Radium isotopes in the Orinoco estuary and eastern Caribbean Sea

Abstract

Radium isotopes provide a means of identifying the source of freshened waters in the ocean and determining the time elapsed since these waters were in the estuary. We present evidence that during April, waters from the Amazon mixing zone pass within 50 km of the mouth of the Orinoco River. These Amazon waters are characterized by a lower 228Ra/226Ra activity ratio (AR) than are waters from the Orinoco at similar salinities. During autumn, the increased discharge of the Orinoco displaces the freshened Amazon waters seaward, yet the two can be distinguished clearly. Within the Caribbean Sea, waters of Orinoco origin carry a characteristic radium signature including excess activities of 224Ra. This isotope may be used to estimate the time elapsed since the waters were removed from contact with sediments. Current speeds based on 224Ra dating ranged from 15 to 33 cm/s during April. The radium isotopes also provide an assessment of sediment mixing in the estuary. During low discharge (April), considerable mixing of older sediment by physical or biological processes or dredging maintained high activities of 228Ra in the estuary and produced the highest 228Ra/226Ra AR's yet measured in any estuary. During high discharge (September), a large fraction of the 228Ra was derived from desorption from fresh sediment rather than mixing of older sediments. Activities of 224Ra were high in the estuary during both high and low discharge, indicating that considerable mixing of recently introduced sediment must occur during each period. The high 224Ra/228Ra AR's measured in September imply that the same sediments were being resuspended during high discharge, and that there was little exposure of older sediments which had been stored in the estuary for months to years. During April, 224Ra and 228Ra activities in the water were about equal, indicating that most of the sediment being resuspended had been stored in the estuary long enough to reestablish radioactive equilibrium in the 232Th decay series (i.e., 20 years).