59th Annual Report on Research 2014

The isotopic composition of authigenic uranium in ancient sediments has been used to reconstruct the paleoredox conditions in the oceans. The rationale is that during deposition of sediments in anoxic, euxinic, suboxic, and oxic sediments, the extents to which uranium isotopes are fractionated differs. Because the residence time of U in the ocean is longer than the mixing timescale by two orders of magnitude, the global ocean is homogeneous in terms of U concentration and isotopic composition. Thus, at steady state, the uranium isotopic composition of seawater should be influenced by the aerial extents of sediments formed under different conditions. Important work has been done over the past several years to constrain uranium isotopic fractionation in sediments formed under various conditions. Furthermore, several recent U oceanic budgets have been proposed (Barnes and Cochran 1990; Morford and Emerson 1999; Dunk et al. 2002; Henderson and Anderson 2003). However, it is difficult to assess whether one budget is more reliable than the other based only on U fluxes. One approach to address this problem is to calculate the isotopic composition of the seawater predicted by these various modern oceanic uranium budgets. Are the sedimentary sinks balancing the riverine input and is the ocean in steady state?

We addressed this first order question by testing the various oceanic budgets against measurements of an array of open ocean water samples from the Atlantic ocean, the Pacific ocean, and the Mediterranean Sea. Our measurements define a homogenous open ocean d²³⁸U value relative to CRM-112a of -0.38 ± 0.02 permil. Using the fluxes and isotopic fractionation factors relevant to the four modern oceanic uranium budgets cited above, we calculated the predicted isotopic composition for modern seawater using a simple mass balance model. We assumed steady state and the riverine input as the only source of U in the ocean. The most recent budget by Henderson and Anderson (2003) fails at reproducing the actual seawater composition and predicts a lighter isotopic composition (-0.49 ‰). However the other three budgets (Barnes and Cochran 1990; Morford and Emerson 1999; Dunk et al. 2002) show very good agreement with the measured oceanic U isotopic. In particular, Dunk et al. (2002) predict a seawater d²³⁸U value of -0.38 ± 0.06 ‰. Because this budget is the only one to include uncertainties on the fluxes and encompasses the budgets of Barnes and Cochran (1990) and Morford and Emerson (1999) we strongly favor the use of the Dunk et al. (2002) budget when looking for quantitative values about the modern U oceanic budget.

The budget comparison also allows us to constrain the ratio F_Anoxic/F_Suboxic in the modern ocean. Indeed, because the anoxic and suboxic reservoirs are the two dominant sinks associated to significant isotopic fractionation (D_Suboxic-SW =+0.2 ‰ and D_{Anoxic/Euxinic-SW} = +0.6 ‰), constraining their relative size is critical in correctly predicting the seawater ²³⁸U/²³⁵U ratio. The three budgets whose predictions are in agreement with the actual seawater isotopic composition estimated the F_Anoxic/F_Suboxic ratio as being in between 0.50 and 0.75. In stark contrast, Henderson and Anderson (2003) estimated this ratio to be 2.2. Because they fail to predict the correct ²³⁸U/²³⁵U ratio, it is safe to conclude that in the modern ocean the amount of uranium sequestered in anoxic sediments is only ½ to ¾ the one incorporated in suboxic sediments, and that the budget of Henderson and Anderson (2003) is incorrect.

A second part of our effort was focused on U isotopes in carbonates. Early work hinted that little to no isotopic fractionation occurred during precipitation of carbonates from seawater, making carbonate an ideal rock to target when trying to access the uranium isotopic composition of the ocean at the time of sample formation, which in turn allows one to determine the amount of anoxic and suboxic sediments in the global ocean.

Because of the low concentration of U in carbonate samples (ppm level or less), microdrilling in pristine areas of the samples is not a viable option and a large masse of samples must be digested to obtain the high precision needed. Furthermore, recent work on uranium isotopes in carbonates has shown that anoxic conditions in pore water can lead to authigenic uranium enrichment in the carbonate, shifting the isotopic composition recorded in bulk rock away from the seawater value (Romaniello et al., 2013). Consequently, we had to develop a new methodology that would allow us to extract the original carbonate isotopic composition from the bulk isotopic composition. For this purpose we developed a step leaching protocol using modern corals. Each leaching step digests ~10% of the total carbonate content of the sample. We found that the first and last few leachates are depleted in U while the intermediate leachates show higher than bulk concentrations. Similarly, the isotopic composition of the U collected in each step is not constant and can differ from the isotopic composition of modern seawater by up to 0.2 ‰ (four times the 2SE of the measurements).

Using this newly established protocol, we are currently in the process of measuring an array of modern carbonates (age from 0 to 1.5 My) taken along a core in the Bahamas bank to assess the effect of early diagenesis on the uranium proxy and validate our methodology at the same time. Older samples will be measured in the coming months to track the rise of the oxygenation of the ocean through Earth’s history.

This grant has supported methodology development, samples characterization and high precision isotopic analyses for one of my PhD student, Francois Tissot, and three chemistry major undergraduates, Benjamin Matthew Go, Magdalena Naziemiec and Garrett Healy. An abstract and a poster presented preliminary results obtained in March 2014 at the LPSC. Results will be presented at the Goldschmidt conference or the AGU conference in 2015.