60th Annual Report on Research 2015

Uranium in marine carbonate rocks provides a unique measure of past oxygenation because ²³⁸U is preferentially reduced from U(VI) to U(IV) (Bigeleisen, 1996) at the sediment-water interface, leaving the residual U(VI) in oxic seawater with a lower d²³⁸U value relative to precipitated uranium in sediments (Brennecka et al., 2010). Consequently, a shift toward more reducing conditions at the sediment-water interface should drive simultaneous decreases in [U] and d²³⁸U is seawater (and carbonate rocks). Moreover, recent studies suggest that microbially mediated uranium reduction is responsible for the observed isotopic fractionation (Stylo et al., 2015). In this project, we are using uranium isotopes and concentrations to study periods in the past characterized by (1) substantial marine anoxia; and (2) the rise of oxygen, as well as (3) modern sediments where U is actively redox cycled.

To characterize the extent of marine anoxia during the recovery period from the end-Permian mas extinction, we measured [U] and d²³⁸U in carbonates from multiple sections in south China and Turkey. Our results show a significant decrease in both d²³⁸U and [U] at the PTB from relatively high Latest Permian values to sustained low values in the Early Triassic, which gradually become more positive in the Middle Triassic (Fig. 1). These changes are best explained by a 100-fold increase in the area of anoxic bottom waters after the extinction that likely resulted in a shallow oxygen minimum zone (OMZ) that shoaled onto the continental shelves, followed by a slow recovery to pre-extinction conditions. we find that the pattern of anoxia in the Early Triassic mirrors that of depressed biodiversity, ecological complexity, and gastropod body size (Fig. 1). In addition, the large negative and positive d¹³C perturbations in the Early Triassic (Payne et al., 2004) coincide with the periods of widespread anoxia. Given these observations, we propose that sea level variations may have modulated the depth of the OMZ, resulting in inhospitable conditions for animals and fluctuations in organic carbon burial fluxes (Lau et al., in review).

Fig. 1: Late Permian to Late Triassic [U] and d²³⁸U data and global records of biogeochemical cycling and biotic recovery. (A) Global sea level curve. (B) d¹³C data. (C) d²³⁸U data. Error bars on d²³⁸U data are 2σ of replicate analyses and are reported relative to CRM-145. (D) [U] data. Vertical gray lines in (C) and (D) are the mean Late Permian values. (E) Carbonate-associated sulfate sulfur isotope data (d³⁴S_CAS). (F) Global trends in sampled-in-bin genus diversity (green open diamonds), modes of life (purple circles), and maximum gastropod body size (black open circles).

To determine if increasing ocean oxygenation levels are linked to the first evolution of metazoans (Cohen and Macdonald, 2015), we have been investigating patterns of oxygenation during the Cryogenian interglacial period using d²³⁸U and [U] in limestone of the Taishir Formation in Mongolia (in two stratigraphic sections that are separated by ~75 km across the basin). Above the Sturtian glacial deposits, d²³⁸U compositions have a mean value that is similar to that of modern seawater. Coincident with a large negative d¹³C shift of 15 per mil known as the Taishir excursion, the d²³⁸U record exhibits a step decrease of ~0.3 per mil, and d²³⁸U remains approximately constant until the erosional unconformity at the base of the Marinoan glacial deposits.  The best explanation for the less negative post-Sturtian values of d²³⁸U is extensive oxygenation of the seafloor. Moreover, the model demonstrates that the higher d²³⁸U values of the post-Sturtian limestones are inconsistent with an increased flux of uranium to the oceans due to post-Snowball weathering as the primary driver of the excursion. Thus, we favor a scenario in which there was a rise in oxygen levels following the Sturtian glaciation followed by a decrease in seafloor oxygenation coincident with the Taishir d¹³C excursion. The U isotopic data, combined with modeling results, challenge the notion of a unidirectional oxygenation history of Neoproterozoic oceans.

An additional component of our project is evaluating the isotopic fractionation associated with U reduction and redox cycling. Because of the difficulty in obtaining sufficient marine porewater, we have analyzed well-characterized sediments from naturally reduced zones within alluvial sediments throughout the Colorado Basin, USA. We observe a strong correlation between U accumulation and the extent of isotopic fractionation, with a d²³⁸U difference of up to +1.6 per mil between uranium-enriched and low concentration zones. The greatest enrichment occurs at the boundary between the water table and unsaturated zone, suggesting redox cycling at oxic-anoxic interfaces can result in substantial enrichment of ²³⁸U in the residual U(IV) pool.  Additional work is underway to compare these values to surrounding groundwater and the speciation of Fe, S and U and to develop reactive transport simulations.

Our efforts in the coming year will focus on (1) the new Neoproterozoic record, (2) expanded data for modern sediments and a reactive transport model linking microbial processes to the uranium and carbon isotopic composition of key phases, and (3) preparing and revising publications.  The later work will further our understanding of the U isotope dynamics at the sediment-water interface and how their linkages to the marine carbon cycle.

References:

Bigeleisen, J. (1996) J. Am. Chem. Soc. 118, 3676-3680.

Brennecka et al. (2011) Proc. Natl. Acad. Sci., 108, 17631-17634.

Cohen, P.A. and Macdonald, F.A., 2015. Paleobiology, DOI: 10.1017/pab.2015.25, 1-23.

Payne, J.L. et al. (2004) Science 305, 506-509.

Stylo, M. et al. (2015) Proc. Natl. Acad. Sci. 112, 5619-5624.