The history of ocean carbonate chemistry indicated by rare earth elements

Due to their relatively short oceanic residence times and coherent chemical behavior, the rare earth elements (REE), preserved in carbonate minerals, have been developed as a geochemical proxy sensitive to water circulation, paleobathymetry, and redox state (Ce-anomaly). In the modern ocean, rare earths develop a shale-normalized enrichment of heavy relative to light elements because lesser complexation of light REE by carbonate ions make them more susceptible to particle scavenging of the free ion. Presuming that this phenomenon dominated REE chemistry in the past, ancient seawater REE composition, namely the degree of HREE enrichment, must have evolved systematically with [CO₃^2-], such that the measurement of rare earths in primary chemical sediments may serve as a proxy for ancient ocean carbonate chemistry.

Here, we present a novel compilation of >3000 REE abundance data from chemical sediments spanning 4 billion years, including diverse carbonate components (bulk limestone/dolostone, cements, microbialites, brachiopods, ooids), iron formations, and phosphates. These measurements exhibit apparent temporal trends in relative fractionation which match model predictions for a [CO₃^2-]-control on oceanic rare earth composition. The REE data also show significant variation with mineralogy, carbonate component type, and geochemical indicators for siliciclastic contamination. From these results we propose updated screening procedures for targeting primary REEs in sediments, and advocate for the use of REE in carbonate sediments for constraining ancient seawater chemistry.