Stability of the Orca Basin Brine Interface Determined Using Radium Isotopes

The Orca Basin is an intraslope basin in the northern continental slope of the Gulf of Mexico, and contains a deep (up to 220 m) anoxic, hypersaline brine lake. The brine interface extends from ocean salinity (at a water depth of 2125 m) to a constant salinity of ~215 psu below a depth of 2250 m. This interface is considered to be among the most stable interfaces on the planet, and contains distinct zones of macrofaunal and microbial assemblages. The brine lake is supported by continued dissolution of exposed rock salt at an estimated rate of 0.5 million t/yr. Such dissolution of salt should serve as a source of radium isotopes, and given their range of half-lives, these isotopes can serve as unique proxies to the location and rate of salt dissolution into this basin. We have collected a series of radon and radium isotope profiles through this brine lake over the past 4 years, which allow us to assess the spatial and temporal stability of this interface. Throughout the brine lake, we observe strong enrichments in unsupported Ra-224, Rn-222, and Ra-226 in a ~10 m thick zone near the base of the brine interface, between 2232 m and 2242 m. The strong enrichment in unsupported Ra-224 in this layer must be supported by a continuous source, presumably the dissolution of exposed salt rock. Various degrees of isotopic enrichment throughout the lake provide an assessment of the rates of lateral and vertical dispersion of salt and associated chemical constituents.