Distribution and source of plutonium in sediments from the southern Gulf of Mexico

Abstract

Here, we report on new data (75 analyses) of plutonium (Pu) isotopes to elucidate activity concentrations, inventories, sources, and their transport from the ocean surface to the seafloor from a collection of six deep-sea sediment cores (depths ranging from 257 to 3739 m) in the southern Gulf of Mexico. Sediment cores collected from the continental shelf and upper slope region of the Gulf of Mexico showed ²⁴⁰Pu/²³⁹Pu ratios of 0.15 to 0.26, and ^239+240Pu-inventories ranging from 14.7 to 33.0 Bq m^-2. Inventories and ratios are consistent with global fallout Pu for this tropical region. In contrast, sediment cores collected from the lower slope region and abyssal plain showed low ²⁴⁰Pu/²³⁹Pu ratios of 0.07 to 0.13 and much lower ^239+240Pu inventories below 6.8 Bq m^-2. This implies that only a small fraction of the expected global fallout Pu has reached the deep-sea sediments. The low ²⁴⁰Pu/²³⁹Pu isotope ratios indicate that fallout from the Nevada testing site was an important source of Pu in deep-sea sediments, and that this Pu was likely more efficiently scavenged from the water column than Pu from global fallout. We estimated that up to 44% of the total inventory of ^239+240Pu in deep-sea sediments is due to the Nevada source. Low values and a progressive decrease of ²⁴⁰Pu/²³⁹Pu ratios and ^239+240Pu inventories with increasing water depth have been previously reported for the Gulf of Mexico. Analysis of Pu isotopes in two sediment traps from the upper slope regions shows ²⁴⁰Pu/²³⁹Pu ratios comparable to those observed in global fallout. These results indicate that global fallout Pu is currently the main source of Pu in sinking particles in the water column. Therefore, a significant fraction of global fallout Pu must still be present, either in a dissolved phase, or as biologically recycled material in the water column, or scavenged on the shelf and shelf break. Our results bring to light important questions on the application of Pu isotopes to establish sediment chronologies in deep-sea sediments, since global fallout features such as the 1963 maximum are not available.

Keywords: 239+240Pu inventory; 240Pu/239Pu isotope ratio; Deep-sea sediments; Global fallout; Nevada Test Site fallout.