A Stable U Isotopic Perspective on the U Budget and Global Extent of Modern Anoxia in the Ocean.

Abstract:

Isotopic fractionation between U⁴⁺ and U⁶⁺makes U stable isotopes potential tracers of global paleoredox conditions. In this work [1], we put the U-proxy up to a test against a highly constrained system: the modern ocean.

We measured a large number of seawater samples from geographically diverse locations and found that the open ocean has a homogenous isotopic composition at δ²³⁸U_SW= -0.392 ± 0.005 ‰ (rel. to CRM-112a). From our measurement of rock samples (n=64) and compilations of literature data (n=380), we then estimated the U isotopic compositions of the various reservoirs involved in the modern oceanic U budget, as well as the fractionation factors associated with U incorporation into those reservoirs. Using a steady-state model, we compared the isotopic composition of the seawater predicted by the four most recent U oceanic budgets [2-5] to the modern seawater value we measured.

Three of these budgets [2-4] predict a seawater isotopic composition in very good agreement with the observed δ²³⁸U_SW, which strengthens our confidence in the isotopic fractionation factors associated with each deposition environment and the fact that U is at steady-state in the modern ocean. The U oceanic budget of Henderson and Anderson (2003) does not reproduce the observed seawater composition because the U flux to anoxic/euxinic sediments relative to the total U flux out of the ocean is high in their model, which our analysis shows cannot be correct. The U isotopic composition of seawater is used to constrain the extent of anoxia in the modern ocean (% of seafloor covered by anoxic/euxinic sediments), which is 0.21 ± 0.09 %.

This work demonstrates that stable isotopes of U can indeed trace the extent of anoxia in the modern global ocean, thereby validating the application of U isotope measurements to paleoredox reconstructions. Based on the above work, we will present the best estimate of the modern oceanic U budget.

[1] Tissot F.L.H., Dauphas N. (2015) Geochim Cosmochim Ac 167, 113-143 [2] Barnes C. E., Cochran J. K. (1990) Earth Planet Sc Lett 97, 94-101 [3] Morford J. L., Emerson S. (1999) Geochim Cosmochim Ac 63, 1735-1750 [4] Dunk R. M., Mills R. A., Jenkins W. J. (2002) Chemical Geology 190, 45-67 [5] Henderson G. M., Anderson R. F. (2003) Rev Mineral Geochem 52, 493-531