn) are elevated at depths < 300 m, where surface pFe:pAl and pMn:pAl both are consistent with lithogenic values, likely implying atmospheric dust input. At the subsurface where DO is below detection limit, elevated pFe:pAl raios are observed. Our data suggests that Fe shuttle may have played a key role in subsurface Ba isotope biogeochemistry in the Peruvian OMZ, where lateral transport of reductively released Fe(II) from anoxic shelf sediments is oxidized and light Ba isotope are co-precipitated with authigenic Fe (oxyhydr)oxides. A simple open steady-state model suggests that most of the upper water column Ba isotope data fall between the arrays defined by barite formation with fractionation factor of a = 1.00058 (Hsieh and Henderson, 2017) and Fe oxyhydroxide adsorption/incorporation with a = 1.00014 (Zhang et al., 2024). We propose that a combination of adsorption of Ba onto suspended particles at the surface, and barite formation as well as incorporation/adsorption onto authigenic Fe (oxyhydr)oxides below the oxic-anoxic interface may induce fractionated Ba isotope ratios seen in our study. Our findings stress that Fe/Mn (oxyhydr)oxides may be an important sink for isotopically light Ba in the ocean and that future Ba isotope studies would need to consider this process in Ba isotope budget calculation.