Evaluating the integrity of the Ce anomaly as a paleoredox tracer using modern marine carbonates

Abstract:

The redox sensitivity of cerium (Ce) relative to other rare earth elements (REEs) and its uptake in marine carbonates makes the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing local redox conditions in the water column. However, although the Ce anomaly is known to be sensitive to detrital inputs, and carbonates are generally vulnerable to multiple styles of post-depositional diagenetic alteration, evaluations of proxy-challenging complications have mostly been limited to ancient carbonates, with few constraints from younger, well-characterized analog facies. Here, we report on analyses of REE concentrations in bulk samples from well-characterized modern marine carbonate samples from the Clino and Unda drill cores in the Bahamas¹. These cores sample multiple alterations: 1) meteoric/mixing zone diagenesis, 2) marine burial diagenesis, and 3) dolomitization, which collectively control secondary carbonate mineral transformations. Ce/Ce* = Ce/Pr²/Nd, calcuated using shale-normalized (i.e., PAAS) Ce, Pr, and Nd concentrations, vary with depth in modern seawater, with a range of 0.3 to 0.8 within the top 200 m depth² and average values of 0.5-0.6 in the upper 200 m. Our results show that the REE patterns in these carbonates are very similar to those of modern seawater, indicating they may reflect primary deposition from seawater. Consistent with this assertion, anomalies in meteorically altered carbonates from the Clino core display values ranging from 0.4 to 0.8. Anomalies in the marine burial zone show larger values with smaller variations (ranging from 0.6 to 0.8) compared to the meteoric zone. This relationship may indicate that the pore fluids in the marine burial zone are more reducing compared to meteoric fluids, or differences in Ce/Ce* can be caused by changes in depositional depth. In Unda core, we observe Ce anomaly variations from an average of 0.65 in the marine burial zone to 0.62 in partially dolomitized zone. Our results, particularly the general agreement with seawater, indicate that the Ce anomaly in shallow marine carbonates has the potential to preserve records of primary deposition even in bulk carbonates with muliple styles of post-depositional diagenetic alteration.

References:

1. Melim et al. (1995) Geology 23; 755-758. 2. Zhang and Nozaki (1998) GCA 62-8; 1307–1317.