南京地区下蜀黄土磁性地层年代与古环境变化

通过对南京江北泰山新村下蜀土剖面所采的144个样品和南京江南燕子矶下蜀土剖面所采的467个样品所进行的古地磁测年、磁化率测量和粒度测定,建立了南京地区下蜀黄土地层220kaB.P.以来较精细的时间标尺;在此工作基础上,将南京下蜀黄土堆积所记录的环境信息同北方经典黄土、深海沉积氧同位素记录进行了对比,发现燕子矶剖面磁化率曲线能够与深海沉积氧同位素曲线1—7阶段进行良好的对比,两者之间的细节变化具有一致性,其中S0表现为高值,可与氧同位素曲线阶段1对比,为一高温期;L1可以划分为两层黄土(L1LL1、L1LL2)和一层弱发育的古土壤(L1SS1),L1LL1和L1LL2可与阶段2和4对比,L1SS1可与阶段3对比;S1古土壤层由3个古土壤亚层(S1SS1、S1SS2、S1SS3)和2个弱成土亚层(S1LL1、S1LL2)叠置而成,磁化率曲线具有明显的三峰夹两谷形式,正好可与氧同位素阶段5对比,表明这一时期的古气候特点是在湿热的背景下有两次干冷波动,其中S1SS1、S1SS2、S1SS3分别对应于5a、5c、5e,S1LL1、S1LL2对应于5b、5d;L2黄土磁化率曲线整体呈谷形,但中下部有一次峰,下部磁化率偏高,与阶段6的曲线特征相吻合;S2古土壤层未见底,目前获得的磁化率曲线特征可与7a和7b对比。以上研究结果确立了江南风尘堆积的古气候全球变化意义。

MAGNETOSTRATIGRAPHICAL DATING OF THE XIASHU LOESS IN NANJING AREA AND ITS PALEOENVIRONMENTAL INTERPRETATION

As one of the best paleoclimatic proxies on the land, Chinese loess holds paleoclimatic information that can be compared with marine oxygen isotope curves. This comparison indicates the global nature of paleoclimatic cycles. Thus the study of Chinese loess plays an important role in global change research. Most geologists believe that Chinese aeolian-dust accumulation extends from N49° to N30°, and its southern boundary lies to the south of the Yangtze River. However, there have been few studies on loess deposits to the south of the Yangtze River——such as the Xiashu loess sequence in Nanjing area, in sharp contrast to the many studies of loess deposits on the Loess Plateau in North China. The goals of this study are to systematically characterize the paleo-geomagnetism, magnetic susceptibility, and grain-size of the Xiashu loess sequence, and to investigate its significance in paleoclimatic reconstruction. On the basis of systematic paleo-geomagnetism, magnetic susceptibility, and grain-size data for two profiles at Taishanxincun and Yanziji, we established a high-resolution time series for the Xiashu loess sequence in the Nanjing area since 220 kaBP. We then compared paleoenvironmental data from Xiashu loess with those from northern Chinese loess and marine oxygen isotope records. We found that the susceptibility curve of Yanziji profile could be compared with the marine oxygen isotope curve. More specifically, S_0 could be compared with MIS (marine isotope stage) 1; L_1 with MIS2-4; and S_1, L_2 and S_2 with MIS5-7. These results support the global impact of paleoclimatic events.