Chemical Weathering And Paleo-environmental Change Since The Holocene In The Daihai Lake Watershed, Inner Mongolia

Due to different geochemical nature between rubidium (Rb) and strontium (Sr), they are always fractionated during chemical weathering on the earth's surface. The Rb/Sr ratio in lake and/or ocean sediments decreases significantly with the enhancement of rock weathering intensity in watersheds, corresponding with its increase in weathering products. The sediments deposited continuously in a closed lake under different paleoclimatic conditions display variations in Rb and Sr distribution, reflecting intensities of chemical weathering in a single watershed by variations in the Rb/Sr ratios.Here we chose the Daihai Lake, Inner Mongolia, Northern China, which is located in a transitional zone between semi-arid and semi-humid conditions, and which is a closed hydrological system. In the first step, a 78cm-long sediment core was recovered from a depth of 12.50m in the central part of the Daihai Lake, which was deposited continuously over the last 500 years according to sedimentary rate by Pb-210. The higher Rb/Sr ratios in the lake sediments indicate a weak chemical weathering during the Little Ice Age, resulting in more Sr into basin. Meanwhile, the Rb/Sr ratio is negatively correlated with magnetic susceptibility of the lake sediments, showing the peak of the Rb/Sr ratio is contrary to the low values of magnetic susceptibility. A very significant feature is that the negative correlation between Rb/Sr ratio and magnetic susceptibility of lake sediment responds to the striking similarity between both in the loess-paleosol sequence. The variations of Rb/Sr ratios in the lake sediment sequence can, therefore, reflect the chemical weathering history. Moreover, in the short-term, silicate weathering is sensitive to change in air temperature and precipitation, therefore the Rb/Sr ratio preserved well in sediments could be a proxy for the regional air temperature and precipitation, especially in watersheds with lower precipitation. Low magnetic susceptibility, low CaCO₃, low organic carbon concentration (C_org), low C/A (Chenopodiaceae/Artemisia) ratio, low Sr concentration and, hence, high Rb/Sr ratio in the lake sediments indicate a cold but moist climate during the global Little Ice Age in the Daihai area.Based upon the paleoclimatic significance of the Rb/Sr ratio in the lake sediments, the Holocene climate was reconstructed by the Rb/Sr ratio, CaCCh content and C_org recorded in a 12.08m-long sediment core also from the Daihai Lake dated by AMS-¹⁴C and ²¹⁰Pb, including some important climate events such as the Medieval Warm Period and Little Ice Age. During 9 to 3.5 ka B.P., the lower Rb/Sr ratios (from 0.5 to 1.0) in the Daihai lake sediments suggest a stronger chemical weathering process was experienced around the Daihai watershed during the early-middle Holocene, indicating a warm and humid climate conditions during the Megathermal (or Hypsithermal, Optimum) characterized by a progressive increase of biologicproductivity and by a highest lake levels, to reach its maximum during about 5ka before present. However, a globally abrupt cooling climate event iccorded in the early llolocene is indicated by a strong increase in the Rb/Sr ratio and by other independent climate indices such as lake level and Corg, which are contemporaneous with the worldwide event identified in the lakes, oceans, land mollusk sequence and polar ice cores at around 8.0-8.5 ka B.P. After ³.5 ka B.P., the neoglaciation period including (he Little Ice Age is demonstrated by significant increase of Rb/Sr ratios and by decrease of CaCOs and Corg and lake levels in the Daihai Lake. The new cooling period was interrupted by a brief recessional event at 0.9-I.2ka B.P. characterized by lower Rb/Sr ratios and higher CaCC>3 and Corg in the sediments, i.e. the Medieval Warm Period, that was a warm and humid environment, though its degree is less than the Megathermal. Finally, the variation of Rb/Sr ratios in the sediments also show an enhancement of chemical weathering under modern warming climate, but its intensity is less than that of the Medieval Warm Period.Our preliminary work suggests that the variation of Rb/Sr ratios in the lake sediment sequence can be used as an effectively geochemical proxy of chemical weathering in watersheds. Furthermore, due to continuous deposition, high-resolution and stable source in the closed basins, the reconstruction of paleoclimate by the variation of the Rb/Sr ratios in the sediments might give us a greater advantage than by that of the ocean sediments and the loess-paleosol sequence.