The influence of seawater carbonate chemistry, mineralogy, and diagenesis on calcium isotope variations in Lower-Middle Triassic carbonate rocks

摘要

AbstractThe geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios (44Ca/40Ca, expressed asδ44/40Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, theδ44/40Ca of carbonate rocks is influenced not only by theδ44/40Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonateδ44/40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawaterδ44/40Ca may be expected based on several large carbon isotope (δ13C) excursions ranging from ?2 to +8‰. Parallel negativeδ13C andδ44/40Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion inδ44/40Ca of ~0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronous between regions and thus cannot be interpreted to reflect secular change in theδ44/40Ca of global seawater. Additionally,δ44/40Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuringδ44/40Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in theδ44/40Ca of seawater)versuslocal controls (e.g., original mineralogy or diagenetic alteration). The [Sr] andδ44/40Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. Using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonateδ44/40Ca values, the relationship between [Sr] andδ44/40Ca holds potential for reconstructing first-order secular changes in seawaterδ44/40Ca composition.]]>