博文
南京石笋记录的末次冰期气候事件与全球对比
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汪永进
论文题目：南京石笋记录的末次冰期气候事件与全球对比
作者简介：汪永进，男，1959年04月出生，1995年09月师从于南京师范大学刘泽纯教授，于2000年06月获博士学位。
摘 要
格陵兰冰芯和北大西洋深海沉积记录揭示了晚更新世以来气候变化的高频振荡特征，这种气候不稳定性进一步得到了高北纬以外地区若干地质记录的证实。然而，要深入理解这些高频气候振荡旋回特征（D/O）和突变事件（Heinrich事件）的形成机制，还有赖于在全球范围内寻找更多的高分辨率气候记录，由此分析各气候子系统对全球事件的正负反馈机制。本文基于南京汤山洞穴石笋24个TIMS测年数据、1224个氧、碳同位素分析数据和显微岩相学研究结果，首次建立了末次冰期中国东部地区最高时间分辨率的气候演化序列，探讨了东亚季风气候事件与全球气候变化的耦合机制。
50万年以来的南京汤山洞穴气候演变过程，为末次冰期亚轨道尺度气候振荡旋回及突变事件的研究提供了洞穴长期气候演化的背景材料。据洞内一支石笋的5个U系年龄和81个氧碳同位素数据，建立了氧同位素期6－13（MIS6－13）的气候变化曲线。与深海DSDP659孔d18O记录对比发现，两者间具有极其相似的变化规律，说明南京石笋氧同位素记录的古气候具有全球性演化特征。此外，石笋氧同位素曲线也反映了东亚季风区气候变化的区域性特征：463－159kaBP期间，石笋d18O曲线暖峰值逐步变小，说明各间冰期洞穴古温度有逐渐下降的趋势，与中国黄土高原堆积序列反映的东亚季风环流强度变化趋势基本一致。
南京洞穴石笋d18O变化信息十分突出地反映了东亚大陆冬、夏季风环流的综合气候效应，增强的夏季风降水和温度效应导致石笋d18O值负向漂移（富16O）。分辨率达100年的末次冰期石笋d18O变化曲线与格陵兰冰芯d18O古气温曲线有着十分良好的对比关系。指示季风气候降温的石笋d18O正向漂移（富18O）事件对应于高北纬气候记录的冰漂碎屑事件（Heinrich事件）和新仙女木事件（YD事件），反之，石笋δ18O值显著负偏响应于格陵兰冰芯记录的百年－千年尺度D/O事件（IS1-20），而且两者之间在曲线形态上具有一一对应关系。石笋冷暖阶的d18O变化幅度（2‰－3‰）基本达到上述50万年来冰期与间冰期的变化幅度。石笋d18O曲线中较长周期（千年至万年）气候变化旋回也与高北纬冰芯和深海记录的Bond旋回具有一致性。在总体变化趋势方面，石笋d18O曲线相似于65ºN夏季太阳辐照曲线和南极冰芯气候记录。南、北极气候变化在南京石笋气候记录中的双重印迹，反映了东亚季风气候的不稳定性和快速变化可能是多种驱动机制共同作用的结果。
石笋高精度热电离质谱（TIMS）测年技术可以直接测定14C测年范围之外的气候事件年龄。4支具有重叠时段的石笋测年结果表明，YD、H1－H6七个降温事件的谷底年龄分别为12100±200、16200±100、24230±100、30540±300、38850±900、48040±470、67080±2500yrBP（日历年），与格陵兰冰芯GISP2时标作对比，5个年轻事件（YD-H4）的年龄基本一致，而H5和H6事件年龄，石笋时标比GISP2时标偏老2000年左右。由于冰芯依据纹层计数和冰流模式建立时标，具有较大的定年不确定性，相距仅30公里的GRIP和GISP2冰芯在标定H1－H5事件年龄上具有较大差异。作者认为，GISP2冰芯年龄应在45－75KaBP之间系统纠偏2000年左右，以便于全球范围内气候事件的对比研究。在精确石笋时间标尺基础上，作者进一步研究了气候事件的发生频率和持续时间，发现Heinrich 事件和D/O事件的发生频率具有一致的变化规律：即末次冰期晚期（40－10KaBP）比早期（75－40KaBP）气候事件发生频率明显加快，其1000－2000年周期接近于北大西洋沉积记录和GISP冰芯记录的1470±500年周期。石笋气候记录的另一重要特征是各间冰段持续时间具有依次缩短现象。如对于在欧洲发现的若干间冰段Orerel（IS15、16、17），Glinde（IS13、14），Hengelo（IS12）和DeneKamp（IS8），其持续时间分别为6，4，3和2Ka，这与极地冰芯记录有显著差异。
