## 5 Interpretation  
Borehole SG-5, located in the western Qaidam Basin, reveals mudstone, clay, siltstone, marl, limestone, gypsum, and halite from a depth of 300 m upwards. Isochronal framework is established according to the age of each standard zones, which connects environment in Taidong area to regional and global paleoclimate. Gypsum and halite begin to appear from around 250 m, corresponding roughly to the K2 standard zone in the Taidong area. The frequency of halite occurrence generally increases as burial depth shallows in the borehole (Figure 8a). Mn concentration data from boreholes SG-1 and SG-1b near SG-5 suggest that the climate has become progressively drier since K3 deposits (\(\sim\)1.7 Ma BP), with two periods of reduced aridity: upper K2 and lower K0 (Figure 8b). Water level and Mg/Ca values in borehole SG-1 were used to reconstruct aridity (Figure 8c) and water salinity (Figure 8d). Both aridity and salinity increased after 1.7 Ma BP, with a sharp escalation occurring after 0.73 Ma BP during K0 deposition. Global temperature anomalies (Figure 8e) and precipitation anomalies (Figure 8f) reflect a continuous decline in both temperature and precipitation, particularly after 0.73 Ma BP. Paleoyardangs found in the Sanhu area at 2.4 Ma BP further indicate persistent hyper-arid conditions since 1.2 Ma BP [33]. Regional climate changes, derived from geological and geochemical data, provide critical insights into interpreting the unique features observed on RGB-fused striatal slices and time-structural maps. The timing and frequency of the sub-circular morphologies align with the evaporite distribution in borehole SG-5 and the regional climate evolution. Sub-circular morphologies, such as those observed at the Bonneville Salt Flats, Utah [34], generally have much smaller diameters--on the order of several meters--compared to the larger morphologies found in the Taidong area. Therefore, the salt flats in meter-scale with pure saline is not proper to interpret those morphologies in Taidong. Considering geomorphology, vertical-frequency structure, average diameter, and associated lithologies, the sub-circular morphologies in Taidong are interpreted as saline pans formed in an eolian/interdunal sabkha (Figure 9a). This is distinct from sub-circular morphologies found in fluvial-lacustrine (Figure 9b) and marine-dominated sabkhas (Figure 9c) [17, 35, 36, 37]. Similar sub-circular morphologies with comparable dimensions are also observed in the northeastern region of modern Lake Chad [17], where the saline water is represented in sky-blue, plants in green, and saline sediment in grey-white. Based on observation and previous studies of Lake Chad, the geomorphology features and their evolution are interpreted. The increase in the restricted degree of water from Lake Chad to its northeastern area follows a distinct pattern (Figure 9a). Freshwater predominates in the open Lake Chad (I), transitioning to semi-restricted brackish water in the northeastern flooded interdunal corridors (II), and finally to restricted brine water in the saline pans of the further northeastern interdunal corridors (III). As salinity increases from west to east, vegetation coverage decreases correspondingly. The water pools exhibit a shift in contour shape, from linear in the flooded interdunal corridors (Figure 10a) to sub-circular in the saline lake (Figure 10b) and the dry saline pan areas (Figure 10c). Linear dunes in the region connect at their ends. The salinity progression across this area starts with fresh water in Lake Chad, moving through brackish and saline water to dry saline pans in the most restricted and evaporite regions. The sub-circular morphologies in northeastern Lake Chad are driven by monsoonal intensity and consist of dunes and saline pans. Mineral paragenesis in this area follows a sequential pattern from Lake Chad to the beach: calcite, aragonite, magnesium calcite, dolomite, huntite, magnesite, sodium carbonates, sodium sulfates, halite, and sylvite [38].