Mercury Stable Isotopes Reveal Deep Methylation of Mercury and its Uptake into the Open Ocean Food Web

Knowledge of the biogeochemistry of methylmercury (MeHg) is required to understand its biomagnification in open ocean food webs. Mass dependent (MDF; δ²⁰²Hg) and mass independent (MIF; Δ¹⁹⁹Hg) fractionation of Hg isotopes provides new insight into sources and transformations of marine Hg. We measured Hg isotope ratios of fish, zooplankton and sinking particles in vertical profiles at Station ALOHA in the North Pacific Subtropical Gyre. Fish display large decreases in δ²⁰²Hg (1.6 to -0.2‰) and Δ¹⁹⁹Hg (5.3 to 0.8‰) between 10 and 800 m mean feeding depth. Zooplankton also display decreases in δ²⁰²Hg (0.2 to -0.3‰) and Δ¹⁹⁹Hg (2.2 to 0.8 ‰) between 25 and 1250 m depth, although not as large as in fish. In contrast, sinking particles (> 53 mm) display little variation in δ²⁰²Hg (-0.1 to -0.2‰) and Δ¹⁹⁹Hg (0.1 to 0.3‰) between 50 and 400 m depth. Precipitation near Hawaii has δ²⁰²Hg (0.1 to 0.2‰) and Δ¹⁹⁹Hg (0.2 to 0.4‰) similar to sinking particles. We developed a preliminary conceptual model to explain these observations. Inorganic Hg (IHg) is deposited to the ocean via precipitation, sorbs to sinking particles and is transported to ≥ 400 m depth without enough methylation or photochemical reduction to shift δ²⁰²Hg or Δ¹⁹⁹Hg values appreciably. MeHg is formed and partially photo-demethylated in the photic zone: a small proportion of this also attaches to particles. Shallow zooplankton bioaccumulate MeHg more efficiently than IHg from particles and therefore acquire δ²⁰²Hg and Δ¹⁹⁹Hg values midway between the values of MeHg (the value of surface feeding fish) and IHg (the value of precipitation). Some methylation also occurs within IHg-dominated particles as they sink to deeper waters; this new MeHg lacks the isotopic signature of photo-demethylation. At about 800 m depth the values of δ²⁰²Hg and Δ¹⁹⁹Hg in fish and zooplankton converge, suggesting that deep IHg and MeHg have similar isotopic values, and that most MeHg in these organisms was formed at depth.