Assessing the reliability of modern marine stromatolites as archives for the uranium isotope paleoredox proxy

verfasst von

Ashley N. Martin, Monika Markowska, Allan R. Chivas, Stefan Weyer

Abstract

Uranium isotopes (²³⁸U/²³⁵U) are commonly utilised in marine carbonates to reconstruct the redox evolution of ancient oceans and the atmosphere. However, questions remain concerning U isotope fractionation during carbonate precipitation and/or early sedimentary diagenesis. Moreover, the stromatolitic nature of many Precambrian limestones and dolostones brings further questions about U isotope fractionation during stromatolite formation due to microbial activity. Thus, it is unclear whether stromatolites are reliable archives for the U isotope composition of seawater. Here we measure the ²³⁸U/²³⁵U ratio of modern stromatolites from the hypersaline Shark Bay, Western Australia, a key study area that represents the largest modern example of stromatolite growth globally. A component-specific approach was utilised to analyse the uppermost crusts of modern stromatolites, older material from the deeper stromatolite laminae and the substrate upon which the stromatolites are forming. Our interpretations of δ²³⁸U data are aided by δ²³⁴U, δ¹⁸O, δ¹³C and trace element measurements, including some mineralogy and total organic carbon determinations for selected samples. Modern aragonitic crusts of stromatolites from the subtidal and intertidal zones around Shark Bay exhibited a narrow range of δ²³⁸U (−0.30 to −0.33‰), corresponding to a small offset of ca. +0.07‰ from seawater. This range of δ²³⁸U values overlaps with that of other primary carbonate precipitates in shallow marine environments such as corals but with a slightly higher mean δ²³⁸U. We investigated if this offset may be associated with the coprecipitation of U with aragonite from hypersaline seawater in Shark Bay, which has a higher proportion of dissolved U present as Ca₂UO₂(CO₃)₃ compared to open seawater. However, this measured offset is much smaller than that predicted using a speciation-dependent isotope fractionation model. Sub-recent material in the deeper stromatolite laminae exhibited higher and more variable δ²³⁸U (up to −0.15‰) compared to the modern aragonitic crusts of stromatolites. The strong inverse correlation between δ²³⁸U and the total organic carbon content (r = −0.83) and positive correlation with Mn concentrations (r = 0.91) suggest that ²³⁸U was preferentially reduced in the deeper laminae in association with organic carbon remineralisation under reducing conditions. The average δ²³⁸U offset of sub-recent stromatolite laminae from modern seawater was +0.13 ± 0.11‰ (1 s.d.), which is slightly lower than, but within uncertainty of, the offset measured for bulk carbonates from the Bahamas (+0.27 ± 0.14‰; 1 s.d.). These results demonstrate that stromatolites are valuable archives of global seawater δ²³⁸U, even in hypersaline restricted basins such as Shark Bay, but an offset to account for syndepositional diagenesis is likely required to accurately reconstruct seawater δ²³⁸U using ancient stromatolites.

Organisationseinheit(en)

Institut für Mineralogie
AG Geochemie

Externe Organisation(en)

Max-Planck-Institut für Chemie (Otto-Hahn-Institut)
University of Wollongong
University of Adelaide

Typ

Artikel

Journal

Geochimica et cosmochimica acta

Band

345

Seiten

75-89

Anzahl der Seiten

15

ISSN

0016-7037

Publikationsdatum

15.03.2023

Publikationsstatus

Veröffentlicht

Peer-reviewed

Ja

ASJC Scopus Sachgebiete

Geochemie und Petrologie

Ziele für nachhaltige Entwicklung

SDG 14 – Lebensraum Wasser

Elektronische Version(en)

https://doi.org/10.1016/j.gca.2023.01.011 (Zugang: Geschlossen)