[M-IS12] Paleoclimatology and paleoceanography

[MIS12-P26] Evidence for an Ancient Giant Salt Lake in Gobi desert

*Akito Nakamura¹, Hitoshi Hasegawa¹, Aki Sakuma², Minoru Ikehara¹, Tomoyo Okumura¹, Chuan-Chou Shen³, Niiden Ichinnorov⁴, Dawadorj Davaasuren⁵, Tsuyoshi Ishikawa⁶, Yoshiro Nishio¹, Asuka Yamaguchi², Trishit Ruj⁷, Goro Komatsu⁸ (1.Kochi University, 2.University of Tokyo, 3.National Taiwan University, 4.Paleontological Institute, Mongolia, 5.Mongolian National University, 6.JAMSTEC, 7.Okayama University, 8.Università d’Annunzio)

Keywords:Gobi desert, Giant Lake, Salt Lake, Stromatolite, Last Glacial, Carbonate

In the Valley of Lakes region of the northern Gobi Desert, southern Mongolia, there are some salt lakes [Boon Tsagaan Lake (average depth of 9.3 meters) and Orog Lake (average depth of 3 meters)] ranging in width from several kilometers to around 20 kilometers. On the other hand, geomorphological studies suggested the possible existence of the ancient giant lake called "Giant Gobi Lake" (over 300 kilometers width and Maximum depth exceeding 200 meters), connecting from Boon Tsagaan Lake to Orog Lake (Komatsu et al., 2001; Lehmkuhl et al., 2018). The development of this giant lake is estimated to have occurred during the MIS6∼5, MIS3, and the mid-Holocene, based on the OSL ages of shoreline and lake sediments (Lehmkuhl et al., 2018). However, the timing of the giant lake development and the climatic mechanism that sustained them remains uncertain.

In July 2023, we conducted a wide-area survey in the Valley of Lakes area and found polygonal landforms, which are generally formed due to the desiccation process in salt lakes, developing at a site 40 km north of Orog Lake (in the shallow lake area of the giant lake). Additionally, we discovered laminated carbonate cemented on the desert pavement scattered over the polygonal landforms. This suggests that “Giant Gobi Lake” may have been a salt lake rather than a freshwater one. Furthermore, age dating and chemical analysis of the laminated carbonate should provide evidence for the timing and causal mechanism of the expansion and disappearance of the giant salt lake. In this study, we analyze the elemental and mineral compositions, stable carbon and oxygen isotopes, and U/Th dating of laminated carbonates in order to elucidate the formation mechanism of the giant salt lakes in the Gobi Desert.

First, we cut a laminated carbonate sample and observed its internal structure. They are composed of following three parts: lower well-laminated part, middle pebbly part, and upper part showing stromatolite and ooid textures. XRD analysis reveals that the upper stromatolitic part is mainly composed of calcite. Elemental composition analysis using µXRF and LA-ICPMS reveals that the laminated carbonate is composed mainly of alternating layers of pure carbonate and carbonate with very fine-grained clastic minerals. In addition, stable carbon and oxygen isotope analysis were performed for micro-scale samples (intervals of 100-200μm) obtained using the micro-milling technique. The carbon isotopic ratio ranges from -1.0 to +8.0‰, and the oxygen isotopic ratio ranges from -6.6 to -3.1‰, reflecting the laminated structure. We will further conduct U/Th age dating to reveal the timing and formational mechanism of the ancient giant salt lakes developed in the Gobi Desert.