Radium Isotopes in the Orinoco Estuary and Eastern Caribbean Sea.

摘要

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 mouth. These Amazon waters are characterized by a lower Ra228/Ra226 activity ratio than are waters from the Orinoco at similar salinities. Within the Caribbean Sea waters of Orinoco origin carry a characteristic radium signature including excess activities of Ra224. This isotope may be used to estimate the time elapsed since the waters were removed from contact with sediments. Current speeds based on Ra224 dating range from 5 to 20 cm/sec during April. 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 Ra228 in the estuary and produced the highest Ra228/Ra226 A. R.'s yet measured in any estuary. During the high discharge (Sept.) a large fraction of the Ra228 was derived from desorption from fresh sediment rather than mixing of older sediments. Activities of Ra224 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 Ra224/Ra228 A. R.'s measured in Sept. 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.