A global reassessment of the controls on iron speciation in modern sediments and sedimentary rocks: A dominant role for diagenesis

The speciation of iron in sediments and sedimentary rocks is a widely used proxy for the chemistry oxidation state ancient water bodies. Specifically, fraction reactive out total (Fe HR /Fe T ) pyrite pool PYR are thought to constrain presence sulfide column, respectively. This approach was developed tested against modern core-top sediments, but application requires consideration effects diagenesis lithification on speciation. Furthermore, deep burial, metamorphism, late-stage alteration during exhumation or sampling (e.g., oxidative weathering) have not been systematically explored. To bridge this gap, we combined new data from four sediment cores ( n = 54) with an extensive literature compilation (2936 measurements 316 cores) (12,173 spanning Neoarchean Quaternary). include both surface buried allowing investigation Depending thresholds distinguish oxic anoxic environments ferruginous euxinic environments, interpretation within existing framework yields incorrect environmental classifications up ≈70% time. In main reason that does represent column. We find correlates porewater it changes progressive burial along three distinctive Fe –Fe arrays, each which represents different set diagenetic processes. suggest similarly stratigraphic variation primarily reflects depositional conditions rather than temporal water-column state. Indeed, analysis geologic reveals no statistically significant trends either Archean present day. arrays identified marine under certain conditions, analyses may be local source(s). Hence, data, together complementary petrographic, mineralogical geochemical constraints, source(s) early late processes, rarely columns.