Changes in chemical concentrations in a Yellowstone borehole along with seismicity, all during May–November 2021

Changes in concentrations of hydrogen (H2), sulfide (HS-), and cells, and the accumulated seismic energy during May–November 2021, when microbial samples were collected from a borehole near West Thumb in Yellowstone National Park. Concentrations of cells, sulfide, and hydrogen all increased with rising earthquake activity, then fell steeply as seismic activity quieted and the total energy accumulated flattened out. Julian Day is the number of the day within the year (so Julian Day 200, for example, was July 19, 2021). From Boyd et al., 2025 (https://doi.org/10.1093/pnasnexus/pgaf344).