Revised Methods for the Application and Interpretation of the Rare Earth Element Paleoenvironmental Proxy

The rare earth elemental (REE) composition of seawater is responsive to numerous factors, including inputs from rivers and hydrothermal systems and the relative effectiveness of scavenging sinks, which are themselves dependent on redox state, pH, and carbonate chemistry of ambient seawater. Due to these sensitivities, REE relative enrichment indices preserved in carbonate sediments represent promising paleoenvironmental proxies. Despite the large corpus of modern oceanic and ancient sedimentary analyses, application of the REE paleo-proxy has traditionally been limited by several uncertainties: 1) the partitioning of rare earths during co-precipitation into various carbonate components; 2) the susceptibility of preserved sedimentary REE compositions to alteration throughout burial history and potential contamination during lab analyses; and 3) the unambiguous interpretation of elemental ratios (e.g., Yb/Nd, Y/Ho) to reflect specific paleoenvironmental conditions and processes.

Here, we present observations from a new literature compilation of REE abundance measurements from modern seawater and carbonate sediment samples, as well as from ancient carbonate samples spanning 4 billion years, to further the development of the REE paleoenvironmental proxy. Based on the apparent partitioning of rare earths between different modern carbonate components and ambient seawater, we propose a set of partition coefficients which reconcile seawater REE reconstructions from biogenic, cement, microbialites, ooid, and bulk carbonate samples. Correlations between siliciclastic and trace elemental proxy data and REE relative enrichment proxies illustrate which indicators are most useful when evaluating alteration of primary REE compositions, and consequently, which proxies are most robust. Finally, we demonstrate the effectiveness of an orthogonal polynomial fitting to REE data to more accurately characterize absolute and relative enrichments. With these proposed methods, the rare earth elemental paleo-proxy can continue to develop and be applied for new insight into local and global paleoenvironmental processes through Earth's history.