Carbon isotopes of pedogenic carbonate are often used to study paleoenvironments, but the existence of detrital carbonate changes the carbon isotopic composition. To develop an experimental method to determine existence of detrital carbonate in carbonate nodules, and to avoid it during isotope analysis, 23 pedogenic carbonate nodules in Miocene loess from the Loess Plateau of China were studied through micromorphology and carbon isotope analysis. The difference in carbon isotopic composition between matrix carbonate (B) and pore carbonate (A) (δ 13C(B-A)) ranges from 0.27‰ to 0.44‰ in nodules containing detrital carbonate and -0.16‰ to 0.13‰ in nodules where detrital carbonate is absent. The latter is within measurement error, but the former is beyond it. Here we propose an isotopic approach to determine if nodules contain detrital carbonate: if δ 13C(B-A) is within the measurement error, the nodules do not contain detrital carbonate, and vice versa. We suggest that it is better to analyze pore carbonate instead of matrix carbonate when using carbon isotope of carbonate nodules to reconstruct paleoenvironments.
通过陇东地区新石器时代2处文化层剖面及5处文化点的花粉、炭屑、种子的综合研究,重建了陇东地区新石器时代农业发展过程及其环境效应.研究显示,陇东地区早期农业经历了由单一黍作农业向黍、粟混作,并结合水稻、大豆等多样化农业发展过程.旱作农作物黍在仰韶文化早,中期占有绝对的优势,而粟在仰韶文化晚期开始明显增加,在齐家文化时期达到最大值.陇东地区仰韶文化晚期大豆开始栽培,稻作农业在陇东地区出现不晚于4800cal a BP,并延续到约4000cal a BP的齐家文化.新石器时代陇东黄土高原塬面以灌丛草原植被为主,沟谷地带存在云杉-铁杉-栎为主的针阔叶混交林,新石器时代农业活动减少自然植被盖度,促使灌丛草原趋向于单一的蒿属草原.农业活动减弱或消退后,以沙棘、蔷薇科、麻黄、豆科、蒿属、藜科为主的灌丛草原植被得以恢复.
Neolithic agricultural development and environmental effects in the Longdong area were reconstructed using a synthetic approach,investigating pollen,charcoal,and seed remains for two cultural layer sections and five flotation sites.Results show that Neolithic agriculture in the Longdong area had a simple organization and was dominated by the production of common millet,especially in the early and middle Yangshao age.After the late Yangshao age,Neolithic agriculture developed into a more complex structure,dominated by both common and foxtail millet and the cultivation of rice and soybeans.The production of foxtail millet gradually increased through the Neolithic period,reaching its highest point during the Qijia culture.Soybeans were first cultivated during the late Yangshao culture,approximately 5000 cal a BP.Rice production began no later than 4800 cal a BP,and continued to exist in the Qijia culture,approximately 4000 cal a BP.Agricultural production in Neolithic Longdong,specifically in the "Yuan" area of the loess plateau,developed as a shrub and grass dominated landscape.Vegetation in the river valleys was partly covered with Picea,Tusga,and Quercus coniferous and broadleaf mixed forests.Agricultural activity during the Neolithic period caused an increase in farmland on the loess tableland and a decrease in the abundance of shrub and grassland in the Longdong area.When farmlands were abandoned,vegetation recovered with Hippophae-,Rosaceae-,Ephedra-,and Leguminosae-dominated shrublands and Artemisia-dominated grasslands.
