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Variations in Grey Scale of a Stalagmite from Lianhua Cave, Hunan and
Its Paleoclimatic Implications during the Mid-late Holocene
ZHANG Huiling, YIN Jianjun1, LIN Yushi
2018, 38 (6):
810-818.
doi: 10.13284/j.cnki.rddl.003090
Based on 42 230Th ages, the results of X-ray diffraction analysis and the grey values of the polishing profile of an aragonite stalagmite, named LH2, which was collected from Lianhua Cave, this paper established the grey scale time series of aragonite stalagmites during the mid-late Holocene in western Hunan. During the mid-late Holocene, the average grey scale value of stalagmite LH2 was 185.7, the maximum was 218 and the minimum was 116. The average grey scale resolution of stalagmite LH2 was 4a and the resolution of grey scale could reach 2a in some time intervals. In general, the change pattern of grey scale of stalagmite LH2 imitated the change of solar radiation at 30°N in summer during the mid-late Holocene. Since 8.6 ka B.P., the amount of summer solar radiation in north Hemisphere has decreased gradually and the grey scale values of stalagmite LH2 decreased synchronously. During the mid-late Holocene, the grey scale of stalagmite LH2 significantly inversely correlated with its 18O values, but positively correlated with its growth rate. Although the grey scale of stalagmite LH2 followed that of summer insolation, it was punctuated by eight low grey scale events lasting about 1~5 centuries occurred at 8.1, 7.2, 5.7, 5.0, 3.7, 2.8, 2.1 and 1.4 ka B.P.. Some research suggest that the cause of the grey scale change of stalagmites was different in different cave. As for Lianghua Cave, Gray values of stalagmite LH2’s polishing profile might relate to crystalline forms of aragonite which were controlled by rainfall and temperature in western Hunan province. During the mid-late Holocene (8.6-0.03 ka B.P.), the history of climate in western Hunan could be divided into three stages. From 8.6 ka B.P. to 5.7 ka B.P., the overall level of gray value was higher, the oxygen isotope value was more negative and the growth rate of stalagmite was higher. The climate was characterized by high temperature and heavy rain, the summer monsoon was strong. From 5.7 ka B.P. to 2.8 ka B.P., the gray values decreased gradually, the oxygen stable isotope values became heavier and the growth rate of stalagmite reduced progressively, which indicated the summer monsoon weakened continuously and the climate turned from warm-wet to cold-dry gradually. From 2.8 ka B.P. to 0.03 ka B.P., the overall level of gray value was lower and fluctuated violently. Meanwhile, the oxygen stable isotope value was heavier and the growth rate of stalagmite was lower and fluctuated strongly. It indicated the climate was cold and dry and existed warm-wet fluctuations. On orbital timescale, variations of gray values of stalagmite LH2 were controlled by the summer insolation in Northern Hemisphere and the location of ITCZ. On centennial timescale, abrupt change of gray values of stalagmite LH2 might relate to ice raft events occurred in the North Atlantic Ocean. When North Atlantic ice raft events occurred, the East Asian summer monsoon weakened, the climate was cold and dry in the western Hunan, and the grey scale of stalagmite LH2 decreased. On inter-decadal scale, solar activity and PDO might regulate gray values of stalagmite LH2. On inter-annual timescale, fluctuations of gray values might be driven by ENSO.
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