Extreme mass-independent fractionation of mercury isotopes driven by atmospheric plasma-induced chemical process

Mass-independent fractionation (MIF) of mercury (Hg) provides a powerful tracer for reconstructing Hg cycling in the Earth system. Although generally attributed to photochemistry, many natural isotope signatures remain inconsistent with the established mechanisms. Here, we present experimental evidence that atmospheric plasma-induced reduction produces odd Hg-MIF with magnitudes and patterns distinct from photochemical processes. Plasma reactions yielded far larger odd-MIF signatures than photoreduction, with Δ¹⁹⁹Hg spanning -78‰ to +28‰, the most negative Hg-MIF across all Hg-reduction experiments. The sign of Hg-MIF reverses with solution pH, yielding strongly negative Hg-MIF at pH ≥ 3.0 but positive Hg-MIF at pH ≤ 2.0. This reversal arises from competition between plasma-generated species, where solvated electrons drive negative magnetic isotope effects (MIE) at higher pH, while hydrogen radicals produce positive MIE at low pH. These findings reveal plasma chemistry as a fundamental but previously overlooked driver of Hg isotope fractionation. Given the ubiquity of natural plasmas such as lightning and aurora, plasma processes need to be integrated into models of Hg cycling in Earth's atmosphere and surface environments.