Tracking Redox with Chromium Isotope

A relatively recent method for tracking the redox state of the Earth is the study of chromium (Cr) isotopes in the rock record. Earth’s redox state (the oxidation of the planet’s atmosphere and ocean) has played an important role in the evolution of life and, ultimately, Earth’s habitability. Understanding redox over various timescales throughout our planet’s history is an important step in determining how the Earth came to support the diversity of life we see today.

The use of Cr isotopes as a redox proxy is limited for now because scientists haven’t built up a clear understanding of the global mass balance of Cr isotopes. One environment in which this mass balance is little-understood is in estuaries. A new study addresses this gap in knowledge by focusing on the Connecticut River estuary, collecting samples over a gradient from salty to fresh water. The study provides the first Cr isotope dataset for this type of environment. The team of scientists examined Cr-containing particulates in the water as well as dissolved Cr. Their results suggest that particles lose Cr as salinity increases, a finding that could have implications for interpreting Cr isotope data from the sedimentary rock record of the Earth.

The study, “Cr isotope systematics in the Connecticut River estuary,” was published in the journal Chemical Geology. The work was supported by NASA Astrobiology through the Exobiology Program.