Experimental study of chromium (III) coprecipitation with calcium carbonate

Chromium (Cr) isotope compositions of marine carbonates have widely been used to trace the evolution of oxygen levels in the atmosphere and ocean during geological history. Recent studies have indicated that Cr found in natural carbonates is dominated by trivalent Cr, also known as Cr(III). However, the incorporation behavior of Cr(III) into carbonates remains undefined. Here, we conducted coprecipitation experiments for Cr(III) with calcium carbonate to constrain the behavior of Cr(III) in carbonate deposition environments. Extended X-ray absorption fine structure (EXAFS) results suggest that Cr(III) is incompatible with the calcite crystal lattice. There are likely to be several Cr species in the coprecipitation samples. Most Cr is adsorbed on the crystal surface of calcite or exists as amorphous Cr hydroxide [Cr(OH)3]. A small fraction of Cr can reside in the calcite crystal, but Cr3+ cannot directly substitute for Ca2+. It may exist in interstitial voids in crystals or occupy the position of Ca2+ in the form of divalent Cr hydroxide cation [Cr(OH)2+], leading to a distorted structure. Moreover, we find that Cr(III) can stimulate polymorph selection of vaterite [a metastable polymorph of calcium carbonate (CaCO3)] during CaCO3 precipitation, likely due to the formation of Cr hydroxide on the surface of vaterite crystals, hindering its transformation to calcite. Because Cr(III) is incompatible with the carbonate crystal lattice, we suggest that most Cr in natural carbonates is adsorbed on the crystal surface and may be acquired at water–sediment interfaces after carbonate precipitation. Therefore, Cr isotope compositions of carbonates may easily be affected by post-depositional diagenetic processes, and Cr isotope variations in sedimentary carbonates should not exclusively correspond to changes in oxygen levels in the atmosphere and ocean. Paleoenvironment reconstruction based on Cr isotope compositions of carbonates should take post-depositional processes into consideration.