Linking mercury, carbon, and nitrogen stable isotopes in Tibetan biota: Implications for using mercury stable isotopes as source tracers

Abstract

Tibetan Plateau is located at a mountain region isolated from direct anthropogenic sources. Mercury concentrations and stable isotopes of carbon, nitrogen, and mercury were analyzed in sediment and biota for Nam Co and Yamdrok Lake. Biotic mercury concentrations and high food web magnification factors suggested that Tibetan Plateau is no longer a pristine site. The primary source of methylmercury was microbial production in local sediment despite the lack of direct methylmercury input. Strong ultraviolet intensity led to extensive photochemical reactions and up to 65% of methylmercury in water was photo-demethylated before entering the food webs. Biota displayed very high Δ199Hg signatures, with some highest value (8.6%) ever in living organisms. The δ202Hg and Δ199Hg in sediment and biotic samples increased with trophic positions (δ15N) and %methylmercury. Fish total length closely correlated to δ13C and Δ199Hg values due to dissimilar carbon sources and methylmercury pools in different living waters. This is the first mercury isotope study on high altitude lake ecosystems that demonstrated specific isotope fractionations of mercury under extreme environmental conditions.

Coprecipitation of Mercury from Natural Iodine-Containing Seawater for Accurate Isotope Measurement

“…Red triangles and green dots represent the seawater samples collected from the Bohai Sea and the Yellow Sea, China, respectively. All gray and black symbols represent the data of representative precipitations (gray triangles), freshwater sediments and soils (gray rhombuses), marine sediments (black squares), forest oils (gray inverted triangles), and coal-impacted rainfall (half-filled black circles). ,,− …”

Transfer of marine mercury to mountain lakes

“…In contrast to Etang Mort, and to some extent Etang du Pla de la Font, Etang Legunabens has no peatland in its direct vicinity, indicating a lower degree of DOC and MeHg input 64 , 65 . This would yield an increased depth of light penetration in the water column resulting in more intense photochemical demethylation of MeHg, and would explain the higher Δ 199 Hg than Etang Mort 58 , 66 . It would also explain the lower Δ 199 Hg than Etang du Pla de la Font as all trout in Etang Legunabens were caught in the lake, i.e.…”

Mercury Isotopes

“…Because different pools of MeHg can have different isotopic signatures, Hg isotopes can be useful in understanding how Hg is getting into the food web. There are many studies of Hg isotopes in freshwater biota (e.g., Bergquist and Blum 2007;Gantner et al 2009;Laffont et al 2009;Perrot et al 2010;Gehrke et al 2011b;Kwon et al 2012;Sherman and Blum 2013;Tsui et al 2014;Kwon et al 2015;Lepak et al 2015;Xu et al 2016) and marine biota (e.g., Senn et al 2010;Blum et al 2013;Zheng et al 2015). Hg isotopes in fish and biota reflect many factors including where Hg is methylated, species of Hg accumulated, trophic level, what depth MeHg is foraged from, the residence time of MeHg, and the source of Hg for methylation.…”