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

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2017-11-05
Lau, Kimberly V.
Maher, Kate
Brown, Shaun T.
Jost, Adam B.
Altıner, Demir
DePaolo, Donald J.
Eisenhauer, Anton
Kelley, Brian M.
Lehrmann, Daniel J.
Paytan, Adina
Yu, Meiyi
Carlos Silva-Tamayo, Juan
Payne, Jonathan L.
The 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 (Ca-44/Ca-40, expressed as delta Ca-44/40) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the delta Ca-44/40 of carbonate rocks is influenced not only by the delta Ca-44/40 of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate delta Ca-44/40 in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater delta Ca-44/40 may be expected based on several large carbon isotope (delta C-13) excursions ranging from - 2 to + 8 parts per thousand. Parallel negative delta C-13 and delta Ca-44/40 excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in delta Ca-44/40 of similar to 0.2 parts per thousand 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 the delta Ca-44/40 of global seawater. Additionally, delta Ca-44/40 values from Turkey are consistently 0.3 parts per thousand lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring delta Ca-44/40 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 the delta Ca-44/40 of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and delta Ca-44/40 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 carbonate delta Ca-44/40 values, the relationship between [Sr] and delta Ca-44/40 holds potential for reconstructing first-order secular changes in seawater delta Ca-44/40 composition.