Post-depositional effects on chromium in black shales and implications for paleo-redox reconstructions based on chromium isotopes

Chromium (Cr) is a redox-sensitive element, and its isotope system has been widely used as a paleo-redox proxy, providing insights into the evolution of oxygen in Earth's oceans and atmosphere. Black shales serve as an important archive for sedimentary Cr isotope data. The precise interpretation of Cr isotope records in black shales relies on a comprehensive understanding of the geochemical behavior of Cr throughout black shale formation and diagenesis. Here, we applied synchrotron-based X-ray spectroscopy, sequential leaching, and Cr isotope analysis to investigate the Cr species in sedimentary black shales from various sites and geologic periods. The results show that most of the authigenic Cr, which is the fraction of Cr precipitated from seawater, is not associated with the usual hydrogenetic phases (e.g., organic matter or iron-rich components) but rather resides in clay minerals. We further demonstrate that post-depositional diagenetic processes can induce redistribution of authigenic Cr in black shales, leading to enrichments of authigenic Cr in silicate minerals. Moreover, these processes were accompanied by significant Cr isotope fractionation, which substantially altered the Cr isotope compositions of the original phases. Similar effects can influence other trace elements, such as vanadium, in black shales. The findings imply that effectively separating authigenic and detrital fractions of these elements in black shales is unlikely. Instead, applying detrital corrections using the concentrations of typical detrital tracers (such as titanium and aluminum) after bulk isotope analysis is a more practical approach. This limitation affects the reliability of paleo-environmental interpretations derived from isotope data in black shales with high detrital contributions.