| 摘要: |
| 基于河南省禹州市马沟洞2支石笋（MG-1与MG-40）24个230Th年龄和1988个氧同位素数据，建立了研究区13.1 — 4.9 ka BP分辨率为2 — 14 a的石笋氧同位素时间序列。马沟洞石笋δ18O的时间序列揭示季风降水在11.2 — 9.1 ka BP时段在波动中逐渐增加，9.1 — 4.9 ka BP季风降水显著波动但无明显长期趋势变化。YD事件、9.3 ka事件、8.2 ka事件记录与其他石笋δ18O记录的一致性揭示末次冰消期—早全新世百年—千年尺度气候突变事件的大范围存在和共同的驱动因子。马沟洞全新世大暖期的δ18O记录中检测出的13个数十年尺度的弱季风事件进一步证实全新世大暖期气候的不稳定性。与大气Δ14C记录、NGRIP冰芯δ18O记录的对比及周期分析揭示，太阳活动引起的太阳辐射变化和北半球高纬气候状况共同影响着亚洲季风的变化，ENSO活动及气候系统内部的相互作用也对东亚夏季风降水产生重要的影响。 |
| 关键词: 石笋 δ18O 气候变化 夏季风降水 全新世 马沟洞 |
| DOI：10.7515/JEE201603004 |
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| 基金项目:中国科学院重点部署项目（KZZD-EW-04）；国家自然科学基金项目（41271229） |
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| Early to mid-Holocene paleoclimatic changes recorded by the stalagmites from the Magou Cave, Henan Province |
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MAO Ruixue, CAI Yanjun, MA Le, CHENG Xing1,2,3
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710054, China
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| Abstract: |
| Background, aim, and scope Henan Province, located in central China, is dominated by monsoonal climate, and has been regarded as the core region of Chinese ancient civilization. Stalagmite has advantages of absolute and high-precision chronology and abundant proxy indexes, and has been widely used to decipher the changes of Asian summer in the past. However, there are few stalagmite records obtained from this region till now and all of them are located in the hilly area of west Henan Province. Here, we present a high-resolution and precisely dated stalagmite δ18O records attained from two stalagmites (MG-1 and MG-40) from Magou Cave, northern Henan Province, Central China. We reconstruct the history of monsoon precipitation during early to mid Holocene, characterize the climate instability of the Holocene Megathermal and investigate the possible driving forces and dynamics of the Asian Summer Monsoon (ASM) in the study region. We also examine the possible links existed between civilization succession and climate change. Materials and methods Two calcite stalagmites, MG-1 and MG-40, have been analyzed in this study. Stalagmite MG-1 is ~26 cm in height and ~9 cm in diameter, and MG-40 is ~4 cm in height and ~8 cm in diameter. We halved these two stalagmites using diamond wire saw. Twenty-four subsamples were drilled parallel to the growth plane using a hand-held carbide dental drill, and were dated with U-series methods on a plasma sourced multi-collector mass spectrometry (MC-ICP-MS) at the Institute of Global Environmental Change, Xi’an Jiaotong University. The chemical procedure is similar to that described in Edwards et al (1987), and the details on instrumental approaches were explained in Cheng et al (2013). Here we use a linear interpolation to establish the chronology, and extrapolate the ages for the portions outside the U-series dates using the growth rate of adjacent segments. Subsamples for stable isotope analysis were collected with a high resolution computer-controlled micro-milling equipment (Micro-mill). A total of 1988 oxygen and carbon isotopic values were obtained on an IsoPrime100 mass spectrometer equipped with the Dual-inlet and connected to the MultiPrep systems in the Institute of Earth Environment, Chinese Academy of Sciences. International standard NBS-19 and inter-laboratory standard TTB1 were run every 10 to 15 samples and arbitrary selected duplicates were run every 10 to 20 samples to check for homogeneity and reproducibility. All oxygen isotopic values are reported in δ notation, the per mil deviation relative to the Vienna Pee Dee Belemnite (VPDB) standard. The standard results show that the precision of δ18O analysis is better than 0.08‰ (2σ). Results All 230Th dates