Dissolved mercury (Hg) speciation in the California Current System from samples collected on R/V Roger Revelle cruise RR2105 in July to August 2021

Monomethylmercury (MMHg) is a neurotoxicant that biomagnifies in marine food webs, reaching high concentrations in apex predators. To predict changes in oceanic MMHg concentrations, it is important to quantify its sources and sinks. Here, we study mercury speciation in the California Current System through cruise sampling and modeling. Previous work in the California Current System has found that upwelling impacts mercury biogeochemistry by transporting mercury-enriched deep waters to productive surface waters. These upwelled waters originate within the California Undercurrent water mass and are subsequently advected as a surface water parcel to the California Current. By comparing the two major water masses, we find that the California Undercurrent contains elevated dissolved total mercury (Hg) and Dimethylmercury (DMHg) concentrations by 57% and 60%, respectively, compared to the California Current. We explain that these differences result from losses during advection, specifically scavenging and DMHg demethylation. We calculate a net DMHg demethylation rate constant of 1.8 ± 0.9% per day; and build an empirically constrained mass budget model to demonstrate that DMHg demethylation accounts for 59% of surface MMHg sources. These findings illustrate that DMHg is a significant source of MMHg in this region, challenging the current understanding of the major sources of marine MMHg. These data are associated with Adams et al., 2024 (doi: 10.21203/rs.3.rs-3909481/v1).