Diagenetic effects on uranium isotope fractionation in carbonate sediments from the Bahamas

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doi: 10.1016/j.gca.2018.06.026
Author(s): Chen, Xinming; Romaniello, Stephen J.; Herrmann, Achim D.; Hardisty, Dalton; Gill, Benjamin C.; Anbar, Ariel D.
Author Affiliation(s): Primary:
Arizona State University, School of Earth and Space Exploration, Tempe, AZ, United States
Louisiana State University, United States
Woods Hole Oceanographic Institution, United States
Virginia Polytechnic Institute and State University, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.237, p.294-311. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 105 refs.; illus., incl. 4 tables, sketch maps
Summary: Uranium isotope variations (δ238U) recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time. However, the effects of diagenetic alteration on the U isotopic composition in carbonate sediments, which are crucial to understand the accurate reconstruction of marine δ238U, are currently poorly constrained. Here we examine the effects of the aragonite-to-calcite transition in the Pleistocene Key Largo Limestone of South Florida, and assess the effects of vadose meteoric, phreatic meteoric, and phreatic marine diagenesis on U isotope fractionation in carbonate sediments from the Bahamas Transect, including the well-studied Clino, Unda, and ODP Site 1006 drill cores.Our results suggest that early diagenetic processes in Bahamas carbonate sediments fractionate U isotopes by an average of 0.27±0.14 ppm (1 SD) heavier than contemporaneous seawater. Downcore variations of δ238U in slope and basin sediments display little, if any, correlation with U concentration and common geochemical indicators of diagenesis (δ13C, δ18O, Mn/Sr, Mg/Ca, Sr/Ca), enrichments of redox-sensitive elements, or rare earth elements anomalies. We propose two possible mechanisms to interpret the positive change in the δ238U during carbonate diagenesis: authigenic enrichment of isotopically positive U(IV) in carbonates and preferential incorporation of isotopically positive aqueous U(VI) species into carbonates. These processes likely operate during early (syndepositional) diagenesis on the banktop. Further diagenesis during deeper burial is limited by the low solubility of U(IV) under reducing pore water conditions. The early diagenetic behavior of U isotopes in Bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past. We suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater δ238U from ancient carbonates. Furthermore, early diagenesis induces significant statistical variability in sediment δ238U values, pointing to the need for large, high resolution data sets in order to average out stochastic variations in individual bulk sediment samples.
Year of Publication: 2018
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 24 Surficial Geology, Quaternary Geology; Actinides; Alkaline earth metals; Aragonite; Atlantic Ocean; Authigenesis; Bahamas; C-13/C-12; Calcite; Calcium; Carbon; Carbonate rocks; Carbonate sediments; Carbonates; Caribbean region; Cenozoic; Diagenesis; Eh; ICP mass spectra; Isotope fractionation; Isotope ratios; Isotopes; Key Largo Limestone; Leg 166; Limestone; Magnesium; Manganese; Mass spectra; Metals; North Atlantic; O-18/O-16; ODP Site 1006; Ocean Drilling Program; Oxygen; Pleistocene; Quaternary; Radioactive isotopes; Rare earths; Reconstruction; Sea water; Sedimentary rocks; Sediments; Spectra; Stable isotopes; Stochastic processes; Straits of Florida; Strontium; U-238/U-234; Uranium; West Indies
Coordinates: N220000 N240000 W0740000 W0800000
Record ID: 2019040202
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands