EXPLORING CHROMIUM SPECIATION IN ANCIENT SEDIMENTARY RECORDS

Recently, chromium (Cr) stable isotopes have successfully been used to investigate the Great Oxidation Event (GOE) and the subsequent oxygenation dynamics during the Proterozoic in ancient sedimentary records. However, processes controlling the burial of this promising paleo-redox proxy are not yet well understood, hampering our interpretation of ancient sedimentary records.

To clarify these processes, the Cr speciation in Proterozoic black shales and ironstones were examined using X-ray Adsorption Fine Structure (XAFS) spectroscopy. XAFS allowed us to determine the oxidation state, coordination chemistry and molecular environment of Cr directly within the rock matrix.

As expected, our preliminary results suggest that Cr is present as the reduced Cr(III) in all our tested samples. Beyond the oxidation state, two different Cr(III) phases were detected, which suggest two distinct pathways controlling Cr burial. The ratio of these two phase changes are based on the type and age of the rock samples, which might be influenced by different processes occurring during transport and burial. Chromium may be buried via precipitation from within water column where anoxic conditions occurred, and/or transported directly from the continental mass without transformation occurring. By identifying processes controlling Cr burial pathways, we can refine our understanding of the early Earth's oxygenation.