RADIUM ISOTOPES IN THE ORINOCO ESTUARY AND EASTERN CARIBBEAN SEA

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 Ra-228/Ra-226 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 Ra-224. This isotope may be used to estimate the time elapsed since the waters were removed from contact with sediments. Current speeds based on Ra-224 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 Ra-228 in the estuary and produced the highest Ra-228/Ra-226 AR's yet measured in any estuary. During high discharge (September), a large fraction of the Ra-228 was derived from desorption from fresh sediment rather than mixing of older sediments. Activities of Ra-224 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 Ra-224/Ra-228 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, Ra-224 and Ra-228 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 Th-232 decay series (i.e., 20 years).