5:15 PM - 7:15 PM
[HTT16-P02] Multi-isotope study of the waters from Baidrag River-Lake Böön Tsagaan system, southwestern Mongolia
Keywords:sulfur isotope ratio, oxygen isotope ratio
The Baidrag River-Lake Böön Tsagaan system in southwest Mongolian Plateau is situated on the southernmost part of the Siberian permafrost and therefore is important for studying permafrost and environmental dynamics in response to climate change. In this study, sulfur isotope ratio (δ34SSO4) and oxygen isotope ratio (δ18OSO4) of sulfate ion (SO42-), oxygen isotope ratio (δ18OH2O) and hydrogen isotope ratio (δ2H) are analyzed for the waters from the river-lake system to evaluate the impact of permafrost on watershed hydrology. In this presentation, we report on the preliminary results.
Lake Böön Tsagaan is located in a hydrologically closed basin system in the Valley of Gobi Lakes which is bounded by the Hangai Mountains in the north and the Mongolian Altai Mountains in the south. The Baidrag River flows down from Hangai Mountain and supplies from the northeastern part into Böön Tsagaan. There are no outflowing rivers and therefore, the lake water has high salinity. The surface area is 252 km2, and an average depth 6.3 m (maximum depth 10 m).
River and lake water samples were observed and sampled at 11 fixed stations (one lake site, six river sites, and four spring sites) along about 175 km from the upstream to the lake. The water qualities such as temperature, pH, EC, ORP etc. were measured. The observations and samples were conducted in 2022 (February, May, July, August, and October), 2023 (February and July), and 2024 (May, June, and September).
Water samples for SO42- isotopic analyses were obtained through a 0.20 μm filter and the SO42- was precipitated as BaSO4. The δ34SSO4 and δ18OSO4 were measured using a stable isotope ratio mass spectrometer connected to a combustion-type elemental analyzer and a pyrolysis-type elemental analyzer, respectively. δ18OH2O and δ2H were measured using a stable isotope ratio analyzer with WS-CRDS method on filtering water samples with a 0.45 μm filter.
During the observation period, the δ34SSO4 of the river and spring waters ranged from 5‰ to 10‰ (except in June) and δ18OSO4 from –14‰ to 1‰, while the δ34SSO4 and δ18OSO4 of the lake waters were about 30‰ and 15‰, respectively. On the diagram of δ34SSO4 vs. δ18OSO4, the SO42- of rivers and springs waters exists in the fields of soil and geological (sulfide oxidation) (Krouse and Mayer 2000). Meanwhile, the δ34SSO4 and δ18OSO4 of lake waters have significantly higher values than those of river and spring waters. Sources of sulfur within the Böön Tsagaan watershed are limited because of the northern catchment area dominated by granitic rocks and the southern catchment area of Mongolian Altai dominated by basalt. SO42- concentrations of river and spring waters ranged from 0.05 mM to 2.02 mM (except in June). In contrast, the SO42- concentrations of lake waters were > 0.2 mM (4.8-17.0 mM). From the observation, the positive anomaly of lake water possibly is considered to have resulted from sulfate reduction of the water column and water-sediment interface (Habicht et al., 2002). However, the positive anomaly of lake water is likely due to incoming groundwater SO42- which was modified by bacterial sulfate reduction. This is because vertical mixing of the lake water occurred during summer and the SO42- concentration was 14.6 mM with δ34SSO4 of 16‰ at downstream of Baidrag River in middle June (melting period).
The δ18OH2O of rivers and springs ranged from –14‰ to –10‰ and δ2H from –107‰ to –81‰, while the δ18OH2O of lake waters ranged from –8‰ to –2‰ and δ2H ranged from –67‰ to –33‰. The d-excess (= δ2H – 8 × δ18O) of river and spring waters ranged from 0 to 11 , which is within the range of the d-excess of rainwaters in this region (–13 to 15; Vanwezer et al., 2021). However, the d-excess of some lake waters (–17 to 0) are lower than those of rainwater. The δ18OH2O of river and spring waters are lower than that of rainwater (–10‰ to –3‰; Vanwezer et al., 2021), while the δ18OH2O of lake water was almost the same value as rainwater. The biogeochemical implications for d-excess and δ18OH2O will be discussed in poster presentation.
Lake Böön Tsagaan is located in a hydrologically closed basin system in the Valley of Gobi Lakes which is bounded by the Hangai Mountains in the north and the Mongolian Altai Mountains in the south. The Baidrag River flows down from Hangai Mountain and supplies from the northeastern part into Böön Tsagaan. There are no outflowing rivers and therefore, the lake water has high salinity. The surface area is 252 km2, and an average depth 6.3 m (maximum depth 10 m).
River and lake water samples were observed and sampled at 11 fixed stations (one lake site, six river sites, and four spring sites) along about 175 km from the upstream to the lake. The water qualities such as temperature, pH, EC, ORP etc. were measured. The observations and samples were conducted in 2022 (February, May, July, August, and October), 2023 (February and July), and 2024 (May, June, and September).
Water samples for SO42- isotopic analyses were obtained through a 0.20 μm filter and the SO42- was precipitated as BaSO4. The δ34SSO4 and δ18OSO4 were measured using a stable isotope ratio mass spectrometer connected to a combustion-type elemental analyzer and a pyrolysis-type elemental analyzer, respectively. δ18OH2O and δ2H were measured using a stable isotope ratio analyzer with WS-CRDS method on filtering water samples with a 0.45 μm filter.
During the observation period, the δ34SSO4 of the river and spring waters ranged from 5‰ to 10‰ (except in June) and δ18OSO4 from –14‰ to 1‰, while the δ34SSO4 and δ18OSO4 of the lake waters were about 30‰ and 15‰, respectively. On the diagram of δ34SSO4 vs. δ18OSO4, the SO42- of rivers and springs waters exists in the fields of soil and geological (sulfide oxidation) (Krouse and Mayer 2000). Meanwhile, the δ34SSO4 and δ18OSO4 of lake waters have significantly higher values than those of river and spring waters. Sources of sulfur within the Böön Tsagaan watershed are limited because of the northern catchment area dominated by granitic rocks and the southern catchment area of Mongolian Altai dominated by basalt. SO42- concentrations of river and spring waters ranged from 0.05 mM to 2.02 mM (except in June). In contrast, the SO42- concentrations of lake waters were > 0.2 mM (4.8-17.0 mM). From the observation, the positive anomaly of lake water possibly is considered to have resulted from sulfate reduction of the water column and water-sediment interface (Habicht et al., 2002). However, the positive anomaly of lake water is likely due to incoming groundwater SO42- which was modified by bacterial sulfate reduction. This is because vertical mixing of the lake water occurred during summer and the SO42- concentration was 14.6 mM with δ34SSO4 of 16‰ at downstream of Baidrag River in middle June (melting period).
The δ18OH2O of rivers and springs ranged from –14‰ to –10‰ and δ2H from –107‰ to –81‰, while the δ18OH2O of lake waters ranged from –8‰ to –2‰ and δ2H ranged from –67‰ to –33‰. The d-excess (= δ2H – 8 × δ18O) of river and spring waters ranged from 0 to 11 , which is within the range of the d-excess of rainwaters in this region (–13 to 15; Vanwezer et al., 2021). However, the d-excess of some lake waters (–17 to 0) are lower than those of rainwater. The δ18OH2O of river and spring waters are lower than that of rainwater (–10‰ to –3‰; Vanwezer et al., 2021), while the δ18OH2O of lake water was almost the same value as rainwater. The biogeochemical implications for d-excess and δ18OH2O will be discussed in poster presentation.