宜昌庙河钙结岩记录的古环境变化

PALAEOENVIRONMENTAL CHANGES RECORDED BY THE CALCRETES AT MIAOHE, YICHANG

摘要: 对位于三峡地区黄陵背斜核部的宜昌庙河钙结岩体进行了沉积学、岩石学分析,识别出土壤钙结岩和地下水钙结岩。对土壤钙结岩进行了胶结物的分离、ESR年龄测试和碳氧稳定同位素分析,结果表明,庙河土壤钙结岩中方解石胶结物的δ¹³C值相对围岩而言明显偏负,多数样品δ¹⁸O值则呈不同程度偏正的趋势。土壤钙结岩主要形成于冰期,计算表明它们形成时期的古气温多为8~12℃,年均降雨量多为760~770 mm,C₄植物占约84%的绝对优势。由于钙结岩的形成过程涉及到诸多影响碳酸盐化学过程和相应土壤碳酸盐稳定同位素组成的因素,得出的认识可能存在一定程度的不确定性。

Abstract: Two types of calcrete at Miaohe, Yichan, i.e., pedogenic calcrete and groundwater calcrete were studied in this paper. Calcite cements were separated from the pedogenic calcrete for ESR dating and δ¹³C and δ¹⁸O analysis. δ¹³C values of calcite cements are obviously lighter than the host sediments, and δ¹⁸O values of calcite cements slightly heavier than those of the host sediments. It's inferred that the palaeotemperature was 8~12℃, the mean annual rainfall was 760~770 mm and C₄ plants accounted for~84% when these calcretes were formed in the glacial stages.