| The Earth experienced a number of glacial-interglacial cycles over the long geological period. A glacial period is an interval of time (thousands of years) withi an ice age that is marked by colder temperatures and glacier advances. Interglacials on the other hand, are periods of warmer climate between glacial periods. The quaternary period, beginning260Ma years ago, witnessed a total of more than fifty such cycles, of which the last two have been investigated intensively at present. The intensive research includes pattern, structure, timing and frequency of millennial-scale events during the last glacial period, sequence of events and associated forcing mechanisms over the deglacial interval, and muti-decadal to centennial oscillations in the warm period, such as interglacials. In order to shed light on the effect of external forcings (e.g. solar insolation), boundary conditions and sea-air coupling on multi-timescale abrupt climate changes, it is vital to provide further insight into the main characteristics of climatic events occurred over older glacial ice ages. Besides, it is of great importance for understanding the mechanisms behind these sharp events to know behaviors of high-resolution monsoonal events particularly during the deglacial, because of tight couplings between Asian monsoon and bipolar climates. During warm interglacials, elucidating the causes for monsoon event variability, for example, in the late Holocene is potential to distinguish the nature forcing from the anthropogenic forcing. Here we address these issues based on isotope and trace element records from Yongxing and Sanbao caves, central China and a new oxygen isotope record from Dongge Cave, southwest China.Stalagmite yx20, from Yongxing Cave and with a length of3.66m, provides a total of3662isotope measurements, with δ18O values ranging from-11.2919‰to-6.649%o. The large amplitude of4.6%o, similar to a glacial to interglacial one estimated from previous speleothem records, reveals that the calcite record is sensitive to the Asian monsoon circulation. A challenge is that the dating uncertainties for the sample are so large that we do not reconstruct a reliable timescale if we do not refer to other generally accepted records. Due to the regional coherency in Chinese stalagmite δ18O signals, it is feasible to tune the yx20δ18O record to intensively and precisely dated Sanbao Cave records. The alignment, based on the mid-point of abrupt δ18O shifts, is further supported by a Greenland temperature reconstruction (Barker et al.,2011). The slightly adjusted age model shows that yx20grew from385ka to365ka, covering a whole insolation cycle. Superimposed on the long-term trend are six significant weak monsoon events, lasting0.4-1.5ka. These oscillations initiated and terminated rapidly, typically within several decades, akin to the DO behaviors during the last glacial period. The similarity between the two cold ages is further exemplified by secondary oscillations, termed precursory events, within the weak monsoon periods. It is plausible to relate the precursory events to abrupt climate change. Nevertheless, the resemblance implies that northern ice sheet might have behaved in the same fashion. By comparing the yx20record to the previously-reported Hulu and Sanbao records covering the last four glacial-interglacial cycles at precessional timescales, we find that only under the condition of the same ice volume is the number of weak monsoon events identical. This finding further extends the theory about the relation between ice volume and Asian monsoon.The trace element information from stalagmite sb61(Cheng et al.,2009) provides an alternative perspective on the glacial termination. All of the multiple proxies (Sr, Sr/Ca and Ba/Ca) show that Termination Ⅲ and Ⅳ (T-Ⅲ and T-Ⅳ) occurred at252.5ka and355ka respectively, preceding the weak monsoon intervals (WMI) inferred from the δ18O record by7.5ka and15ka. The observation is in contrast to a classic Northern Hemisphere summer insolation intensity trigger for an initial retreat of northern ice sheets, a traditional viewpoint supported by the calcite δ18O record (Cheng et al.,2009). However, my result is consistent with the finding that the Indian monsoon begins to increase before global ice volume reaches a maximum (An et al.,2011). In comparison with Antarctic records, including δD, CH4and CO2, we find that the onsets of T-Ⅲ and T-Ⅳ are concurrent with early CO2increasing relative to the other two records. This finding implies that CO2plays a key role in modulating low-latitude moisture balance and has a potential to push the Earth to another mode.Over the Holocene, the forcing factors may be different. A new stalagmite record from Dongge Cave, southwest China provides more robust evidence for solar forcing on Asian monsoon variations. Without tuning to the solar signal, our data show a visual peak-to-peak correlation with cosmogenic nuclide14C, total solar irradiance and sunspot number at multi-decadal to centennial scales. Cross-wavelet analyses for the calcite δ18O and atmospheric14C records show a strong coherence at three periodicities of～80,200and340yr, suggesting an important role of these solar cycles in monsoon changes. Along with the observation of the better resemblance of our record to atmospheric14C than total solar irradiance, we propose that a solar forcing of centennial-scale monsoon variations during the late Holocene may involve influences of cosmic rays (possibly by cloudiness) and ocean circulations.In brief, the context has three clear paleoclimatic conclusions or implications. First, the transitional characteristics and interval oscillations for the millennial-scale monsoon events during the older glacial period, reveals that boundary conditions, such as a combination of insolation and global volume, controlled their character and pacing. Second, the strong correlation between CO2and trace element records highlights the predominant role of CO2in producing monsoonal terminations. Third, the strong evidence for solar forcing contributes to the model studying and climate projecting. |
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