合肥下蜀土地球化学特征及其古气候意义

GEOCHEMICAL CHARACTERISTICS AND PALEOCLIMATE IMPLICATIONS OF HEFEI XIASHU LOESS

合肥地处长江中下游北岸,下蜀黄土广泛分布,通过对合肥地区BK2钻孔剖面岩性、氧化物含量及其地球化学风化参数的分析,研究合肥地区风成沉积物地球化学元素特征及其气候变化规律。结果表明:野外钻孔岩芯剖面显示地层较好连续性,结合年龄数据划分了该孔的第四纪沉积框架,Qh底界为1.20m,Qp^3底界为5m,5~35.10m属Qp^2中晚期地层;合肥下蜀土主要化学成分(SiO2+Al2O3+Fe2O3)的平均含量之和达88.99%,这种显著的富硅铝铁现象表明了该区气候较为湿润;化学风化程度较强,下蜀土的CIA平均值及其脱Ca、Na、K的程度均大于洛川黄土,说明其堆积期古气候比同期堆积的洛川气候温湿,较宣城干凉,与南京、镇江气候较为接近;近0.5Ma以来总体经历由湿热-干冷的变化,大致可分为35~14.50m、14.50~4.50m、4.50~1.20m和1.20~0m四个阶段,气候变化由温暖湿润-温暖偏干-冷凉偏干-温暖湿润,显示了区内更新世中晚期以来的气候变迁具有全球一致性特征。

Hefei is located in the north bank of the middle and lower reaches of the Yangtze river and Xiashu loess is widely distributed.Through the analysis of lithology, oxide content and geochemical weathering parameters of BK2 borehole section in Hefei area, the geochemical characteristics and climate change rules of the eolian sediments in Hefei area are studied.The core section of the field borehole shows good continuity of the strata, and the quaternary sedimentary framework of the hole is divided according to the age data.The bottom boundary of Qh is 1.20 m, the bottom boundary of Qp^3 is 5 m, and the depths range from 5 m to 35.10 m belong to middle and late Qp^2.The values of SiO 2 and TiO 2 in Hefei loess gradually decrease with depth, whereas those of K2O, Na2O, CaO, MgO gradually increase and those of Al2O3, Fe2O3, P2O5, MnO show no obvious increase.The average content of the main chemical constituents (SiO2 +Al2O3+Fe2O3 )of Xiashu loess in Hefei is up to 88.99%, which indicates that the climate in this area is relatively humid.The degree of chemical weathering is relatively strong.Weathering index and residual coefficiency decline gradually from bottom to up while Ba value and de-alkali increase from bottom to up, which demonstrate the weathering intensity decreases from bottom to up.These imply that the wetness degree decreased gradually during the sedimentation of Xiashu Formation;The average CIA value of Xiashu loess and the degree of removing Ca, Na and K of Xiashu loess are all higher than that of Luochuan loess, indicating that the paleoclimate of Xiashu loess was warmer and wetter than that of Luochuan at accumulational stage, drier and cooler than that of Xuancheng, and closer to that of Nanjing and Zhenjiang.Geochemical feature of sedimentary records from borehole section in Hefei shows that since 0.5Ma, it has experienced the change from wet heat to dry cold, which can be roughly divided into four stages: 35 m to 14.50 m, 14.50 m to 4.50 m, 4.50 m to 1.20 m, and 1.20 m to 0 m.The climate has changed from warm wet-warm dry-cold dry-warm wet, indicating that the climate change in the region has a global consistency since the middle-late Pleistocene, which matches with MIS curve.