分辨率达10年的石笋氧同位素曲线，反映了全球气候事件发生时东亚季风气候的转型特征和内部结构差异。H1事件具有快速降温和阶梯式缓慢升温的转型特征，这种异常结构的降温事件既不同于其它H事件的内部结构，又不同于更长周期的Bond 旋回，这一特殊类型的变冷事件进一步得到了具有年际分辨率的石笋年层厚度变化曲线的支持。根据石笋年层厚度变化曲线，识别出末次盛冰期（18.2－14.4KaBP）6个旋回的异常结构降温事件，每一旋回（本文称MG旋回）持续500－800年不等。指示了北半球冰流开始后撤背景下，短时间尺度东亚季风环流形式对全球气候事件的特殊响应方式，有可能说明H型气候事件存在着不同的形成机制。石笋高分辨率气候记录同时提供了高纬与低纬高频气候－火山事件遥相关的强有力证据。根据印尼TOBA火山喷发年龄、火山事件地层对比资料，作者认为TOBA火山诱发了72KaBP左右的D/O旋回，其冷暖变幅可达10℃以上，这一事件对中国东部地区和格陵兰极地气候都产生了深刻的影响，进一步证明D/O事件的多成因特征及其大气传输机制。对Bond 旋回的详细研究发现，在H和D/O事件背景上叠加有一系列数十年－百年尺度的气候变化旋回，表明D/O事件内部的不稳定性特征，其d18O变幅一般在0.5‰－1‰范围内，也许太阳黑子数变化周期对这类不稳定性气候变化的发生有重要贡献。
通过对洞穴石笋碳同位素的研究，作者指出：洞穴石笋d13C的变化不能完全反映地表植被类型的更替，地表年平均温度、地表雨水的PH值以及洞穴围岩的溶蚀组分都对d13C变化有所贡献：在50万年以来的长期的气候演化序列中，d13C值在MIS6/7界面附近的显著正向漂移可能指示了洞穴上覆土壤侵蚀事件；末次冰期石笋d18O与d13C在千年尺度上呈负相关，即当全球气温下降时，C3型植物丰度相对增加；而当全球气温增高时，C4型植物相对丰度增加，d13C这种变化规律与目前普遍认可的东亚季风气候特征相矛盾，或许，在末次冰期时，东亚季风气候在响应全球气候事件过程中呈现一种独特的方式，当发生Heinrich事件时，东亚季风气候降水相对充沛，当D/O暖事件发生时，大气降水相对减少。
对于石笋年层厚度气候意义，作者也提出了新的认识：石笋年层厚度变化响应于百年尺度的洞穴古气温变化趋势，而与降水关系较为复杂。
最后，根据钙板、石笋的测年数据以及地层层序分析，作者重新认识了南京直立人的生存时代问题。研究表明I号头骨与II号头骨分布于不同层位，I号头骨的年龄至少老于500KaBP，而II号头骨的形成时代目前尚无法确切判别，但其年龄范围应该在529～239Ka之间。
关键词： 末次冰期， 气候事件， 全球变化， 洞穴石笋，南京。
Abstract
High-resolution paleoclimatic records of the Greenland ice cores and North Atlantic deepsea sediments revealed the general high-frequency instability of Late Pleistocene climate. Recently, increasing geological evidence from several areas outside the northern Atlantic have shown that the millennial scale warmings and coolings have global signatures. Understanding forcing mechanism of these rapid climatic oscillations (D-O cycle and Heinrich events) and processes of positive and negative feedback to Earth’s climate system still requires globally extended investigations of these events in various high-resolution climate records. Here the author firstly present the highest resolution climate record of the last glaciation in East China, based on 24 TIMS dating ages and 1224 data of stable isotopic compositions as well as lithological microsequences of stalagmites from a cave at Nanjing city, to investigate a general relationship between high frequency variability of East Asian monsoon and the global events.
