Figure 1. A chromium (Cr) isotope record of evolving Earth surface redox. Chromium isotope values in marine sediments (iron formations, ironstones, and siliciclastic sediments) through time show a significant increase in Cr isotope fractionation during the Neoproterozoic. This increase in Cr isotope values coincides with a period of major eukaryotic diversification reconstructed from the fossil record (vertical green bar) as well as a series of major biological and ecological events in the late Proterozoic (upper panel). These values are significantly higher than the high-temperature crustal range of Cr isotope values (horizontal gray bar), suggesting the initial onset of significant coupled O2-Mn-Cr cycling.