Chromium isotopic compositions of a reef in the South China Sea: Biological effects and influence from early diagenetic Mn-oxides reduction

Global climate fluctuations have been inextricably linked with changes in the marine environment and ecosystems' evolution during the Cenozoic. The chromium isotopic compositions (δ53Cr) of marine carbonates have been used to study changes in ocean redox conditions. However, whether δ53Cr of carbonates faithfully reflect the information of ambient seawater at the time of deposition remains controversial. Here, we report major and trace element concentrations, as well as the Cr isotopic compositions for samples from a Neogene–Quaternary biogenic reef (Core NK-1) in the South China Sea. The samples exhibit highly variable Cr concentrations and δ53Cr values. The Cr concentrations range from 0.76 μg/g to 8.58 μg/g with an average of 3.96 μg/g, and the δ53Cr values vary from −0.54‰ to 0.68‰ with an average of 0.06‰, which are generally lower than the reported Cr isotopic compositions for global seawater. The fluctuation of δ53Cr values is unlikely to be controlled by changes in the redox conditions of seawater. Biogenic processes may have affected some samples with relatively low Cr concentrations and δ53Cr values. Other samples may have been affected by post-depositional addition of Cr from pore water. Early diagenetic reduction of ferromanganese oxides is likely another significant control factor for the variation of δ53Cr values. The results suggest that the factors influencing the Cr isotopic compositions of biogenic carbonate reefs are complex, and the variation of Cr isotopic compositions should not be directly linked to the redox state of the ocean. After evaluating the potential influences of multiple factors, the inferred Cr isotopic compositions of syn-depositional fluids of the carbonates from Core NK-1 are similar, which may suggest a relatively stable redox environment in the South China Sea during the Neogene to Quaternary period.