Unequal anthropogenic enrichment of mercury in Earth’s northern and southern hemispheres

Abstract

Remote northern (NH) and southern hemisphere (SH) lake sediment and peat records of mercury (Hg) deposition show a ×3 to ×5 Hg enrichment since pre-industrial times (<1880AD), leading to the common perception that global atmospheric Hg enrichment is moderate and uniform. Anthropogenic Hg emission in the NH is, however, approximately four times higher than in the SH. Here we reconstruct atmospheric Hg deposition to four remote SH peatlands and review sediment and peat Hg records from both hemispheres. We observe a ×4 enrichment in SH Hg deposition from pre-anthropogenic (<1450AD) to late 20th century periods, which is lower than the large ×16 all-time enrichment in NH Hg deposition. We attribute this difference to lower anthropogenic Hg emissions in the SH, and higher natural atmospheric SH Hg concentrations, supported by ×2 higher natural background Hg accumulation in SH peat records. We suggest that the higher SH natural Hg concentrations reflect the SH land-ocean distribution, with higher marine SH Hg emissions driven by transport of NH Hg to the SH by the Ocean conveyor belt. Our findings suggest that Hg background levels and anthropogenic enrichment in both hemispheres are different and must be taken into account in international Hg assessments and environmental policy.

Spatiotemporal Characterization of Mercury Isotope Baselines and Anthropogenic Influences in Lake Sediment Cores

“…The present‐day unremote sediments showed 2.9‐ to 25‐fold increases in THg relative to their respective pre‐industrial sediments. The present‐day remote sediments, without Laguna Negrilla, Peru, showed 1.3‐ to 5.1‐fold increases in THg, consistent with those reported from other lake sediment cores collected from remote regions (3–5 fold) (Amos et al., 2015; Engstrom et al., 2014; Fitzgerald et al., 2005, Li et al., 2020). Among the remote sediments, a reduction in THg (0.53‐fold) was observed in Laguna Negrilla, Peru (Figure 1a), consistent with the cessation of widespread historical mercury mining for pigment and precious metal extraction (1532–1821 AD) (Cooke et al., 2013).…”

“…The mercury isotope ratios of the pre‐industrial sediments appear to reflect both natural and anthropogenic mercury delivered via atmospheric deposition and runoff from the adjacent watershed. Previous evaluations of anthropogenic mercury enrichments in sediment and peat cores have suggested that historical anthropogenic activities (since 1450 AD) such as Spanish colonial mercury and silver mining (Amos et al., 2015) as well as deforestation and biomass combustion (Li et al., 2020) have liberated substantial amounts of mercury into the atmosphere. The resultant Hg 0 would have accumulated within foliage prior to watershed runoff or deposited to lake sediments upon atmospheric oxidation.…”

“…(2015) and Li et al. (2020) using empirical assessments of mercury in various natural archives. Note that year 2014 is the earliest mercury isotope measurement available in our compiled data.…”

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

“…Modern anthropogenic Hg emissions to air (2400 Mg a -1 ) outweigh natural Hg emissions (340 Mg a -1 ) by an order of magnitude. 1,2 Hg emissions are mostly in the form of gaseous elemental Hg (GEM), which has a long (3 months) atmospheric life-time against oxidation to divalent reactive Hg (RM) forms that include gaseous oxidized Hg (GOM) and particulate bound Hg (PBM). 3 RM is rapidly deposited to continental and marine ecosystems where microbial and abiotic activity transforms a fraction of inorganic Hg into the bioaccumulating and toxic methylmercury form.…”

Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs

“…In line with the spatial distribution of Hg emissions, observations show higher atmospheric Hg 0 levels in the northern hemisphere compared to the southern hemisphere ( 9 ). Sediment and peat records of atmospheric Hg deposition also suggest a larger anthropogenic Hg enrichment in the northern hemisphere ( 10 ). Anthropogenic Hg emissions are thought to have tripled the concentrations of total Hg in the thermocline waters (100 to 1,000 m) of the global ocean relative to deeper older waters ( 11 ), yet still unclear is how these anthropogenic Hg inputs are converted into methylmercury in oceans.…”