en
Scientific article
English

Biological effects on uranium isotope fractionation (238U/235U) in primary biogenic carbonates

Published inGeochimica et Cosmochimica Acta, vol. 240, p. 1-10
Publication date2018
Abstract

Determining whether U isotopes are fractionated during incorporation into biogenic carbonates could help to refine the application of 238U/235U in CaCO3 as a robust paleoredox proxy. Recent laboratory experiments have demonstrated that heavy uranium (U) isotopes were preferentially incorporated into abiotic aragonite, with an isotope fractionation of ±0.10‰ (238U/235U). In contrast, no detectable U isotope fractionation has been observed in most natural primary biogenic carbonates, but the typical measurement precision of these studies was ±0.10‰ and so could not resolve a fractionation of the magnitude observed in the laboratory. To resolve this issue, we have developed a high precision 238U/235U method (±0.02‰, 2 SD) and utilized it to investigate 238U/235U in primary biogenic carbonates including scleractinian corals, calcareous green and red algae, echinoderms, and mollusks, as well as ooids from the Bahamas, Gulf of California, and French Polynesia. Our results reveal that many primary biogenic carbonates indeed fractionate U isotopes during U incorporation, and that this fractionation is in the same direction as observed in abiotic CaCO3 coprecipitation experiments. However, the magnitude of isotope fractionation in biogenic carbonates is often smaller than that predicted by abiotic CaCO3 coprecipitation experiments (0.00–0.09‰ vs. 0.11 ± 0.02‰), suggesting that one or more processes suppress U isotope fractionation during U incorporation into biogenic carbonates. We propose that closed-system behavior due to the isolation of the local calcificiation sites from ambient seawater, and/or kinetic/disequilibrium isotope fractionation caused by carbonate growth kinetics, explains this observation. Our results indicate that U isotope fractionation between biogenic carbonates and seawater might help to constrain U partition coefficients, carbonate growth rates, or seawater chemistry during coprecipitation.

Keywords
  • Primary carbonates
  • Biological effects
  • 238U/235U
  • The Bahamas
Funding
Citation (ISO format)
CHEN, Xinming et al. Biological effects on uranium isotope fractionation (238U/235U) in primary biogenic carbonates. In: Geochimica et Cosmochimica Acta, 2018, vol. 240, p. 1–10. doi: 10.1016/j.gca.2018.08.028
Main files (1)
Article (Published version)
accessLevelRestricted
Identifiers
ISSN of the journal0016-7037
442views
0downloads

Technical informations

Creation10/11/2018 2:56:00 PM
First validation10/11/2018 2:56:00 PM
Update time03/15/2023 8:45:48 AM
Status update03/15/2023 8:45:48 AM
Last indexation05/02/2024 9:25:02 PM
All rights reserved by Archive ouverte UNIGE and the University of GenevaunigeBlack