黄土高原和南海陆架古季风演变的生物记录与Heinrich事件

PALEOMONSOON EVOLUTION AND HEINRICH EVENTS:EVIDENCE FROM THE LOESS PLATEAU AND THE SOUTH CHINA SEA

宝鸡黄土中０．１５Ｍａ以来植物硅酸体研究表明，年均温度、１月份和年均降水量的变化与深海氧同位素变化有很好的一致性，而７月份降水量的变化与印度洋反映季风强弱的粒度变化相类似。南海陆架孢粉研究发现，１４７００—１３９００ａＢ．Ｐ．和２２９００一２０６００ａＢ．Ｐ．气候变冷期与北大西洋沉积物中Ｈ１、Ｈ２变冷事件存在成因上的联系。全球冰量通过冬季风等因素在“轨道尺度”上可能影响了夏季风对特定地区的控制时间，从而控制了黄土区气候的年均状况，但没有影响夏季风固有的变化周期和振幅。由低纬夏季太阳辐射控制的夏季风以其准２００００ａ周期叠加在冬季风所具有的准０．１Ｍａ周期上。冬、夏季风的演化行为具有较强的相对独立性，冬、夏季风同时减弱或增强的情况是存在的。Ｈｅｉｎｒｉｃｈ事件在东亚地区不仅影响了冬季风而且影响了夏季风。

In the recent paleoclimatic studies, the variations in loess magnetic susceptibility and organic contents are used as proxy indicators of Asian summer monsoon strength,and those of the grain-size as the proxy of winter monsoon. The grain size record is dominated with a 0.1Ma period. These led to the interpretation that the climate in Asian monsoon area is controlled largely by the variations in global ice volume.However,this is inconsistent with a strong 20000a period in the loess-paleosol sequences.Thus,the major factor controlling paleoenvironmental changes of the East Asian monsoon zone is still a contentious point. The difficulty arises from the fact that the proxy index previously used for summer monsoon is not the seasonal climatic indicator and it is therefore difficult to distinguish the effect of summon monsoon from that of the winter monsoon.This paper presents results of phytolith research from the Baoji loess section during the last 0.15Ma to address this problem.It has been known that since phytolith assemblage has great advantage in seasonal reconstruction of paleoclimate changes, especially in steppe area where vegetation community is dominated by herbaceous plants, phytolity is incomparably superior to pollen. In dry steppe, phytolith production consists of 60%-90% of the total dry production of vegetation community. As most of the herbaceous plants are annul or seasonal growing plants, the evolution of grassland community in steppe area is generally last for about 10 years or a little more, by which phytolith can be used as the seasonal indication of short-term climatic changes.We have established the regressive relation of phytolith assemblages from 153 surface soil samples of China with the modern climatic parameters and obtained six climatic factor transfer functions for January, July and annual mean temperature and precipitation. Fig 1 shows the reconstructed variations in January, July, annual mean precipitation and annual mean temperature in the Baoji loess section in the last 0.15Ma and the correlation with the grain-size record from the Indian Ocean and the oxygen isotope record from deep sea sediments.It is shown in Fig 1 that variations in annual mean temperature, precipitation and January precipitation are in good agreement with that of the oxygen isotopic record. In contrast, July precipitation shows similar variations to the grain-size curve from the Indian Ocean core Rc27-61, which reflects changes in strength of the indian Ocean monsoon.The essence of monsoon variation is thought to have the following two aspects: 1) astronomic effect, 2) topographical and thermal difference. We take into account that the global ice volume may influence the duration time of summer monsoon on a given area through the effect of winter monsoon at orbital scales, and as a result, controlled the annual climatic pattern of the Loess Plateau. But, it did not change the initial periodicity and amplitude of summer monsoon with the quasi-20000 year period related with the low-latitude summer solar insolation. The last one is inlaid in the quasi- 0.1Ma period of the winter monsoon. Our results do not support the notion that the winter monsoon controls the summer monsoon or the summer monsoon weakens the winter monsoon.We also analyzed 58 pollen assemblages taken from the upper 42m of Q4 core in the continental shelf of the South China Sea (1 15°25'22' E,21°00'22'N ).Fig 2 shows parts of typical pollen species diagram and climatic variations in relation with the Heinrich events.Two cooling events have been documented which approximately correspond to the H1 (about 15000-13500aB.P.) and H2 (about 23000-21000aB.P.) events recorded in the North Atlantic Ocean.Similar events took place in the Loess Plateau. The results obtained from grainsize and weathering index in loess-paleosol records indicated that the Heinrich events happened preferably during the climatic change period in relation with the orbital periodicity. We may infer that the Heinrich event is an response of the climate system to the Earth's orbital forcing