Uranium and radium diffusion in organic-rich sediments (sapropels)

Abstract

Among the late Quaternary Mediterranean sapropels, the S5 (125 ka) is one of the best preserved due to its high organic carbon content that has limited postdepositional oxidation. The high uranium content in this sapropel, >40 dpm g -1, makes this layer interesting for studying uranium series disequilibrium in organic-rich sediments. For this reason, the present work provides isotopic measurements of the U decay series in a S5 sapropel by applying more precise mass spectrometric methods, TIMS/MC-ICPMS, and gamma spectrometry. Assuming that U in the sapropel mostly originated from seawater the ( 234 U/ 238 U), ( 230 Th/ 238 U), ( 226 Ra/ 230 Th) and ( 231 Pa/ 235 U) activity ratios show systematic deviations from the theoretical values for a closed-system evolution of the U series over the 125 ka since sapropel formation. The radiogenic 234 U rad and 226 Ra show clear evidence of migration in the sapropel with modeled diffusion coefficients of (7.1 1.1) × 10 -12 cm 2 s -1 and (1.6 0.2) × 10 -10 cm 2 s -1, respectively. The diffusion of 234 U rad cannot explain the high ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) activity ratios observed in the sapropel. Two possible mechanisms or a combination of both are proposed for explaining the irregular ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) activity profiles in sapropel S5. The first one is an enhanced export flux of 230 Th xs and 231 Pa xs excesses exceeding the production rate in seawater, during the time of sapropel formation, and the second one is diffusion of authigenic U auth in the sapropel. However, the ambiguous determination of 230 Th xs and 231 Pa xs in the sapropel and the poorly understood processes that might lead to U auth migration in anoxic sediments still limit a final explanation for the deviation of ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) activity ratios from their expected theoretical values. Copyright 2011 by the American Geophysical Union.