Constraints in using Cerium-anomaly of bulk sediments as an indicator of paleo bottom water redox environment: A case study from the Central Indian Ocean Basin

Abstract

The total rare earth elements (∑REE) abundance along with major and a range of trace element chemistry of twenty-eight sub-sections in a 5 m long sediment core (AAS-05/GC-02) from the Central Indian Ocean Basin shows a distinct REE fractionation. Sediments from the top ∼1 m of the core have comparatively low ∑REE abundance (close to average shale), with a sample / shale REE ratio very close to 1 strongly suggesting a dominantly terrigenous source. The down-core ∑REE abundance shows a nearly two fold increase (∑REE = 167–341 ppm) with depth and the REE strongly co-varies with Mn, Ti, P and smectite. Inter-element correlations and the shale-normalized REE patterns suggest that light, middle and heavy REE are selectively carried by terrigenous, Mn oxide and authigenic phosphate phases respectively suggesting a distinct REE fractionation. These fractionation effects become more pronounced with increasing depth in the core. The Cerium (Ce)-anomaly in marine sediments is used as one of the promising tools to trace paleo bottom water redox conditions and this varies in the sediments from 0.10 to − 0.08. The Ce-anomaly in general is positive in the top 4 m but the top ∼1 m have higher Ce-anomaly, Mn / Ti and Ce / Ti suggest sediment deposition relatively under more oxic bottom water condition. The negative Ce-anomaly between 4 and 5 m depth, which could be interpreted to indicate a suboxic/anoxic environment. The redox environment inferred by the Ce-anomaly is compared to a number of other redox sensitive parameters to test its reliability. These include Mn content, total organic carbon, U / Th, authigenic Uranium, Cu / Zn and V / Cr ratio that all suggestive of deposition of sediments under oxygenated bottom water condition. Thus, the negative Ce-anomaly observed between 4 and 5 m core depth does not represent a suboxic/anoxic environment, but is more likely due to the retention of a negative Ce-anomaly caused by authigenic phosphate and authigenic Fe-rich smectite which is formed by the reaction between iron oxyhydroxide and biogenic opal suggests an early diagenetic process and is in equilibrium with seawater REE. Early diagenetic process is responsible for the part of negative Ce-anomaly observed here. Therefore, we suggest that the Ce-anomaly of bulk sediments as an indicator of paleo-ocean bottom water redox conditions needs to be used with a caution.