Tracing the Amazon component of surface Atlantic water using ²²⁸Ra, salinity and silica.

High ²²⁸Ra/²²⁶Ra activity ratios characteristic of waters in the Amazon estuary provide a sensitive indicator of the presence of these waters in the Atlantic Ocean. A conservative mixing model utilizing the ²²⁸Ra/²²⁶Ra activity ratio (AR) tied to absolute measurements in the estuary allows us to estimate that 20-34% of the surface water east of the Antilles during June, 5-9% from the same area during December, and 15-20% of the eastern Caribbean surface water during December are derived from the Amazon estuary. Differences in ²²⁸Ra input occur in response to variable stratification of water near the river mouth. During high discharge, intense vertical mixing enriches the water in the estuary in ²²⁸Ra. A large fraction of this water moves to the north and east of the Antilles, where its relatively high ²²⁸Ra/²²⁶Ra AR distinguishes it over 1500 km from its source. During low discharge (northern hemisphere fall) a significant fraction of river water passes northwest of the zone of intense mixing into a vertically stratified region where ²²⁸Ra gain is lower. This water is transported by the Guiana Current along the coast of South America and into the Caribbean. -Authors