Modern carbonate ooids preserve ambient aqueous REE signatures

Abstract

Copyright © 2019 The Authors. Skeletal and non-skeletal components of marine sedimentary rocks have been analyzed for the purpose of reconstructing the rare earth element (REE) and yttrium (Y) compositions of paleo-seawater, but skeletal carbonates frequently have proven to be unreliable recorders of seawater chemistry. Here, we present a systematic multi-technique assessment of rare earth and other trace elements in ooid sands from the modern Great Bahama Bank (GBB–marine) and Great Salt Lake (GSL–continental) based on strong-acid (hydrofluoric and nitric) and weak-acid (acetic) digestions, as well as laser ablation (LA) of ooid cortices and nuclei. The results show that Bahamian ooid cortices possess shale-normalized REE + Y features nearly identical to those of shallow seawater, including limited contamination from siliciclastic REEs. An admixture of even 0.2% of detritally sourced material can modify the primary marine REE + Y patterns by, for example, increasing the light REE (LREE) content. Mean values of LA data for Bahamian ooid cortices exhibit similar REE + Y signatures to those produced by acetic acid digestion, but LA data are generally noisier, primarily as a result of low REE concentrations and the small volume of carbonate ablated in analysis. Screening out samples with ΣREE <0.9 ppm reveals more uniform, seawater-like REE + Y patterns in individual ooid cortices as well as high, uniform REE distribution coefficients (116 ± 21, from La to Lu) with respect to ambient seawater. Ooid laminae show no distinct alternation between oxic and anoxic pore fluid characteristics, suggesting that growing ooids primarily formed under oxic conditions in carbonate shoal settings, even when buried during their resting phase. Unlike Bahamian ooids, shale-normalized REE + Y patterns of GSL ooids digested in weak acid show slightly depleted LREE compositions that may deviate from ambient fluids owing to release of REEs from an included clay fraction based on comparison between digestion and LA methods. Multiple ablation spots within the same cortex allowed direct comparison of more and less contaminated portions of a single ooid. Nearly uniform positive Ce anomalies may be related to strongly alkaline water conditions. Individual ablation spots that are partially overprinted by siliciclastic-derived REEs contain variable Zr contents (from 0.096 to ~4 ppm) and flattened shale-normalized LREE patterns. The least-contaminated GSL ooid cortex yields an LREE-depleted REE + Y distribution. Normalization of the REE + Y distributions of least-contaminated GSL ooids using reliable GBB ooid distributions (i.e., with ΣREE > 0.9 ppm) returns a flat pattern, suggesting similar degrees of LREE depletion controlled by carbonate complexation under similar aqueous alkalinity conditions in Great Salt Lake and Bahamian waters. In summary, ooids can be a reliable proxy for REE + Y characteristics of ambient surficial waters when adopting suitable analytical methods, including laser ablation, that allow the identification and isolation of a contamination signal from siliciclastic detritus.