Fig. 5: Conceptual model showing the deep cycling of mercury on Earth (not to scale).
From: Mantle Hg isotopic heterogeneity and evidence of oceanic Hg recycling into the mantle
Photoreduction of Hg(II) produces gaseous Hg(0) with negative Δ199Hg values and gaseous Hg(II) with positive Δ199Hg values. Gaseous Hg (0) is preferentially accumulated in soil and vegetation (light gray area), whereas gaseous Hg(II) is readily deposited into the marine reservoir (light blue area). In subduction zones, oceanic crust (white-gray area) is subducted underneath the continental crust (dark gray area) and delivers large amounts of Hg from marine sediments (light blue circles) into the mantle. Most of the subducted Hg is released and cycled back to the surface environment through arc volcanism, forming island arc basalts (IAB, purple area) with positive Δ199Hg values. The remaining Hg in the subducting slab is cycled into the upper mantle (light orange area) and released at mid-ocean ridges by mid-ocean ridge basalt volcanism (MORB, highlighted in blue), which displays positive Δ199Hg values. Some of the subducted Hg may also be cycled into the lower mantle (orange area), resulting in Δ199Hg signals in some ocean island basalts (OIB)1; however, given that OIBs and CFBs mostly show near-zero Δ199Hg values (highlighted in red), this recycled Hg may represent only a small proportion of Hg in the lower mantle.