The climatic changes from Nanjing cave since 500KaBP provide the information of the long-term climatic changes for the study of the climatic oscillations and rapid events on sub-orbital scales during the last glaciation. A curve of climatic changes during the oxygen isotopic period (MIS) 6-13 is established on the basis of 5 U-series ages and 81 oxygen and carbon isotopic data of a stalagmite. Compared with the d18O record in deepsea DSDP659 core, the oxygen isotope curve of the stalagmite has a very similar pattern to that of DSDP659 core, suggesting that the monsoonal paleoclimate changes recorded by Nanjing stalagmite have global signature. On the other hand, the oxygen isotopic curve also shows the regional climatic changes. Successively increase in value of stalagmite d18O at interglacial periods from 463ka to 159ka indicates a general decrease tendence in air temperature, which is consistent with the East Asian monsoon records from loess deposits on the Loess Plateau, central China.
Oxygen-18 signal of the cave speleothems well reflects East Asian climate changes characerized by seasonal alternations of summer and winter monsoons. Strengthened effects of precipitation and temperature induced by monsoon wind circulations would cause cave calcite to become 18O-depleted isotopic composition. A high-resolution (equivalent to a resolution of 100 years on average) d18O record of the stalagmtes developed in the last glacial period from Nanjing Cave bears a high similarity to the d18O-paleotemperature record of the Greenland ice cores. Generally, positive shifts of the stalagmite d18O value responding to decreased air temperature of monsoonal climate are in phase with the Heinrich and YD events in the high northern latitude climate records. Whereas strong summer monsoon-induced highly negative values of d18O(18O-depleted) in the stalagmites correspond to the millennial-century scale D/O events(interstadial stage 1~20) of Greenland ice core d18O records. Similar to each of these warming peaks(from IS1 to IS20) in the Greenland ice core record, all equivalent peaks in our record are indentifiable. It is also prominent that each of longer-term cooling cycles in our record agrees well with the so-called Bond cycles recorded in North Atlantic deep sea sediments and the Greenland ice cores. Meanwhile a general pattern of the stalagmite d18O curve follows the track of July insolation at 65º N and the climatic records of Antarctic ice cores. The stalagmite-based climatic record imprinted both from the North and South polar climate changes suggests that instability and rapid changes in East Asian monsoon climate would be an effect of multi-driven mechanisms.
Perhaps one of fruitful researches in this paper is to determine ages of these events with the high-precise thermal ionization mass spectrometer techniques,especially considering that some older events beyond the 14C dating limit have not been directly so far. TIMS dating results of four overlapped stalagmites demonstrate that stronger cooling of the monsoon events corresponing to YD and Heinrich H1-H6 occurred at the age of 12100±200, 16200±100, 24230±100, 30540±300, 38850±900, 48040±470, 67080±2500 yrBP, respectively. Compared with GISP2 ice core chronology-determined ages of Heinrich events, five events of YD-H4 are essencially same while the other two, H5 and H6, are about 2000 years older than their equivalent events expressed in GISP2. Considering that GISP2 ice core were dated with annual lay-counting and an ice-sheet flow model and with large uncertainty in the range beyond 14C dating limit , the author suggest that the time-scale of GISP2 should be corrected by +2000 years in the time interval of 45000-75000yrBP. If it would be the case, synchronous climatic events in global scale could be well correlated.Using the well-dated stalagmite time-scale, we could inspect frequency and duration of these events. Like the pacing of Heinrich events(H1-H6), stadial pulses in our time-scale increase in general with decreasing age. In the early period(from 75 to 40kaBP) of the last glaciation, the stadial events happended at a slow pacing. Compared with the earlier events, late stadial events(40-20kaBP) went on a significantly high frequency of 1000-2000 yaers, close to a cyclicity of 1470±500 years in North Atlantic deepsea and GISP2 climate records. Another intriguing feature of climate events is successively shortened duration of some broader interstadial events with decreasing age. From the older to the younger interstadial events discovered in Europe, Oerel(a bundle of IS15, IS16, IS17), Glinde(a bundle of IS13, IS14), Hengelo(IS12) and Denekamp(IS8) lasted about 6, 4,3 and 2 thousand years, respectively. That record in the stalagmite is quite different from those of the Greenland ice core.
