Mercury stable isotope composition of seawater suggests important net gaseous elemental mercury uptake

Abstract

Human exposure to toxic mercury (Hg) is dominated by the consumption of seafood. Earth system models suggest that Hg in marine ecosystems is supplied by Hg(II) deposition, with a 3x smaller contribution from gaseous Hg(0) uptake, and that photochemical reduction of marine Hg(II) drives important Hg(0) evasion to the atmosphere. Observations of marine Hg(II) deposition and gas exchange are sparse however, leaving the suggested importance of air-sea exchange unconstrained. Here we present the first Hg stable isotope measurements of total Hg (tHg) in surface and deep Atlantic and Mediterranean seawater. We use an isotope mass balance to estimate that sea water tHg can be explained by the mixing of 41% atmospheric Hg(II) deposition and 59% Hg(0) uptake. In the particulate Hg (pHg) fraction, which includes phytoplankton at the base of the marine food web, and in a compilation of marine fish Hg isotope data, we estimate similarly important marine Hg(0) uptake fractions of 73% and 49%. We observe no photochemical odd Hg isotope anomalies in tHg, which calls into question the large model Hg(0) evasion flux. Our findings indicate that direct atmospheric Hg(0) uptake is important and has implications for our understanding of atmospheric Hg dispersal and marine ecosystem recovery.

Mercury Accumulation in Marine Sediments – A Comparison of an Upwelling Area and Two Large River Mouths

“…It has been estimated that approximately 96% of the deposited Hg to the ocean is lost through evasion from the surface, and only 30% of the Hg flux that reaches the deep ocean is preserved in sediments (Mason and Sheu, 2002). However, a recent study of isotope mass balance suggests that evasion flux estimations from models are likely overestimated (Jiskra et al, 2020). The current estimate of deep-ocean Hg accumulation also shows that Hg flux in the deep ocean is higher than the currently accepted deep-ocean average Hg flux (Sanei et al, 2021).…”

Mass-Independent Fractionation of Even and Odd Mercury Isotopes during Atmospheric Mercury Redox Reactions

“…Our new free tropospheric RM Δ 200 Hg observations of 0.15 ± 0.06‰ allow us to account for the large global RM dry deposition flux to land and oceans. We include additional nonurban aerosol and fog Δ 200 Hg observations from previous studies to derive a global mean RM Δ 200 Hg of 0.12 ± 0.05‰ ( n = 66). − Marine and terrestrial Hg re-emissions are taken into account using published median Δ 200 Hg of 0.05 and −0.01‰ for these pools, respectively. , We assume here that Δ 200 Hg of the Earth surface Hg 0 emissions equals the Δ 200 Hg of the Earth surface pools, as the fractionation of Δ 200 Hg takes place exclusively during upper atmospheric redox reactions. Total Hg emissions in the model are 8540 Mg/y and deposition 8500 Mg/y, confirming the steady state.…”

“…[51][52][53] Marine and terrestrial Hg re-emissions are taken into account using published median Δ 200 Hg of 0.05 and -0.01 ‰ for these pools respectively. 28,54 We assume here that Δ 200 Hg of Earth surface Hg 0 emissions equal the Δ 200 Hg of the Earth surface pools, as the fractionation of Δ 200 Hg takes place exclusively during upper atmospheric redox reactions.…”

“…24 Because even-MIF does not seem to occur measurably during Hg transformation at the Earth's surface, it has Non-peer-reviewed EarthArXiv pre-print 3 become a useful tracer for atmospheric deposition pathways, or sources of Hg to land and oceans. 27,28 Yet, the exact even-MIF mechanism and even-MIF inducing Hg transformation(s) remain essentially unknown.…”

“…Large odd-MIF signatures imparted on inorganic Hg and methyl Hg by photochemistry can be traced into all Earth surface environments, that is, sediments, biota, atmosphere, soils, and so forth, and help quantify sources and Hg transformations. − More unusual, even isotope MIF (Δ 200 Hg and Δ 204 Hg signatures) has been observed in oxidized Hg forms in rainfall and snowfall globally − and has been speculated to originate in GEM photo-oxidation at and above the tropopause . Because even-MIF does not seem to occur measurably during Hg transformation on the Earth’s surface, it has become a useful tracer for atmospheric deposition pathways or sources of Hg to land and oceans. , However, the exact even-MIF mechanism and even-MIF-inducing Hg transformation(s) remain essentially unknown.…”