南京葫芦洞石笋生长速率及其气候意义讨论

一支连续发育4?000年纹层的末次盛冰期葫芦洞石笋的年际生长速率与高分辨率δ¹⁸O曲线对比分析表明,年际生长速率在不同时间尺度上对年均温和地表有效湿度的变化有着复杂的响应关系。对该洞内持续发育3~4万年的两支单体石笋实测了33个²³⁰Th年龄,其生长曲线显示MIS 2比MIS 3阶段平均生长速率增大了40%以上。以5000年为步长的同一洞穴17支石笋生长频率支持平均生长速率的研究结果,说明在轨道尺度上较大的石笋生长速率并不完全指示较暖湿的气候条件。作者认为,年际尺度的连续生长速率是决定石笋平均生长速率的关键因素。盛冰期条件下本区洞穴内外的温差效应导致了24~14ka B.P.年际尺度的石笋连续生长。

Approach to the Growth Rate and the Climatic Significance of Stalagmites in Hulu Cave,Nanjing

The timing of growth phase in 17 stalagmites from Hulu Cave, Nanjing, has been precisely dated by thermal ionization mass spectrometric ²³⁸U-²³⁴U-²³⁰Th dating with 69 samples. Based on the comparison between the high-resolution oxygen isotopic records and the thickness of annual layers, we obtained some interesting relationship between the two records. The stalagmite YT which continuously growth for 4000 years during the last glaciation shows the closely and complex relationship between the thickness of annual layers in different time scales and the temperature as well as the effective humidity of the earth's surface above the cave. The growth rate of stalagmite MSD shows the negative variability with the δ¹⁸O. That is, the variability of the growth rate abruptly changed in about 28 ka B.P. During 28-18 ka B.P., the average growth rate was 14.27 mm/ka, while it was 9.73 mm/ka during 53-28 ka B.P. So the growth rate of stalagmite MSD was higher during the dry cold marine isotopic stage 2 (MIS2) than during the wet warm marine isotopic stage 3 (MIS3). However, the other stalagmite named MSL during 72-31 ka B.P., which overlapped the marine isotopic stage 3 (MIS3) and stage 4 (MIS4), shows the reversed results. The average growth rate reached to 11.14 mm/ka in the wet warmer period MIS3, while 5.63 mm/ka during the dry colder MIS4. Although the MSD and MSL showed the reversed growth rate feature during the warmer and colder period, but the absolute average thickness of annual layers is comparatively.In progressively, we analyse growth frequency of 17 stalagmites with 5000 year-step in the same cave. The results show the highest accumulative growth frequency during the 25-5 ka B.P., being 52% of the whole growth quantity and the lower accumulative growth frequency during the period 60-30 ka B.P., the wet warmer MIS3, which takes up only 30% of the whole growth quantity. Generally, the growth rate and quantity of the Cave stalagmite could reflect the variability of the outer cave climate. In Hulu Cave, Nanjing, the continuously growth of the annual layer controlled the average growth rate. Difference temperature effects of outer and inner cave results in the continuous growth of stalagmite during 24-14 ka B.P.. So the higher average growth rate could not entirely represent the wet warmer climate.