洛川黄土地层的再划分及其L_9、L_(15)古环境意义的新解释

PEDOSTRATIGRAPHIC SUBDIVISION OF THE LOESS-PALEOSOL SEQUENCES AT LUOCHUAN AND A NEW INTERPRETATION ON THE PALOENVIRONMENTAL SIGNIFICANCE OF L_9 AND L_(15)

洛川剖面的午城黄土是可以在野外进行细分的 ,洛川的午城黄土含有 1 8层古土壤和 1 8层黄土。其中 ,S1 5～S2 3的土壤组合与前人划分的WS1 相当 ,L2 4 与WL1 相当 ,S2 4 ～S2 8与WS2 相当 ,L2 9与WL2 相当 ,S2 9～S32 与WS3相当 ,L33与WL3相当。黄土高原的两个粉砂层L9和L1 5,虽然形成于第四纪的冷期 ,但并非代表了第四纪时期最为干冷的气候条件。笔者的解释是 :其形成与这两个时期青藏高原及其周边山体的快速隆升导致的沙漠和黄土物源的快速增加有关。特别是 1 .1MaB .P .前后 (L1 5)的隆起 ,使中国黄土记录对全球变化的响应显著增强 ,也正是在这次运动之后 ,中国黄土古气候记录可以与深海氧同位素很好对比 。

The Early Pleistocene Wucheng Loess Formation at Luochuan was previously divided roughly into six units (WS 1, WL 1, WS 2, WL 2, WS 3 and WL 3), but on the basis of our recent studies it is subdivided into eighteen loess beds and eighteen soils, similar to those in the Baoji loess section. Among the new subdivided units S 15～S 23, L 24, S 24～S 28, L 29, S 29～S 32, and L 33 correspond to the previous divided units WS 1, WL 1, WS 2, WL 2, WS 3 and WL 3, respectively. The two sandy loess beds of L 9 and L 15 marked by the greatest thickness and the largest particle size had been used as marker layers for spatially pedostratigraphic correlation throughout the Loess Plateau, and they had been interpreted as representing two cold and dry extremes of the Quaternary. However, this interpretation causes the discrepancy between the terrestrial loess and the marine oxygen isotope records, because the marine records do not imply largest ice volumes during the formation of L 9 and L 15. Here, we have a new interpretation: although L 9 and L 15 were accumulated during the cold episodes, the climatic conditions to form them were not the most cold and dry of the Quaternary, and their formation could be associated with an increased dust supply in response to quick uplift of the Tibetan Plateau and its surrounding mountains 1.1 and 0.9 Ma B. P. The elevation of the uplifted plateau seems to reach in its uplift history a threshold value that made the coupling of the regional-scale loess deposition with the global-scale ice volume variation significantly enhanced. The Chinese loess records just after this uplift can be well correlated with the marine records, whereas the Early Pleistocene Wucheng Loess cannot match the recorded climatic fluctuations in deep seas. $$$$