Nitrate and silicate fluxes at the sediment–water interface of the deep North Pacific Ocean illuminated by 226Ra/230Th disequilibria

Abstract

By taking advantage of recent analytical advances, we herein develop the ²²⁶Ra/²³⁰Th isotope systematics as a novel tool for quantifying nitrate and dissolved silicate fluxes across the sediment–water interface of the deep-ocean floor. Sediment cores were retrieved from the seabed between 4927 m and 5951 m in the North Pacific Ocean. Downcore profiles of ²³⁰Th and both dissolved and total ²²⁶Ra were measured using a high-sensitivity inductively coupled plasma mass spectrometer. At all study sites, a marked deficit of total ²²⁶Ra with respect to ²³⁰Th was observed between 0 and 20 cm, indicating active migration of soluble ²²⁶Ra from the sediment into the overlying seawater. By constructing the mass balance of ²²⁶Ra in the sediment column, the flux of dissolved ²²⁶Ra across the sediment–water interface was estimated to range from 461 to 1320 dpm m⁻² yr⁻¹. When coupled to a diffusive transport model as developed by early investigators, these flux values of ²²⁶Ra enabled us to calculate the flux of any dissolved constituent of interest by measuring their bottom water concentrations and pore water “saturation” concentrations. Based on the ²²⁶Ra/²³⁰Th disequilibrium approach, the derived fluxes vary between 4.1 and 10.5 mmol m⁻² yr⁻¹ for nitrate and between 11 and 49 mmol m⁻² yr⁻¹ for dissolved silicate. A compilation of nitrate and silicate fluxes from the seabed in the deep Pacific Ocean shows that these values are consistent with historical flux measurements based on the conventional core incubation method in the same study region. In addition, both nitrate and silicate fluxes exhibit a clear depth-dependent trend. Overall, our results suggest that sedimentary diagenetic alterations at the North Pacific Ocean floor below ∼ 5000 m are efficient so that only < 2 % of the particulate organic carbon and < 12 % of the biogenic opal raining to the seafloor are ultimately preserved in the sediment.