Mercury isotope fractionation during biogeochemical processes

Abstract

Mercury is a naturally existing toxic trace element that can bioaccumulate to elevated levels in wildlife and humans. While studies of mercury biogeochemical processes have been relatively comprehensive, number of processes that are relevant to linking environmental sources of mercury to ecosystem fate are still subject to significant uncertainty. Measurements of natural abundances of mercury stable isotopes is becoming a powerful tool for understanding sources and complex biogeochemical processes of mercury in natural environment. Unlike the traditional light isotope systemㄴ of carbon and nitrogen, mercury stable isotopes can also undergo two types of fractionation pathways known as mass-dependent and mass-independent fractionation. The mechanism of mass-dependent fractionation is similar to the traditional light isotope systems such that the degree of fractionation is dependent on the nuclear mass of the isotopes. Thus far, many environmental processes have shown to cause mass-dependent fractionation. In particular, microbial methylation or the production of highly toxic and bioaccumulative form of mercury known as methylmercury has shown to cause significant mass-dependent fractionation in aquatic sediments, wetlands, and in the open ocean water column. In contrast, mass-independent fractionation occurs primarily in the odd mass number isotope (199Hg, 201Hg) and only during photochemical reduction and degradation of inorganic mercury and methylmercury. Photochemical reduction and degradation are thought to be the dominant mechanisms responsible for ecosystem removal of inorganic mercury and methylmercury, respectively. Moreover, a recent discovery suggests that there is absence of significant mercury isotope fractionation during methylmercury bioaccumulation and trophic transfer in ecosystem food webs. Collectively, this knowledge has shed light to the underlying mechanisms responsible for various biogeochemical processes as well as our ability to precisely link environmental sources of mercury to ecosystem fate. In this seminar, I first illustrate how various biogeochemical processes lead to fractionation of mercury stable isotopes. Subsequently, I show how understanding of mechanisms and extent of mercury stable isotope fractionation can aid a precise source-fate relationship in aquatic environments.