Measurements of radioactive and stable sulfur isotopes at Mt. Everest and its geochemical implications

Abstract:

The Himalayas were recently identified as a global hotspot for deep stratosphere-to-troposphere transport (STT) during spring [1]. Although STT transport in this region may play a vital role in tropospheric chemistry, the hydrological cycle and aquatic ecosystems in Asia, there is no direct measurement of a specific chemical stratospheric tracer to verify and evaluate its possible impact. Here, cosmogenic ³⁵S tracer (half-life: ~87 days) produced in the stratosphere was measured for the first time in surface snow and river runoff samples collected at Mt. Everest in April 2013 using a low-noise liquid scintillation spectroscopy [2]. Strikingly, we find extraordinarily high concentrations of ³⁵S in these samples (>10 times higher than the southern Tibetan Plateau), verifying the Himalayas as a gateway of springtime STT. In light of this, two studies were conducted: a) Measurements of ³⁵SO₂ and ³⁵SO₄^2- at the southern Tibetan Plateau reveals that the oxidative life time of SO₂ is reduced to 2.1 days under the influence of aged stratospheric air masses from the Himalayas. A concept box model for estimating the influence of STT on surface O₃ using ³⁵S tracer is proposed. b) Quadruple stable sulfur isotopes in a sediment core (~250 years) from the Gokyo Lake (the world’s highest freshwater lake) [3] near Mt. Everest are being measured to investigate the possible impact of STT on sulfur budget at the Himalayas. The absence of sulfide suggests that bacterial sulfate reduction may be negligible in this lake. Enrichment of uranium (EF ≈ 10) in 20th century samples highlights the impact of atmospheric deposition. S-isotope sulfate anomalies are not found (∆³³S and ∆³⁶S ≈ 0‰), implying that sulfate in this lake may be mainly contributed by eolian dust or derived from rock. This is also supported by the low enrichments of most trace elements (EF ≈ 1). Rare earth elements will be used to assist in identifying the potential sources and interpreting the variation of δ³⁴S. To better resolve the impacts of STT on sulfur cycle, sulfur isotope measurements in aerosols or ice cores collected at Mt. Everest in the future are recommended.

References: [1] Skerlak, B. et al. (2014) Atmos. Chem. Phys., 14, 913-937. [2] Brother, L. et al. (2010) P. Natl. Acad. Sci. 107, 5311-5316. [3] Sharma, C. et al. (2012) Limnology 13, 181-192.