A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes

Abstract

Recent studies demonstrate a short 3-6-month atmospheric lifetime for mercury (Hg). This implies Hg emissions are predominantly deposited within the same hemisphere in which they are emitted, thus placing increasing importance on considering Hg sources, sinks and impacts from a hemispheric perspective. In the absence of comprehensive Hg data from the Southern Hemisphere (SH), estimates and inventories for the SH have been drawn from data collected in the NH, with the assumption that the NH data are broadly applicable. In this paper, we centre the uniqueness of the SH in the context of natural biogeochemical Hg cycling, with focus on the midlatitudes and tropics. Due to its uniqueness, Antarctica warrants an exclusive review of its contribution to the biogeochemical cycling of Hg and is therefore excluded from this review. We identify and describe five key natural differences between the hemispheres that affect the biogeochemical cycling of Hg: biome heterogeneity, vegetation type, ocean area, methylation hotspot zones and occurence of volcanic activities. We review the current state of knowledge of SH Hg cycling within the context of each difference, as well as the key gaps that impede our understanding of natural Hg cycling in the SH. The differences demonstrate the limitations in using NH data to infer Hg processes and emissions in the SH.

Keywords: Background; Geogenic sources; Litterfall; Methylation processes; Oceans; Soils.

Fig. 1

Conceptual overview of the key differences in the natural mercury (Hg) cycle in the Southern Hemisphere and Tropics (relative to the NH)

Fig. 2

Mercury concentrations in uncontaminated soils (top five centimeters) of the Southern Hemisphere and Tropics. Data and sources of information are given in Tables 2 and S1. Basemap from the World Wildlife Fund, Terrestrial Ecosystems of the World, version 2.0 (Olson et al. 2001)

Fig. 3

Gridded Hg deposition through litterfall from observed and modelled fluxes. Reprinted from Wang et al. , Assessment of Global Mercury Deposition through Litterfall, Environmental Science & Technology 50: 8548–8557, Copyright 2016, with permission from Elsevier

Fig. 4

Yields of riverine mercury exports in global river basins. Yields of riverine Hg export (flux divided by drainage area) from different river basins: (a) total Hg and (b) MeHg. Reprinted by permission from Springer Nature: Nature Geosciences vol. 14. Reprinted from Liu, M.; Zhang, Q.; Maavara. T; Liu, S.; Wang, S.; Raymond, P.A. Rivers as the largest source of mercury to coastal oceans worldwide, pp. 672–677. Copyright 2021

Fig. 5

Schematic view of Hg methylation hotspots (highlighted in yellow) in aquatic zones in the tropics vs. the temperate zone. In the tropics, methylation hotspots occur in the flooded forest and in the periphyton on submerged surfaces of macrophytes. In temperate zones, methylation hotspots occur in sediments, epilithon and epiphyton. Note that in the temperate zones, Hg methylation rates are at a minimum in winter. Letters at the bottom of the graphics refer to the months of the year

Fig. 6

Global distribution of volcanoes, tectonic plates, mercuriferous belts and mercury productive deposits. Image combines information from USGS, (Bailey et al. ; Gustin et al. 1999). Map background by CartoDB and Stamen Design, under CC BY 3.0. Data by OpenStreetMap, under ODbL. Map projection: Robinson