are all in stratigraphic order and it appears that MG-1 and MG-40 grew continuously throughout this period. The calculated growth rate of stalagmites MG-1 and MG-40 varied from 4.7 μm ∙ a−1 to 58.6 μm ∙ a−1 and from 1.5 μm ∙ a−1 to 6.3 μm ∙ a−1 respectively. The growth rates of both stalagmites are similar in general, they are relatively low during the early Holocene, then increasing and holding steady. The 1988 isotopic data obtained generate a 2 — 14 years resolution to the oxygen isotope profile during the 13.1 — 4.9 ka BP. A careful examination of the time-series of speleothem δ18O ratios finds that the δ18O of stalagmite MG-1 fluctuated around −10.5‰ (mean value) from 9.1 to 4.9 ka BP, varying from −12.7‰ to −9.2‰. The δ18O of stalagmite MG-40 fluctuated around −10.4‰ (mean value) from 11.1 to
7.1 ka BP, varying from −11.9‰ to −8.4‰. The Magou δ18O profile can be divided into three periods:
(1) 13.1 — 11.2 ka BP, which possible correspond to Allerød Warming and Younger Dryas; (2) a gradual increasing precipitation interval between 11.2 — 9.1 ka BP; and (3) a relatively stable interval between 9.1 —
4.9 ka BP. The δ18O value fluctuated around −10.51‰ (mean value) from 9.1 to 4.9 ka BP, corresponding to the Holocene Megathermal. Discussion Based on instrumental observations (winter precipitation accounts for only ~37.7% of the amount of annual precipitation in this region) and previous stalagmite studies in this region and southern China, we interpret the stalagmite δ18O from Magou Cave as a qualitative proxy of ASM precipitation, with heavier values indicating a weak ASM and less summer monsoon precipitation, and vice versa. The Younger Dryas, 9.3 ka and 8.2 ka events can be identified in the Magou δ18O profile and are in good agreement with other speleothem records from Asian monsoon regions. Thirteen significant weak monsoon events on centennial to decadal timescales were singled out within the period of 8.0 —
4.9 ka BP. The comparisons between MG-1 δ18O record and atmospheric Δ14C record and the NGRIP δ18O record (during the period of 9.1 — 4.9 ka), and the spectrum analysis reveal largely consistent changes among these records. Conclusions The occurrence of Younger Dryas, 9.3 ka and 8.2 ka events in the Magou δ18O profile are in good agreement with other speleothem records from Asian monsoon regions, suggesting that these abrupt events have affected the vast monsoon regions at least in East Asian and South Asia. Thirteen significant centennial-decadal weak monsoon events identified within the period of 8.0 — 4.9 ka BP, confirm the instability of monsoon climate during the Holocene Megathermal. The concordance of MG-1 δ18O record and atmospheric Δ14C record and the NGRIP δ18O record (during the period of 9.1 — 4.9 ka), as well as the spectrum analysis, reveal that not only did both changes in solar radiation and climatic changes at high northern latitude affect the Asian summer monsoon variations during the Holocene, bout also ENSO and the internal interaction of climate sub-systems have significant impacts on East Asian summer monsoon changes. We note that the flourishing of the Yangshao Culture at the central China during the 7200 — 6000 a BP was coincident with the relatively stable warm-humid climate within the Holocene Megathermal. It is then not implausible that the climate optimum plays an important role in the development of earlier civilizations (e.g., Yangshao Culture). Recommendations and perspectives This paper provides a new high resolution stalagmite record for palaeoenvironment and environmental archaeological research of Henan Province from early to mid-Holocene. |
| Key words: stalagmite δ18O climate change summer monsoon precipitation Holocene Magou Cave |
