Responses of Sediment Properties to Paleoclimatic Changes since the Holocene in Ancient Milan River Channel, Northwest China

Abstract

In this paper, the sediment grain size, organic matter C/N ratio, and isotope δ 13 C of the ancient Milan River channel, Northwest China are used as indicators, and their responses to paleoclimatic changes since the Holocene were analyzed. The results show that the dominant grain size of the surface sediments of the ancient Milan River channel is silt, and the soil particle size is smaller than that of the floodplain, indicating that the hydrodynamics were weak in the late paleochannel. The surface sediments of the paleochannel contain a wind created layer of sand, indicating that the sedimentary environment was affected by a two-phase function of wind and water. The C/N ratio of the sediments varies from 5 to 19.6 with an average value of 12.3. The δ 13 C values range from -27.383‰ to 21.58‰, indicating that the organic matter in the sediment was dominated by river organic matter but mixed with some terrestrial organic matter. The dates of the Optically Stimulated Luminescence (OSL) and the variation in the measured values of each element in the vertical section indicate that since 5000 aBP, the paleoclimate in the study area has experienced six stages of evolution. In stage I, 5000-4500 aBP, the climate was mainly warm and dry. During this period, the climate environment fluctuated frequently, and the cold and warm periods alternated. During stage II, 4500-3900 aBP, the climate gradually decreased and then rebounded slightly. The overall climate was cold and humid, and the water volume increased significantly during this period. During stage III, 3900-2800 aBP, the climate experienced a large temperature increase and cooling, and a warm front appeared around 3000 aBP, indicating typical warm and dry climate characteristics during this period. In stage IV, 2800-1800 aBP, the temperature continued to decrease, and the climate became cold and humid, which was conducive to the growth of crops. In stage V, the first half of the period from 1800 to 1000 aBP was relatively warm and dry, and the temperature dropped briefly causing a cold period. In stage VI, 1000 aBP to present, the temperature experienced a small fluctuation, the climate became dry and