过去64万年以来湖北三宝洞石笋生长速率变化及其古气候意义

Variation of stalagmite growth rate and its paleoclimatic significance in Sanbao cave,Hubei Province over the past 640,000 years

利用石笋平均生长速率变化来重建古气候环境变迁已成为洞穴古气候研究的重要领域。文章以湖北三宝洞横跨22~64万年的5支石笋167个230Th年龄数据为材料,结合过去的工作,重建了晚更新世64万年以来长江中下游地区东亚夏季风降水变化过程。结果表明:深海氧同位素(MIS)1,5.3,5.5,7.3,7.5,9,15.1,15.5阶段石笋生长速率显著增大,指示东亚夏季风强度增强,降水增多;相反,冰期阶段生长速率缓慢或者不发育,指示夏季风强度减弱,降水明显减少,但平均生长速率指标并不能定量指示夏季风强度变化。当石笋生长速率低于10μm·a^(−1)时,不能有效地指示冰期—间冰期旋回变化。在轨道尺度上,平均生长速率所揭示的冰期—间冰期波动可能归因于全球冰量和太阳辐射共同作用的结果。

In the past few decades,great progress has been made in the study of stalagmite paleoclimate.Compared with the geochemical indicators such as stalagmiteδ18O,the average growth rate of stalagmite is a physical indicator,which can directly reflect the wet and dry changes of the external climate,and is one of the important means to reconstruct the past regional hydrological changes.Sanbao cave(31°40′N,110°27′E),the study area,is located in Shennongjia National Nature Reserve,Hubei Province,on the north bank of the Three Gorges of the Yangtze River and adjacent to the southern edge of the Loess Plateau.Mainly affected by the East Asian summer monsoon,the average annual temperature in this area is 8-9℃with the annual precipitation of 2,000 mm.In summer,warm and humid air from the equatorial ocean enters the hinterland of the mainland,bringing about 60%of the annual precipitation.In winter,the dry and cold air from Siberia controls the climate in Shennongjia area,so it is particularly sensitive to the seasonal advance and retreat of East Asian summer monsoon.Sanbao cave,1,900 m above sea level,is located on the northern slope of Shennongjia Mountain in Hubei Province,covered with limestone layer about 300 m thick.The thickness of the surface layer is about 2-3 m,mainly distributed with trees,shrubs and weeds.The research objects are five stalagmites(numbered SB12,SB14,SB32,SB58 and SB61)from Sanbao Cave,with a cumulative height of about 2.991 m,of which SB61 was deposited in 229.4-384 ka B.P.,corresponding to MIS8-MIS10;SB14 grew and developed from 299.6-622.8 ka B.P.,spanning from MIS9 to MIS15;SB12 and SB58 were respectively deposited at 425.1-462.7 ka B.P.and 426.7-464.7 ka B.P.,corresponding to MIS12;SB32 grew from 514.3-638.2 ka B.P.,and spanned from MIS13 to MIS15.167230Th age data of these five stalagmites was obtained by the analytical instruments of Thermo-Finnigan Element and Thermo-Finnigan Neptune in Minnesota Isotope Laboratory,the USA.The age analysis error was±2σmeasurement statistical error with the error accuracy of 0.5-2.0 ka.For the calculation of average growth rates of stalagmites,firstly,the data of stalagmite age reversal in stratigraphy has been excluded.Then,the curve of the measured age point of stalagmite and its corresponding growth depth has been established.The new results are basically consistent with the growth rate curve in the original literature.Finally,the average growth rates of stalagmites have been obtained by dividing the depth difference between two adjacent points by the measured age difference.Based on 167230Th age data from 5 stalagmites spanning 220,000 to 640,000 years in Sanbao cave,and combined with the past work,this paper has reconstructed the precipitation change process of East Asian summer monsoon(EASM)in the middle and lower reaches of the Yangtze River over the past 640,000 years in the late Pleistocene.The results show that the growth rates of stalagmites in Marine Isotope Stages(MIS)1,5.3,5.5,7.3,7.5,9,15.1,15.5 increased significantly,indicating that the intensity of EASM increased significantly with more precipitation in the interglacial stage.On the contrary,the slow or undeveloped growth rates in the glacial stage indicate the weakening of the summer monsoon intensity with less precipitation.Based on statistical analysis of the growth rates of 22 stalagmites,we hold that the average growth rate index cannot quantitatively indicate the change of the monsoon intensity.Moreover,when the stalagmite growth rate is less than 10μm·a−1,it also cannot effectively indicate the glacial-interglacial change.On the orbital scale,the glacial-interglacial fluctuation revealed by the average growth rate may be attributed to the joint action of global ice and solar radiation.Finally,it should be noted that in different time scales,the controlling factors of stalagmite growth rate are complicated,and the noise generated by the environment difference inside the cave may cover up or weaken the transmission of the climate signal from the outside to the inside.Therefore,the possible influencing factors must be carefully considered when we reconstruct the paleoenvironment by using the average growth rates of stalagmites.