OSTRACODE OCCURRENCE, WATER CHEMISTRY, AND GROUNDWATER IN WESTERN MONGOLIA

In August of 2004, 125 samples from 111 sites in western Mongolia were collected for ostracodes. The area included the Valley of Great Lakes (ca. 1100 m ASL) and northeastern foothills of the Altai Mountains (1500~2500 m ASL). Lakes, playas, streams, springs, and groundwater seeps were sampled. Water temperature, pH, and TDS ranged from 2.5 to 25°C, 6.7 to 10.4, and 35 to 7750mg/L plus 3 sites with up to 350g/L, respectively. All 63 sampled waters had carbonate alkalinity (meq) to Ca (meq) ratio (alk/Ca) >1.

The climate of the region is semi-arid with temperature extremes typical of mid-continent. The geology of the Altais and associated Jargalant Uul is characterized by Devonian granites, Paleozoic and Mesozoic siliciclastic sediments while the Valley of Great Lakes area is underlain by Mesozoic siliciclastics, Late Proterozoic to Early Paleozoic granitic rocks, Quaternary basin fill, and local Archean mafic volcanics (Yanshin, 1989). Carbonate rocks are absent in the sampled region.

Every spring and groundwater seep except for one at 2230m ASL in a broad flat valley contained live ostracodes. Groundwater chemistry was on the whole dilute with 8 out of 12 sites measuring <400mg/L. Four others ranged from 600 to 1000mg/L. All were characterized by alk/Ca ≤ 3. Among lakes, those with TDS 50~3500mg/L, Mg/Ca ≤10, and alk/Ca ≤ 15 had live ostracodes, whereas those with any one of the three parameters above those values had none. Eight high elevation sites (> 2000 m ASL) did not fit this pattern: 4 lakes and wetlands with TDS 47~500 mg/L and alk/Ca 1.2~3.0, and 4 sites (1 lake and 3 of the 4 stream-connected ponds) with TDS 1000~3000 mg/L and alk/Ca 1.2~3.1 had no ostracodes. For 6 of the 10 sites, there were ostracodes in nearby springs but not in the streams flowing out of them into the sampled lakes, ponds and irrigation fields. Without further study, the reason for the absence of ostracodes at these sites remains a puzzle. In the Valley of Great Lakes, 3 lakes with TDS < 150 mg/L and alk/Ca <3.0 had no ostracodes. Very low dissolved oxygen (6% of saturation) or high H₂S were noted in these 3. Three small evaporating pools with estimated TDS upward of 40g/L contained Heterocypris incongruens or Limnocythere inopinata.

Reference Yanshin AL (1989) Map of geological formations of the Mongolian People's Republic. Academia Nauka USSR, scale 1:1,500,000.