Chromium isotope composition of organic-rich marine sediments and their mineral phases and implications for using black shales as a paleoredox archive

Chromium (Cr) stable isotopes in marine sediments, in particular iron formations, have been used to track changes in Earth's surface redox conditions. In recent years several studies have been published that focus on shale-hosted Cr isotopes to further constrain the evolution of atmospheric oxygen levels. As shales are composed of a mixture of detrital and authigenic components, it is critical to know which phases host Cr and to determine any variations in Cr concentration and degree of fractionation between the different host phases. Here we assess whether shales have the potential to record Cr-isotope values that are meaningful to paleoenvironmental studies, focusing on representative examples of organic-rich marine sediments (black shale, phosphate and chert samples) from the Cambrian-aged Jiumenchong and Ediacaran-aged Liuchapo Formations (South China). We use a novel approach to isolate authigenic components which potentially record primary marine redox-fluctuations. Bulk sediment digests and sequential leaches (0.5 M HCL, 6 M HCl, HF and aqua regia), combined with X-ray diffraction and microbeam mineral mapping (Nanomin) characterisation of unreacted and leached sediment samples, allow us to identify potential Cr-hosting phases and their isotopic signatures. The black shale bulk sediment δ53Cr values vary between 0.02‰ ± 0.15‰ and 0.56‰ ± 0.10‰ and are thus slightly positively fractioned compared to the igneous Earth signature of −0.124 ± 0.101‰. The highest δ53Cr value was measured for a chert sample (0.98‰ ± 0.13‰). The leaching test results reveal that the majority of Cr (75–85%) is hosted in the silicate mineral fraction of the sediment, and is liberated during the 6 M HCl and HF acid leaching steps. Further, carbonates/phosphates and pyrite could be isolated as potential Cr hosts by our sequential leaching approach (liberated by the 0.5 M HCl and aqua regia leaching steps respectively). The δ53Cr values of each sequential leach step were generally within error of their respective bulk δ53Cr value. This is surprising as Cr derived from detrital phases is expected to have a different δ53Cr value from Cr in authigenic phases, which are thought to record δ53Cr of Cr in the water column, and suggests that the δ53Cr value of detrital minerals might deviate from the narrow range of values documented for igneous rocks. The similar δ53Cr values observed for the authigenic and detrital mineral phases suggest whole-rock diagenetic homogenisation, which complicates the application and interpretation of Cr stable isotopes in organic-rich black shales for paleoredox conditions.