The geochemistries of 210Po and 210Pb in waters overlying and within the Orca Basin, Gulf of Mexico

Abstract

Vertical profiles of dissolved 210Po, 210Pb, and 226Ra were obtained from waters overlying and within the Orca Basin; an anoxic, hypersaline and virtually non-sulfide bearing basin in the Gulf of Mexico. The station had a depth of 2410 m. The seawater-brine interface was located at 2230 m. In oxic waters, near-surface (0–250 m) concentrations of dissolved 210Po and 210Pb ranged from 6 to 8 dpm/100 kg and decreased to about 3 dpm/100 kg at 740 m. Below this depth concentrations remained relatively constant. Near-surface 226Ra concentrations varied from about 7 to 9 dpm/100 kg, and then increased with depth to a relatively uniform deep-water value of approximately 13 dpm/100 kg, in agreement with previously reported data for the Gulf of Mexico. The slight deficiency of 210Pb in the surface waters may result from mixing with inner shelf waters that are enriched in 226Ra. At the seawater-brine interface, sharp maxima were observed in the profiles of 210Po, 210Pb, and 226Ra, where concentrations reached 56.0, 28.3, and 840 dpm/100 kg, respectively. These are among the highest values reported for the marine environment to date. Below the interface, concentrations decreased to about 9, 6, and 210 dpm/100 kg for 210Po, 210Pb, and 226Ra, respectively. These sharp maxima may be caused by a combination of the accumulation of particles in the strong pycnocline, the subsequent dissolution of particulate hydrous Mn and Fe oxides and hydroxides, and the decomposition of biogenic particulate material. The residence time of 210Pb in the brine is about one year, a value similar to that reported for the anoxic waters of the Cariaco Trench even though the concentration of 210Pb in that basin is much lower.