URANIUM ISOTOPIC COMPOSITIONS OF CARBONATE SEDIMENTS AS A POTENTIAL REDOX PROXY

Redox changes in the global ocean have played a major role in the evolution of life. In recent years, new proxies have been developed to track these changes through Earth history and to understand the underlying causes. However, most of these proxies are tied to black shale deposits. So far no proxy exists that reliably tracks redox changes of the global ocean in carbonate rocks. A carbonate-based proxy would allow access to a more complete geologic record and a wider range of depositional environments than proxies based on analyses of black shales.

One potential proxy is the U isotope system. Uranium is an abundant trace element in carbonate sediments and carbonate rocks. Recent work has revealed significant variations in the natural isotope composition of U isotopes, driven by isotope fractionation. Because the residence time of U in the oceans far exceeds ocean mixing times, this isotope system could provide a proxy that integrates global average ocean conditions. Previously published results suggest that biogenic carbonates from corals record the U isotopic composition of seawater. Hence, carbonate sediments could track changes of the U isotopic composition of seawater. Here we report U isotope values from non-biogenic ooids (from Joulters Cay) and sediments (mixed biogenic and non-biogenic) from an ODP core of Bahamas slope deposits.

The ooids are slightly heavier (~ -0.26‰) than previously reported average seawater values (average -0.41‰). Similarly, slope sediments from ODP samples (Leg 166) are also slightly heavier (average -0.23‰). These results suggest that unlike biogenic carbonates, carbonate sediments that are not entirely derived from organisms do not reflect seawater values. Potential explanations are (1) that these deposits contain phases (e.g., organics) that have a heavier U isotopic composition than seawater, (2) that the isotopic composition of the seawater of the Bahamas bank differs from open ocean water, or (3) that abiotic precipitation of calcium carbonate leads to isotopic fractionation. Further study is needed to understand these isotopic offsets in order to fully exploit U isotopes in non-biotic carbonates as a paleoredox proxy.