CHROMIUM INCORPORATION INTO CALCIUM CARBONATE MINERALS AND ASSOCIATED ISOTOPIC FRACTIONATION: IMPLICATIONS FOR THE CR ISOTOPE PALEOPROXY

The chromium (Cr) isotope system has recently gained interest as a paleoproxy for tracking atmospheric oxygen levels due to the presence of large isotope fractionations in association with redox reactions. One important assumption in the current framework is that redox reactions cause the most significant Cr isotope fractionations, and that these variations are faithfully preserved during the formation and burial of Cr-containing sedimentary rocks. Carbonate sediments are commonly used for Cr isotope paleoproxy studies due to their stratigraphic ubiquity and the potential that they record isotopic signals from shallow seawater. However, in order to accurately interpret Cr isotope signals preserved in carbonate sediments, it is critical to investigate and understand potential isotope fractionations caused by Cr incorporation into carbonate minerals.

We investigated Cr isotope fractionation during Cr(VI) co-precipitation with different calcium carbonate phases (aragonite, calcite, and amorphous calcium carbonate), with a specific focus on the effects of precipitation rate, reaction time, and aqueous Cr concentration. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to determine the mineral phase and morphology of the solid phase precipitates. X-ray adsorption spectroscopy (XAS) was used to determine oxidation state and the local bonding environments of incorporated Cr ions. Chromium isotope fractionation was measured using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS).

Incorporation into calcite resulted in larger isotope fractionations than both aragonite and amorphous calcium carbonate. This difference in fractionation signature is likely due to differences in the structural constraints imposed on incorporated Cr ions into the lattices of calcite, aragonite, and amorphous calcium carbonate. Higher concentrations of Cr in initial solution resulted in more positive isotope fractionation. The isotope fractionation signatures measured from this preliminary study are significant when compared to previous Cr isotope fractionation measured in carbonate sediments, and should be considered in further attempts to better constrain isotopic mass balance of Cr in modern and ancient oceans.