Cycling of fallout (Pu, ²⁴¹Am, ¹³⁷Cs) and natural (U, Th, ²¹⁰Pb) radionuclides in Washington continental slope sediments

Abstract

We report activity profiles of Pu isotopes, ²⁴¹Am, ¹³⁷Cs, ²¹⁰Pb, U and Th isotopes in Washington continental slope sediments from 332-1415 m water, identify major processes responsible for them, and compare results with those previously reported for Washington shelf sediments. Specific activities of all nuclides but ¹³⁷Cs are higher in slope surface sediments than shelf surface sediments. These higher specific activities, which are due to more efficient scavenging by finer grained particles and higher concentrations of dissolved nuclides in slope waters, do not cause proportionally higher slope sediment radionuclide inventories, because they are counterbalanced by 5-10 fold lower sediment accumulation rates on the slope. Profiles of natural U and Th series isotopes reveal that fallout nuclide profiles are not markedly affected by greater exchange of pore and overlying waters, but they are dependent upon rate and depth of solid phase mixing. ^239,240pu activities inventories in slope sediments correlate with those of excess ²¹⁰Pb, as in shelf sediments, but with higher ratios of ²¹⁰Pb/Pu in slope deposits. ²⁴¹Am/^239,240Pu activity and inventory ratios in slope sediments average twice ratios in shelf sediments, due primarily to greater particle affinity and preferential downward transport of ²⁴¹Am. Am/Pu activity ratios change little with depth in slope sediments, suggesting insignificant separation of the two nuclides during diagenesis. ^239.240Pu/¹³⁷Cs ratios are 2-3 times greater in slope than shelf sediments, due to a combination of preferential scavenging of Pu and greater ¹³⁷Cs loss by decay and desorption. Pu/Cs activity ratios decrease with depth in more porous slope sediments, due to downward migration of ¹³⁷Cs. ¹³⁷Cs is found in slope sediments from all water depths, even though it is barely detectable below about 700 m in North Pacific waters.