甘肃灵台邵寨红粘土的磁性地层及其色度记录

对位于甘肃灵台的邵寨红粘土剖面进行了系统的磁性地层研究，并对色度的古气候意义进行了探讨。磁性地层研究结果表明，邵寨剖面典型风尘堆积的底界年龄约为5.23Ma B.P.； 色度测试结果显示，红粘土的红度和黄度值具有较好的相关性，是反映东亚夏季风强度变化的理想指标； 邵寨红粘土的亮度值变化在2.6~3.6Ma B.P. 期间与成壤强度的相关性较好，但3.6Ma B.P. 以后，两者相关性较差，可能与该时期形成的大量非成壤碳酸盐有关。进一步的研究并结合与已有研究成果的对比分析揭示出，邵寨红粘土剖面的红度、黄度及磁化率指标完整的记录了上新世4.4Ma B.P. 以来东亚夏季风阶段性增强的演化过程，该过程与青藏高原西北缘的主体隆升及巴拿马海峡关闭在时间上是一致的，这二者可能是该时期东亚夏季风增强的主要驱动因素。

MAGNETOSTRATIGRAPHY AND CHORMA RECORDS OF A RED CLAY FORMATION NEAR LINGTAI COUNTY OF GANSU PROVINCE

The Shaozhai red clay section(34°59'17.5"N, 107°46'26.5"E) is 120.25m thick and located near Lingtai County of Gansu Province. The present-day climate at Lintai is semi-arid with a mean annual rainfall of 500mm and an annual mean temperature of 8.6℃. This section is lithologically composed of three parts:the type loess-paleosol sequence at the upper part(0~13.7m), the red clays interbeded with carbonate layers at the middle part(13.70~95.65m) and the underlying water-reworked loess sediment(95.65~120.25m). In this study, 488 oriented samples at 25cm intervals were taken for Paleomagnetic measurements, 4811 samples at 2.5cm intervals were collected for magnetic susceptibility measurements, and 477 samples at 20cm intervals were selected from the upper and middle parts(0~95.65m) for chroma tests. Magnetostratigraphic results show that the M/G and Ga/Gi magnetic reversals are found at the depths of 4m and 35.5m, respectively. Extrapolation with accumulation rates suggest that the top and basal ages for the type eolian deposits in this section are about 2.48Ma B.P.and 5.23Ma B.P., respectively. The color of soil is mainly affected by hematite and goethite. These iron oxides mainly come from pedogenesis, the hematite appears red and the goethite is yellow-brown Therefore, the higher redness and yellowness values of red clay indicate stronger weathering intensity. The chroma results of the Shaozhai red clay section show that the values of redness and yellowness have a good correlation and also coincide with the pedogenic intensity, indicating that they are ideal proxies for reflecting the changes of East Asia summer monsoon intensity. The Shaozhai red clay formation can be divided into three parts on the basis of redness, yellowness and susceptibility. The lower part(5.23Ma to 4.80Ma)have the higher redness, yellowness and susceptibility values, which are about 8.77, 23.88 and 67.75×10-8m3/kg on average, respectively. For the middle part(4.8~4.4Ma), the average values of redness, yellowness and susceptibility are about 7.92, 21.94 and 65.8×10-8m3/kg respectively, which are relatively lower compared with those of the lower part. From 4.4Ma to 2.6Ma, the redness, yellowness and susceptibility gradually increase, ranging from 6.14, 20.79 and 30×10-8m3/kg to 13.12, 29.26 and 243×10-8m3/kg and with average values of 9.64, 25.11 and 102.01×10-8m3/kg, respectively. These results imply that the East Asia summer monsoon strengthened stepwise since 4.4Ma in the Pliocene, which may be related to uplift of the Tibetan Plateau and closure of the Panama Strait.