Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

Abstract

The oceans are an important global reservoir for mercury (Hg), and marine fish consumption is the dominant human exposure pathway for its toxic methylated form. A more thorough understanding of the global biogeochemical cycle of Hg requires additional information on the mechanisms that control Hg cycling in pelagic marine waters. In this study, Hg isotope ratios and total Hg concentrations are used to explore Hg biogeochemistry in oligotrophic marine environments north of Hawaii. We present the first measurements of the vertical water column distribution of Hg concentrations and the Hg isotopic composition in precipitation, marine particles, and zooplankton near Station ALOHA (22°45′N, 158°W). Our results reveal production and demethylation of methylmercury in both the euphotic (0–175 m) and mesopelagic zones (200–1,000 m). We document a strong relationship between Hg isotopic composition and depth in particles, zooplankton, and fish in the water column and diurnal variations in Δ199Hg values in zooplankton sampled near the surface (25 m). Based on these observations and stable Hg isotope relationships in the marine food web, we suggest that the Hg found in large pelagic fish at Station ALOHA was originally deposited largely by precipitation, transformed into methyl‐Hg, and bioaccumulated in situ in the water column. Our results highlight how Hg isotopic compositions reflect abiotic and biotic production and degradation of methyl‐Hg throughout the water column and the importance of particles and zooplankton in the vertical transport of Hg.

Methylmercury produced in upper oceans accumulates in deep Mariana Trench fauna

“…In contrast, the trench sediments have negative δ 202 Hg (−0.96 ± 0.27‰, 1 SD, n = 5) and very small, positive odd-MIF (Δ 199 Hg = 0.20 ± 0.07‰, Δ 201 Hg = 0.18 ± 0.04‰, 1 SD, n = 5) (Supplementary Table 3). The even-MIF of trench fauna (amphipod and snailfish) is characterized by positive Δ 200 Hg (0.02-0.13‰) and negative Δ 204 Hg (−0.18 to −0.03‰), which is very similar to those of trench sediments, and upper marine particles and fishes from the upper NPO 16,32 ( Supplementary Fig. 4).…”

“…Hg isotope compositions. The most striking observation of Hg isotopes is that both MDF (δ 202 Hg: −0.05 to 0.54‰, n = 28) and odd-MIF (Δ 199 Hg: 1.26 to 1.70‰, Δ 201 Hg: 1.01 to 1.37‰, n = 28) of trench amphipods are comparable to those of NPO fishes at 300-600 mbs 16,32 , despite their dramatic difference in depths (Fig. 2).…”

“…Significant even-mass number MIF (even-MIF, represented by Δ 200 Hg and Δ 204 Hg) has also been observed primarily in atmospheric precipitations, which is proposed to be related to photochemical oxidation of gaseous Hg(0) in the upper atmosphere 27,28 . Due to lack of significant Hg isotope fractionation during accumulation and trophic transfer of MMHg in aquatic food chains 29,30 , Hg isotope compositions of marine biota have demonstrated great potential in tracing the sources, production and degradation of MMHg in oceans 16,31,32 .…”

A Global Model for Methylmercury Formation and Uptake at the Base of Marine Food Webs

“…Elevated concentrations of oxidants for Hg 0 (including Br, HO 2 , and NO 2 ) and high precipitation rates mean the tropical oceans (30°S–30°N) account for 49% of global Hg II deposition (Horowitz et al, ). These high atmospheric inputs lead to higher seawater total Hg concentrations (~1.5 pM) and MeHg concentrations (500–700 fM) in the upper ~1,000 m of these regions, as observed on the RITS and SHIPPO cruises (Kim et al, ; Mason & Fitzgerald, ) and ALOHA station ( Motta et al, n.d .). For instance, total Hg concentrations as high as 1.5 pM and MeHg concentrations up to 800 fM have been reported in the tropical part of the SHIPPO cruise (Kim et al, ).…”

Soil-atmosphere exchange flux of total gaseous mercury (TGM) in subtropical and temperate forest catchments

“…Due to the long longevity, atmospheric Hg 0 is able to undergo over long distances to areas without anthropogenic emissions (Kamp et al, 2018;Slemr et al, 2018). Global long-range atmospheric transport and deposition is the main pathway of Hg input to remote ecosystems (Obrist et al, 2018;Zdanowicz et al, 2018;Motta et al, 2019). Soils account for more than 90% of Hg stored in terrestrial ecosystems (Obrist, 2012), with global top soil Hg pools (0-40 cm) estimated at > 300 000 Mg (Hararuk et al, 2013;Zhou et al, 2017a).…”