A geochemical framework for retrieving the linked depositional and diagenetic histories of marine carbonates

In modern shallow seawater carbonate platforms, successive changes in diagenetic zones are found as the result of a drop in sea level. Such a relationship is important in identifying diagenetic processes, in interpreting global carbon isotope shifts and in finding the reasons for positive correlations between C and O isotopes in marine carbonates. However, it is not readily recognized in ancient marine carbonates because evidence for sea level changes can be cryptic in the rock record. Because of the fall of sea level, fresh water may add to precipitating marine carbonates on continental substrates, resulting in geochemical mixtures between shallow seawater and fresh water. To resolve this issue, we carry out a combined study of rare earth elements and yttrium (REE + Y) and CO isotopes in Carboniferous-Triassic marine carbonates from the Lower Yangtze platform in China. The relationships between inter-REE + Y ratios such as Y/Ho vs. (Nd/Yb)PAAS for pure carbonates strongly indicate mixing between fresh water and shallow seawater. The REE + Y patterns for pure carbonates show temporal variations, reflecting changes in the proportion of fresh water addition into shallow seawater, which were likely due to changes in relative sea level. Changes in diagenetic zones indicated by the relationships between carbonate C and O isotopes as well as other evidence follow the changes in relative sea level suggested by the REE + Y patterns. Therefore, a combined study of the REE + Y and CO isotopes in marine carbonates can provide robust constraints on the changes in the linked depositional environments and diagenetic processes on continental substrates.