Deep-time paleosols record a vegetation-driven transition in terrestrial mercury cycling

Mercury (Hg) isotopes are sensitive tracers of atmosphere–land interactions, yet the processes controlling Hg isotope fractionation in deep-time terrestrial environments remain poorly constrained. Here we report Hg concentration and isotope data from Devonian and Jurassic–Cretaceous paleosols in southwestern China, spanning major changes in vegetation structure, climate, and pedogenic regimes. The studied paleosols display limited odd-MIF variability overall (Δ 199 Hg = −0.11‰ to +0.17‰; Δ 201 Hg = −0.12‰ to +0.12‰) and near-zero even-MIF (Δ 200 Hg = −0.05‰ to +0.09‰), while δ 202 Hg varies more substantially from −2.40‰ to −0.16‰. Coupled δ 202 Hg–Δ 199 Hg–Δ 201 Hg systematics further suggest contrasting pedogenic Hg transformation pathways: Lower Devonian paleosols show relatively constrained δ 202 Hg values and more variable odd-MIF signatures with Δ 199 Hg/Δ 201 Hg slope of 1.01, consistent with enhanced photochemical Hg(II) reduction under open-canopy and light-exposed soil surfaces. In contrast, Jurassic paleosols show weak Δ 199 Hg–Δ 201 Hg covariation (R 2 =0.19) but steeper slope of 2.38, and Cretaceous samples show a stronger positive Δ 199 Hg–Δ 201 Hg relationship with slope of 1.17 (R 2 =0.7). The Δ 199 Hg–Δ 200 Hg relationship in Lower Devonian paleosols (y = 0.50x + 0.02, R 2 = 0.63) further suggests a greater contribution from precipitation-derived Hg(II) to ancient soil system during early land plant colonization, before dense vegetation substantially modulated atmospheric Hg deposition through canopy uptake and litterfall during Mesozoic. Consistent with paleoclimate evidence and Earth system model simulations, these results demonstrate that climate-driven vegetation structure may modulate pedogenic Hg transformation pathway in ancient soils. Our findings indicate that terrestrial Hg odd-MIF signatures were not consistently negative through geological time, challenging simple terrestrial endmember assumptions for interpreting Hg isotope records.