Sedimentary records of mercury stable isotopes in Lake Michigan

Abstract

Mercury (Hg) concentrations and Hg isotopic composition were investigated in three sediment cores in Lake Michigan (LM). Two cores were collected from Green Bay, a region heavily impacted by Hg contamination and one core from an offshore region of LM absent of direct point source Hg. Historical trends of Hg influxes suggest increased Hg deposition began in the 1890s in Green Bay and in the early 1800's in offshore LM. Recently deposited sediment reflecting more anthropogenic influence shows similar d202 Hg values (-1.0 to -0.5‰) for all three cores however, deep core sediments, reflecting pre-industrial eras, show much lower d 202Hg values (-1.7 to -1.2‰). Using a binary mixing model based on d 202Hg signatures, the proportion of anthropogenic Hg was estimated. Model output confirms that Green Bay is more contaminated by local point source than the offshore LM. An increase in positive D199 Hg values (-0.02 to +0.27‰) was observed from inner Green Bay to the offshore of LM, which may indicate increased input of atmospheric Hg and decreased watershed inputs along this transect. Overall, this study suggests that sedimentary Hg isotopes maybe a useful tracer in understanding Hg sources and history of Hg contamination in large lakes.

Mercury Inputs to Chinese Marginal Seas: Impact of Industrialization and Development of China

“…The isotopic composition of the uppermost sample of core C30 ( δ 202 Hg: −1.34‰; Δ 199 Hg: 0.07‰) was within the range of our previous results about the PRE sediments ( δ 202 Hg: −2.80‰ to −0.68‰; Δ 199 Hg: −0.15‰ to 0.16‰) (Yin et al, ). In previous studies, the variations of Hg isotopes in sediments have been attributed to variations in Hg sources (Balogh et al, ; Cooke et al, ; Donovan et al, ; Gray et al, ; Guédron et al, ; Ma et al, ; Mil‐Homens et al, ; Sonke et al, ; Yin et al, ). In our study, the isotopic compositions of Hg varied over time and among different sampling sites.…”

“…Mercury influxes increased rapidly since the 1980s, which coincided with the launching of China's Economic Reform policy in 1978. Mercury influxes for cores M7, F0306, and C0803 increased by a factor of 1.5–2, whereas lake sediment cores in China (even in Tibet Plateau) and other sites of the world increased by a factor of 3–5 (Balogh et al, ; Cooke et al, ; Donovan et al, ; Ma et al, ; Mil‐Homens et al, ; Gray et al, ; Guédron et al, ; Sonke et al, ; Yang et al, ; Yin et al, ). This may be explained by the effective soil conservation practices and extensive dam and impoundment constructions at the higher stream of Chinese rivers in recent decades.…”

“…By plotting Δ 202 Hg versus Δ 199 Hg signals, the three Hg sources are characterized by distinct “ δ 202 Hg‐Δ 199 Hg” signatures (Figure ). The isotopic signature of industrial Hg can be estimated using Hg polluted sediments with their locations in the vicinity of known industrial sources (Balogh et al, ; Bartov et al, ; Bonsignore et al, ; Cooke et al, ; Donovan et al, ; Guédron et al, ; Ma et al, ; Mil‐Homens et al, ; Gray et al, ; Perrot et al, ; Sonke et al, ; Wiederhold et al, ; Yin et al, ), and hydrothermal ores which are the major source of Hg used in industry (Blum & Bergquist, ; Cooke et al, ; Feng et al, ; Foucher & Hintelmann, ; Gray et al, ; Smith et al, ; Stetson et al, ; Wiederhold et al, ; Yin et al, ). Based on the summarization of previous data, we estimated that industrial Hg has mean δ 202 Hg of −0.53 ± 0.51‰ and mean Δ 199 Hg of −0.02 ± 0.11‰ (σ, n = 481).…”

“… Triple mixing of industrial, watershed, and atmospheric Hg for sediment cores. The solid red circle ① represents industrial Hg ( δ 202 Hg: −0.53 ± 0.51‰; Δ 199 Hg: −0.02 ± 0.11‰; σ, n = 481) (Balogh et al, ; Cooke et al, ; Donovan et al, ; Gray et al, ; Guédron et al, ; Ma et al, ; Mil‐Homens et al, ; Sonke et al, ; Yin et al, ); the solid yellow circle ② represents soils collected from pristine sites ( δ 202 Hg: −1.82 ± 0.39‰; Δ 199 Hg: −0.29 ± 0.12‰; σ, n = 156) (Biswas et al, ; Demers et al, ; Jiskra et al, 2017; Wang et al, ; Woerndle et al, ; Zhang et al, ; Zheng et al, ); the blue dashed circle ③ represents precipitations ( δ 202 Hg: −1.22 ± 1.08‰; Δ 199 Hg: 0.38 ± 0.33‰; σ, n = 117) (Chen et al, ; Demers et al, ; Gratz et al, ; Sherman et al, ; Wang et al, ; Yuan et al, ); and the solid blue circle ④ represents precipitation‐derived Hg with a shift of −0.6‰ in δ 202 Hg.…”

“…However, it needs to be recognized that the use of Hg isotopes for source tracing may lead to ambiguous interpretations, due to the fact of the complexity of Hg isotope fractionation processes as well as the variability of different source signatures. In recent years, mercury isotopes in sediment cores have been utilized to understand Hg deposition history in terrestrial lakes and coastal sites (Balogh et al, ; Cooke et al, ; Donovan et al, ; Gray et al, ; Guédron et al, ; Ma et al, ; Mil‐Homens et al, ; Sonke et al, ; Yin et al, ). In contrast, only few studies reported high‐resolution isotopic data on offshore cores of the major ocean, most likely due to low concentrations of Hg and low sedimentation rates (Gehrke et al, ; Gleason et al, ).…”