Patterns and internal structures of the monsoon events linked to global changes have been further investigated with higher resolution (10 years a sampel) d18O curves of the stalagmite. The author have observed that H1 event in stalagmite records displays a pattern with abruptly cooling event is quite different not only from a general climate shift pattern of other H-style events but also from the longer term Bond's cycles. Like the d18O singal, paleoclimatic indicator inferred thichness changes of annual microbanding of stalagmites shows the same pattern and internal structure for H1 event. Based on a curve of annual thickness vs. age of stalagmite, the author has detected 6 cycles of the unique cooling events including H1 during the Last Glacial Maximum (from 18.2 to 14.4kaBP). Each of these cycles (so-called MG cycles in this paper) lasted 500~800 years or so. These particular cooling cycles, the author considered, would be short-term scale circulation effects of East Asian mosoon winds responding to retreat. This suggests that various H-style events may be arose from diferrent driven mechanism. Most surprisingly, the GISP2 and the stalagmite records all show an extremely cooling valley between IS 19 and IS20 peaks, for which lacking an equivalent Heinrich in North Atalantic marine sendimens. This extremely cooling event recorded in stalagmite d18Ocurve provides a strong evidence of linkage between high latitude climte and low latitude volcanic eruption. Based on dated ages of the Toba volcanic eruption in Indonesia and correlation of the volcanic event stratigraphy, the author believed that the Toba eruption triggered Dansaard-Oeschger cycle at 72KaBP. This event yielded the strongest influence on the climates in Greenland and East Asian regions, causing over 10 air temperature decreased,suggesting that D/Ocycles should have different driven forces and that atmospheric circulations might be responsible for passing signal of cooling events. Our further investigation on a longer cooling cycle, the so-called Bond cycle, demonstrates that superimposed on the D/O cycles were a series of decadal-century climate oscillations with amplitude of 0.5-1‰ in d18O, suggesting a general instability in the internal D/O events. Perhaps Sun-spot activities should be important contributions to formation of the instability in climate.
The study of carbon isotope shows that changes of d13C can not be fully explained with flora assemblages having different types of photosynthetic pathway, the annually meam surface temperature, PH value of precipitation and isotopic composition from parent limestones all play important roles in carbon isotope changes of cave calcite. It is evident that the soil erosion above the cave at the boundary of MIS6/7 influence the long-term carbon isotopic record over the past 500 Ka; Our records also show that the d13C and d18O of the stalagmite bears a similar pattern to each other in millennial scale, suggesting both of the indicators are the synchronous response to the climate changes. Generally speaking, d13C reflects the changes of the amount of C3 and C4 vegetation above the cave. Negative d13C values indicate a relative increase of the C3 vegetation, which reflect the humid environment. The d13C of the stalagmite displays a negative correlation to variations of d18O in millennial scale. This implies that C3 vegetation increase when the temperatures decrease, and vise versa. In conclusion, our results exhibit a climate pattern with cold- humid and warm- dry conditions. This climatic pattern is quite different from the present climate conditions of the East Asian monsoon. It is now difficult to explain this particular climate condition with some current climatic models. Perhaps a large amount of precipitation did occur just during Heinrich events on the conditions of particular monsoonal wind circulation.
The author puts forward new explannations for paleoclimatic significance of annual microbanding of cave stalagmite. Compared with oxygen isotopic curve, variations of thickness of annual microbanding are sensitively response to century-scale cave temperature changes. However, changes in annual thickness have complex relationship with the amount of precipitation.
U-series ages of a set of flowstones from the Hulu cave range from older than 500 Ka BP to around 14 Ka BP, providing basic data to establish a geochronologic sequence for the cave sediments. Together with the buried layers of two hominid craniums, the authors argue that the two fossil craniums, probably transported into the cave, deposited in different geological periods. The ages of one flowstone sample and stalagmites show that the geochronology of fossil cranium I is at lest more than 500 Ka BP. On the basis of the stratigraphic sequence, Fossil cranium II is younger than fossil cranium I. In spite of being much uncertain of age during which cranium II deposited, its age range should be located to the period from 529 to 239Ka BP.
Keywords: Last glaciation, Climatic event, Global chang, Cave stalagmite, Nanjing
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