定量重建黄土高原末次间冰期土壤呼吸CO2浓度

成壤碳酸盐气压计(paleosol carbonate CO₂ barometer)是定量恢复古大气CO₂浓度的重要方法, 但是目前重建结果仍然存在较大的不确定性, 主要原因是土壤呼吸产生CO₂(简称S (z))的数值分布范围较大。S (z)在古土壤中不能直接测量, 它受古降水影响具有明显的空间分异特征。中国黄土高原广泛分布的成壤碳酸钙结核是土壤呼吸作用和淋溶再沉淀的产物, 具有恢复长时间序列古大气CO₂浓度的潜力, 但是目前各指标方法恢复的S (z)存在差异。本研究利用黄土高原7个末次间冰期(对应古土壤S₁层)剖面的成壤碳酸钙结核恢复黄土高原S (z)值的空间变化规律, 并对比不同指标方法恢复的S (z)。结果显示: 黄土高原S (z)呈现出南部高于北部的特征, 南部剖面S (z)的平均值为848±170ppm, 北部剖面S (z)的平均值为539±87ppm。这个趋势与暖季平均降水量的空间变化趋势一致。本研究确定黄土高原S (z)的空间变化范围, 为利用黄土碳酸盐重建古大气CO₂浓度提供可靠的参数限制。

Quantify last interglacial soil respired CO2 on the Chinese Loess Plateau

The paleosol carbonate CO2 barometer is an important method to quantitatively reconstruct the pCO2. However, at present, there are still large uncertainties in the reconstruction results. The main reason is that the soil respired CO2 (S(z)), which is the most uncertain parameter when reconstruct pCO2, has a large distribution range. The S(z) cannot be measured directly in paleosols. Importantly, S(z) is characteristic by obviously spatial differentiation, because of the influence of paleo-precipitation. Calcium nodules widely distributed in the Chinese Loess Plateau(CLP) are the product of soil respiration and re-precipitation process. It has the potential to reconstruct long time paleo-atmospheric CO2 concentrations, but the discrepancy of S(z) exists among different proxy-based methods. Thus, this study tries to reconstruct the S(z) values from the carbonate nodules below seven last interglacial(paleosol S1) to explore the spatial variation trends of the S(z) in the CLP, and then compare the results between the geological records from the last interglacial period and other proxy-based results. The seven sections belong two transect, eastern transect ordered from north to south includes Yanchang(YC), Luochuan(LC), Tongchuan(TC) and Weinan(WN) sections; western transect ordered from north to south includes Beiguoyuan(BGY), Xifeng(XF) and Lingtai(LT) sections. Bulk samples were collected at 2cm intervals in Beiguoyuan, Xifeng, Weinan sections; at 5cm interval in Yanchang, Luochuan, Tongchuan sections and at 10cm interval in Lingtai section. Within each section, δ13CPC and δ13COM are measured. The S(z) are back-calculated based on the paleosol carbonate CO2 barometer and assuming the pCO2 are known from ice core pCO2 records over the past 800ka. Back-calculate results show that S(z) in the Loess Plateau changed from 505ppm to 865ppm. Average S(z) value in the southern sections is 848±170ppm, which is higher than the average S(z) value in northern sections of 539±87ppm. This spatial variation characteristic is consistent with the results calculated based on the climofunction between S(z) and magnetic susceptibility, but lower than the results calculated based on the relationship between S(z) and Mean Annual precipitation. This may because the mean warm month precipitation is more important for S(z). This work determined the spatial variation range of S(z) in the CLP, which provides accurate and reliable parameters for pCO2 reconstructing by using carbonate nodules. By this work, we know that the S(z) in the CLP is lower than before assumed 10000ppm, 5000ppm and 2500ppm.