Tracing the advection of organic carbon into the subsurface Sargasso Sea using a 228 Ra/ 226 Ra tracer

Abstract

During a 3‐year record of 228 Ra/ 226 Ra ratio profiles (from nine cruises) in the Sargasso Sea, two profiles in the Spring (April 1997 and 1998) show distinct subsurface maxima. Such maxima in the radium isotope ratio record the subsurface advection of surface waters, such as subtropical mode waters. The April 1997 maximum occurred below the euphotic zone (between 300–400 m) and was also coincident with that of POC and dissolved oxygen, supporting the subsurface advection of surface waters. This single event represents a large isopycnal injection (23 g m −2 ) of organic carbon into the subsurface ocean. Although we cannot quantify carbon export by this process on a basin scale based on these single‐site measurements, it may be an important pathway for short‐term oceanic carbon export.