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

作者：Chen Xinming^*; Romaniello Stephen J; Herrmann Achim D; Hardisty Dalton; Gill Benjamin C; Anbar Ariel D

来源：Geochimica et Cosmochimica Acta, 2018, 237: 294-311.

DOI：10.1016/j.gca.2018.06.026

• Diagenesis
• Paleoredox proxy
• Uranium isotopes
• Carbonates

Uranium isotope variations (delta U-238) 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 delta U-238, 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 parts per thousand (1 SD) heavier than contemporaneous seawater. Downcore variations of delta U-238 in slope and basin sediments display little, if any, correlation with U concentration and common geochemical indicators of diagenesis (delta C-13, delta O-18, 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 delta U-238 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 delta U-238 from ancient carbonates. Furthermore, early diagenesis induces significant statistical variability in sediment delta U-238 values, pointing to the need for large, high resolution data sets in order to average out stochastic variations in individual bulk sediment samples.

• 出版日期2018-9-15