2.5Ma以来大陆风化强度的演变

Variations in Continental Weathering Intensity over the Past 2.5 Ma

本次研究得出了2.5 Ma BP以来洛川剖面黄土-古土壤序列的酸不溶物^(87)Sr/^(86)Sr、Rb/Sr、磁化率和沉积速率的变化曲线。黄土和古土壤酸不溶物^(87)Sr/^(86)Sr、Rb/Sr和磁化率的变化具有类似的阶段性和周期性,这与晚新生代以来古气候的主要变化以及行星轨道参数相一致。研究证明,黄土和古土壤酸不溶物^(87)Sr/^(86)Sr值是化学风化强度的替代指标,沉积速率是物理风化强度的替代指标。自2.5 Ma BP以来在喜马拉雅山的北面风化强度的变化分为两个阶段,其界限在约1.2 Ma BP。自2.5 Ma BP以来化学风化强度在总体上呈下降趋势,而物理风化强度呈上升趋势。物理风化强度总的增长可能是导致海水^(87)Sr/^(86)Sr迅速上升的主要原因。

In this paper, 87Sr/86Sr and Rb/Sr ratios of acid-insoluble residues, and magnetic susceptibilities and deposition rates of loess and palaeosols from the Luochuan loess section are presented. The 87Sr/87Sr, Rb/Sr, and magnetic susceptibilities of palaeosol residues are generally higher than those of loess residues. This is attributed to the difference in chemical weathering intensity related to palaeoclimate, which leads to the formation of either loess or palaeosol. Temporal variations in these parameters can be divided into two stages. The first stage, from -2. 5 Ma to 1. 2 Ma, is characterized by low-frequency oscillations of 87Sr/86Sr, Rb/Sr and magnetic susceptibility and low deposition rate, while the second stage, from -1. 2 Ma to the present, is characterized by high-frequency oscillations of 87Sr/86Sr, Rb/Sr and magnetic susceptibility and high deposition rate with a cycle of -100 ka. The second stage can be further divided into two sub-stages with the boundary at -0. 6 Ma. This investigation reveals a long-term decrease in residue 87Sr/86Sr ratios and a long-term increase in deposition rates from 2. 5 Ma to the present. This may imply a general rise of mechanical erosion intensity coupled with a general decline of chemical weathering intensity over the past 2. 5 Ma. These long-term trends may be a combined result of the uplift of the Himalayas and Qinghai-Tibet Plateau, general intensification of glaciation and a decline of the Asian summer monsoon.