Historical Records of Mercury Stable Isotopes in Sediments of Tibetan Lakes

Abstract

The Tibetan Plateau (TP), known as the “Third Pole”, is a critical zone for atmospheric mercury (Hg) deposition. Increasing anthropogenic activities in the globe leads to environmental changes, which may affect the loading, transport and deposition of Hg in the environment. However, the deposition history and geochemical cycling of Hg in the TP is still uncertain. Our records of Hg and Hg isotopes in sediment profiles of the two largest lakes in the TP, Lake Qinghai and Nam Co, show increased Hg influx since last century, with the maximum Hg influx enrichment ratios of 5.4 and 3.5 in Lake Qinghai and Nam Co, respectively. Shifts in negative δ 202Hg in Lake Qinghai (−4.55 to −3.15‰) and Nam Co (−5.04 to −2.16‰) indicate increased atmospheric Hg deposition through rainfall, vegetation and runoff of soils. Mass independent fractionation of both even-Hg (∆ 200Hg: +0.05 to +0.10‰) and odd-Hg (∆ 199Hg: +0.12 to +0.31‰) isotopes were observed. Positive Δ 200Hg suggest high proportion of precipitation-derived Hg in the TP, whereas the positive Δ 199Hg results from Hg(II) photo-reduction. Both lakes show increasing Δ 199Hg since the 1900 s, and we conclude that with the decrease of ice duration, Hg(II) photo-reduction may have been accelerated in these TP lakes.

Supplemental Material: Mercury isotopes track the cause of carbon perturbations in the Ediacaran ocean

“…The analytical uncertainties of Hg isotopic compositions were evaluated by repeated analysis of the isotopic compositions of NIST SRM 3177 (n = 39), Lichen CRM BCR 482 (n = 10) and NIST SRM 2711 (Montana soil, n = 5). The measured Hg isotopic compositions of NIST SRM 3177, Lichen CRM BCR 482, and NIST SRM 2711 are presented in Table S1 and agree with previously published results (Biswas et al, 2008;Estrade et al, 2010;Yin et al, 2016). The analytical uncertainty of Hg isotopic composition in this study is the larger 2σ value of either the NIST SRM 3177 or repeated analysis of the sediment sample over different analytical sessions.…”

Sources of mercury in deep-sea sediments of the Mediterranean Sea as revealed by mercury stable isotopes

“…4). Laboratory experiments on Hg 2+ photoreduction revealed δ 202 Hg/∆ 199 Hg of 0.83 49 , while for Tibetan Lakes the ratio was much higher with δ 202 Hg/∆ 199 Hg = 8.88 and 5.75 50 . Further no correlation between ∆ 199 Hg and HgT (P > 0.05), was observed, which may suggest that Mediterranean sediments may be more influenced by in-ocean processes.…”

Mercury Evidence of Intense Volcanism Preceded Oceanic Anoxic Event 1d

“…However, Hg released into the environment can experience many complicated physical, chemical, and biological reactions, which may lead to ambiguous interpretations (Blum et al., 2014), we therefore do not interpret MDF δ 202 Hg signatures here and focus instead on MIF Δ 199 Hg signatures of these data. On geological timescales, Hg emitted by volcanoes or derived from geogenic sources has no MIF (Δ 199 Hg = 0 ‰; Yin et al., 2016; Zambardi et al., 2009). However, the MIF signature of geogenic Hg can be altered by aqueous Hg photoreduction in cloud droplets and surface waters, which imparts positive Δ 199 Hg values in the residue Hg 2+ phase and negative Δ 199 Hg values in Hg° (g) (Bergquist & Blum, 2007).…”