CHOICE OF CARBONATE DIGESTION METHOD IMPACTS THE ABSOLUTE VALUES OF ORGANIC CARBON ISOTOPE STRATIGRAPHY – AN EXAMPLE FROM CRETACEOUS OAE2

Before organic carbon isotopes can be analyzed, the carbonate component of the sample must be removed first. There is no standard protocol for this step and a wide range of methodologies have been used in the literature. HCl is the most commonly used acid, but the strength and time of dissolution vary widely. Ten percent, 6N, and even as low as 0.5N have all been reported in the literature. The length of digestion also varies in the literature from overnight in a single digestion step to as little as 30 minutes with up to four digestions in weaker acid. All reported organic carbon isotope data include drying and pulverization steps before final analysis, but even the drying step has been reported to range from 40 degree centigrade to 85 degrees centigrade.

Here, we compare two organic carbon isotope data sets from a single core in Iowa that spans the Upper Cretaceous Oceanic Anoxic Event OAE2. A legacy data set had been produced utilizing strong HCl that was allowed to react overnight, and a new data set has been produced that utilized 0.5N HCl introduced in 20mL washes four times for a total reaction time of less than two hours. The two data sets show identical shapes of the signal demonstrating that they both faithfully record the timing of the onset, peak, and end of OAE2. However, the absolute values of the data vary considerably with the stronger acid digestion method showing generally lighter (i.e., more negative) values. This study demonstrates the impact that the carbonate digestion method can have on the absolute values of organic carbon isotope data and provides a cautionary note in over-interpreting the absolute values of organic carbon isotope stratigraphy.