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

Abstract

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.