Researchers have developed a high precision method for measuring the fractionation of uranium isotopes, specifically the ratio of 238U to 235U (238U/235U). The work was undertaken to help in assessing the use of 238U/235U in calcium carbonate (CaCO3) as a paleoredox proxy. Previous studies using the 238U/235U method on natural biogenic carbonates resulted in no detectable U isotope fractionation as the precision was not high enough to pick up fractionation levels observed in laboratory experiments.

The new, high precision method was used to study biogenic carbonates from a variety of organisms, and multiple environmental sites. The results suggest that uranium isotopes are indeed fractionated as they are incorporated into biogenic carbonates. The fractionation occurs in the same direction as observed in abiotic experiments, but at a smaller magnitude.

The study, “Biological effects on uranium isotope fractionation (238U/235U) in primary biogenic carbonates,” was published in the journal Geochimica et Cosmochimica Acta. The work was performed at the NSF/NASA Center for Chemical Evolution (CCE) at the Georgia Institute of Technology in Atlanta, Georgia. The CCE is a collaborative program supported by the National Science Foundation (NSF) and the NASA Astrobiology Program.