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• W2920717081 abstract "Hydrological data are useful in reading past climate variability. Analysis of long-term, mid-continent lacustrine sediment records has demonstrated an acute sensitivity to insolation fluctuation in interior continental Asia including, for example, Lake Baikal (BDP-98 Members, 2001; BDP-99 Baikal Drilling Project Members, 2005). Another example is the Darhad basin, now mostly a dry area in northeastern Mongolia, which has been suggested to have been filled with water to form a lake as large as Lake Hovsgol (Krivonogov et al., 2012). Eventually, the Darhad paleolake disappeared, possibly due to the retreat of a damming glacier (Batbaatar & Gillespie, 2016; Komatsu et al., 2009). In central Mongolia (Bayankhongor Province), there is a valley where lakes and remnants of paleolakes concentrate, called “Valley of the Gobi Lakes,” in which lakes repeatedly emerged and disappeared during the Quaternary (Lehmkuhl, Grunert, Hülle, Batkhishig, & Stauch, 2018). Currently, water is supplied mainly from the Khangai mountains in the north where precipitation is approximately 200 mm/year. While, in the valley, none of the main lakes (e.g., Böön Tsagaan Lake) have an outflow river reflecting the modern dry climate, Komatsu, Brantingham, Olsen, and Baker (2001) reconstructed a large lake suggesting a wet climate in the past. However, the connectivity between the lake and the upstream watershed is not well understood. To reconstruct the ancient hydraulic environment of this area, an investigation of the upstream basin of Böön Tsagaan Lake, which we refer to as the Olgoi Basin, is presented here. Olgoi Basin is located in the southern part of the Khangai Mountains (Figure 1a). The river flowing in the Olgoi Basin is one of the major tributaries of the Baydragiyn Gol River, which is a main conveyer of water to the modern Böön Tsagaan Lake. Quaternary lake sediments are distributed throughout the Olgoi Basin (Mineral resources authority of Mongolia, 1998), which suggests that the Olgoi Basin, in the past, included a larger lake that supplied water to Böön Tsagaan Lake. Topographic features found in the eastern part of Olgoi Basin in observations from Google Earth imagery (Figure 1c) were found to be comparable to those reported by Komatsu et al. (2001) for other major lakes in the Valley of Lakes. These features are composed of several curved lineaments parallel not only to each other, but also to the outline of the basin. Similar features were also identified in the western portion of the basin, and it is noted that both exist at similar elevations. The curved topographic features likely represent paleoshorelines of a large lake in Olgoi Basin. Using GIS software (QGIS), it was found that these paleoshorelines fall within in the range of 2 045 m to 2 054 m above present-day mean sea level, and the lowest part (the western outlet) is 2 022 m. The Olgoi paleolake was reconstructed on the map assuming the maximum water level as 2 054 m above sea level (Figure 2a). The estimated lake area is approximately 200 km2, which is two orders of magnitude larger than that of modern Olgoi Lake at 3 km2. This mapping analysis is the first to show the existence of the large paleolake as no previous reference to this paleolake was found in the past studies. Presumed paleo-hydraulic conditions at the time of flooding were calculated in the upstream Olgoi Basin (Figure 2b) and constrained according to the following methodology. A measurement was taken on the largest members of rounded cobble (Figure S1) observed at a site in the center of the paleochannel in the middle of the calculation area (red circle in Figure 2b). Almost all observable cobbles and gravels are partially buried in the ground supported by a matrix of finer grains. An approximate mean value of 10 cm in diameter for the cobbles was determined, and it was assumed that these rounded cobbles were transported when the river was in flood stage. The critical shear stress necessary to transport grains of this size was then used to estimate river discharge. According to Lane (1953) who compiled the experimental data of sediment transport in various cases, the bottom shear stress necessary to convey 10 cm cobbles is 80 N/m2 in water flow containing mixed sediments with various sizes. Although we adopt this value (80 N/m2) for the estimation, a nonlinear effect of mixed-size sediment on large grain transport might reduce the critical shear stress for the cobbles, which is suggested by Wilcock and Crowe (2003) who showed the critical Shields number for large grains decreases to about 0.02 with increase in the content of finer grains. The case of 30 N/m2 (corresponding to 0.02 of the Shields number for the cobbles) was calculated as well in this study. The water level at the downstream end as the boundary condition was defined as the reconstructed lake water level (2 054 m above sea level). The calculation of the flow was carried out using the software iRIC Ver.2 provided by the International River Interface Cooperative (Nelson, Shimizu, Takebayashi, & McDonald, 2010). The topographic data used for the calculation was a 90 m mesh digital elevation model from NASA Shuttle Radar Topography Mission Version 3.0 (SRTM-3). Tributaries were ignored. Multiple permutations of the hydrological model were calculated varying the discharge from the upstream end to match the above criteria. Calculation area is shown in Figure 2b. Other information about calculation is presented in File S1 and Table S1. Figure 3a shows the calculated map of shear stress for the case of 6 000 m3/s, at which the shear stress first attains the required state (80 N/m2) to transport the observed cobbles, suggesting the lower-limit discharge of the paleoflood condition. At this hydraulic condition, the flow width was about 1–4 km, a maximum depth was about 4 m, and the maximum flow velocity was about 3 m/s (Figure 3b). Results of the cases of less and more than 6 000 m3/s are shown in Figure S2. When setting the critical shear stress to be 30 N/m2, taking account of the nonlinear effect due to finer grains that render cobbles easier to move (Wilcock & Crowe, 2003), the estimated discharge becomes about 30 % smaller (Figure S3). The water depth at the sampling point of the cobbles estimated in the two cases, i.e. 80 N/m2 and 30 N/m2 for the critical shear stress, is 2.7 m and 2.5 m, respectively. These values are more than twice the values estimated by applying the simple equation for shear stress under the assumption of uniform flow (called depth-slope product rule), i.e. 1 m and 0.4 m for 80 N/m2 and 30 N/m2 respectively, where the river bed slope is about 0.0085 (for the 1–10 km reach around the sampling point). This is considered to be because the calculation with the depth-slope product rule ignores surrounding topographies and we took particular note of spatial continuity of the area that satisfies the criteria for the cobble transport from upstream. The estimated paleoflow appears to be much larger than the modern inflow river into the Olgoi Basin. However, further direct evidence, such as dating analysis of lacustrine sediments of Olgoi paleolake, is required to better evaluate the timing and magnitude of this type of event. The result of the present study favors a humid climate at the age of expanding lakes, which is different from the present climate in the Olgoi Basin. This is the first result concerning paleoclimate in the Olgoi Basin, which is approximately 130 km upstream of and approximately 700 m higher than the Valley of the Gobi Lakes where the climate used to be more humid than the present. The authors thank the members of the survey in Mongolia for their contribution to our fieldwork. We especially thank Y. Tanaka from Kyung Hee University for his cooperation in communication and observation. Our study was partially supported by JSPS KAKENHI to NE (Grant Number 16K01216) and to NH (Grant Number 16H05643). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article." @default.
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• W2920717081 date "2019-02-27" @default.
• W2920717081 modified "2023-10-16" @default.
• W2920717081 title "Paleolake reconstruction and estimation of paleo‐inflow in the Olgoi Basin, Mongolia, based on GIS and hydraulic analyses" @default.
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• W2920717081 doi "https://doi.org/10.1111/iar.12299" @default.
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