The near-surface distribution of radium in the Gulf of Mexico and Caribbean Sea : temporal and spatial variability and hydrographic relationships

Abstract

Only a few radium measurements have been made in the near-surface (0--600m) Gulf of Mexico and Caribbean Sea. Published 228Ra data from 1968 and 1973 reveal a significant increase of 228Ra activity in the near-surface Gulf of Mexico. The increase has been attributed to factors affecting the water prior to its entry into the Caribbean-Gulf system. It has been proposed that 228Ra can be used to study thermocline mixing rates, but few detailed studies have been made to test the validity of this hypothesis. Physical oceanographic evidence indicates that vertical mixing significantly impacts the upper 100-150 meters of the water column in the Gulf of Mexico. This dissertation reports and discusses the results of 123 measurements of the 228Sa:228Ra activity ratio (radium activity ratio), and 73 measurements of 226Ra in the 1975-1976 near-surface water columns of the Gulf of Mexico and Caribbean Sea. 228Ra activities are calculated from the activity ratio and 226Ra data. Radium activity ratios in excess of 1.0, with corresponding 226Ra and 228Ra specific activities in excess of 10 dpm/100kg, are widespread throughout the northern shelf region of the Gulf of Mexico. There are significant spatial variations of all three radium parameters associated with the transition from shelf waters to interior oceanic waters. A significant increase of 226Ra activity from 1973 to 1976 is observed in the upper ~100m of the Gulf of Mexico. At depths greater than 100m, the vertical distribution of 226Ra is the same for 1973 and 1976. 22BRa in the near-surface Gulf of Mexico is shown to be time variant in concentration as well as vertical distribution. Comparisons of surface radium data from the Gulf of Mexico, Caribbean Sea, and Atlantic Ocean lead to the conclusion that there is a significant internal source of radium within the Gulf of Mexico. Scatter plots of the 1976 radium data substantiate this conclusion, and indicate that the source of excess radium is the northern shelf of the Gulf of Mexico. Estimates of the shelf water component in the upper 100m of the water column in the central Gulf range from 7-19%, and 1-8% on the northern Campeche Bank.