A Quantitative Model of the Global Rare Earth Element Cycle and Implications for Variability in the REE Composition of Ancient Oceans

Rare earth elements (REEs) represent valuable proxies due to their coherent chemical behavior and mass fractionation in response to complexation and particle scavenging. These processes induce a shale-normalized enrichment of heavy relative to light REEs in the modern ocean, while deviations from this fractionation may be indicative of freshwater mixing or anomalous aqueous geochemistry. The REE composition of carbonate minerals has also been utilized as a proxy for water circulation, paleobathymetry, and redox state in ancient oceans. However, these data are subject to scrutiny because the low levels of REEs in carbonates make them susceptible to overprinting when contaminated by REE-rich siliciclastics, phosphates, and ferromanganese crusts. A common screening practice for local effects rejects data which do not exhibit a positive La anomaly, high Y/Ho, and heavy REE enrichment - characteristics of the modern ocean. This approach assumes that seawater REE composition does not vary over geological time, but without a theoretical framework to suggest otherwise, analytical techniques will be unable to test for evolution in REE fractionation.

In this study, we simulate trends in global oceanic REE reservoirs and fractionation with a simple box model. This treatment calculates a mass balance between input fluxes of REEs (terrestrial weathering, aerosol deposition, hydrothermal fluids, and sedimentary diagenesis) and output fluxes by scavenging. Additional parameters such as climate and weathering, hydrothermal activity, and ocean alkalinity are incorporated as correction factors. By approximating climate and geochemical conditions, REE patterns in ancient oceans may be estimated. Furthermore, we simulate environmental perturbations in Earth's history to measure the amplitude and turnover of the REE response. These preliminary results suggest dynamic global trends in REE fractionation over geologic timescales. Although local effects such as siliciclastic contamination must still be considered when interpreting REE data, future sampling for stratigraphic variation of REE enrichments in pristine carbonates will test our model hypotheses of temporal trends. Through the union of modelling and analytical techniques, REEs may continue to develop as a proxy for paleoclimate and ocean geochemistry.