最后两个冰期黄土中记录的Heinrich型气候节拍

洛川、宜川及渭南黄土剖面古风化强度的研究揭示出，在Ｌ１和Ｌ２中记录了一系列干冷气候事件。它们多数发生于黄土与古土壤的交界处，以５０００—１００００ａ的间隔叠置于明显的准２００００ａ岁差周期上。前六个事件的时代与北大西洋的Ｈｅｉｎｒｉｃｈ事件大致相当。上述结果表明，Ｈｅｉｎｒｉｃｈ事件在东亚季风区有明显印迹，且类似事件在相当于深海氧同位素阶段６的黄土中也存在。最后两个事件时我国的古环境状况可能比末次盛冰期还要干冷些。这些事件出现的地层层位似乎表明，它们易发生于由地球轨道变化引起的气候转变时期。

HEINRICH-RHYTHM PULSES OF CLIMATES RECORDED IN LOESS OF THE LAST TWO GLACIATIONS

Luochuan, Yichuan and Weinan loess sections in the Loess Plateau of China are studied to characterize the variations of the East Asian monsoon climates in the Late Quaternary period. Paleo-weathering profiles based on two weathering indexes reveal a series of spatially correlative peaks of lower weathering intensity (LW) in the last glacial and late penultimate glacial loess, indicating significant cooler or/and dryer conditions. Most of them are identified at the transitional boundaries between loess and soil units except the LW-3, suggesting that the events preferably occurred in relation with major climatic boundaries.Twenty-eight radiocarbon and thermoluminescence measurements combined with Kukla's magnetic susceptibility age model date these peaks at about ￣ 14000 a B.P., ￣ 21000aB .P ., ￣ 29000a B.P ., ￣ 38000a B .P ., ￣ 52000aB .P .,￣ 71000 a B .P., ￣0.13 Ma B .P. and ￣ 0.145 Ma B.P. with ￣5000-10000 a frequency inlaid in the orbitally induced ￣ 20000 a periodicity. The six peaks in the last glacial period are therefore timely correlated to the 'Heinrich layers' in the North Atlantic Ocean resulted from massive discharges of icebergs. These results suggest that 'Heinrich events' had clear imprints in the east Asian monsoon zone, as has also been indicated by Porter and An based on the grain-size time series from Luochuan sections. However, our timescale based on a great number of radiocarbon and TL dates suggests a different correlation pattern from that proposed by Porter and An for the older events. Moreover, our data reveal that similar events also existed in the penultimate glaciation. Heinrich events have affected both summer and winter monsoons. Both weathering records from the Loess Plateau region and published pollen record from the South China Sea suggest that the climate conditions during the Heinrich events 1 and 2 were cooler than during the Last Glacial Maximum (￣ 18000 a ).The stratigraphic position of the LW peaks in the soil-loess sequences may be helpful for understanding the mechanism that driven these events. Since the soil-loess sequences closely correlate with marine δ18O records, the occurrence of LW peaks at the transitional boundaries between loess and soil units seems indicate that Heinrich events preferably occurred in the courses of ice volume growth and deglaciation. This seems consistent with the case of the last deglaciation and some modeling results that one type of ice surges occurred during ice-sheet growth. If these were the cases, the ￣ 50000 a B .P. age of the lower boundary of the soil complex L1 SS1, significantly younger than that of the marine isotope stage 3 and coinciding with the LW-5 peak, seems indicate that this event may have significantly delayed the shift of monsoon climate to interstadial conditions that was already underway. From another point of view, since the paleosols in the loess of the last climatic cycle closely match the orbital precession cycles, the stratigraphic position of the LW peaks may also imply that these events might be an internal response of the climate system to the orbital forcing. The mechanisms linking the Heinrich events and the monsoon climate may be similar to that driven the Younger Dryas event which has been identified in a large number of records in China and in the surrounding oceans.