Decadal-to Centennial-scale Asian Monsoon Variability Since The Last Glacial Maximum And Forcing Mechanisms

Chinese stalagmite δ18O records, spanning the past 500 ka, shed light on the detailed evolution history of the paleo-Asian monsoon. We know that the Northern Hemisphere insolation modulated the long-term trend of the Asian summer monsoon, and triggered the occurrence of deglaciation; it is demonstrated that glacial build-up deceased the duration of millennial-scale Dansgaard-Oeschger events but increased their frequency; it was verified that solar activity played a predominant role in centennial monsoonal variations besideds the influence of the North Atlantic climate. At present, these precisely dated records have become important benchmarks for correlating and calibrating other climate records.The earth experienced three completely different climatic and environmental conditions from the Last Glacial Maximum (LGM) through the succeeding deglacial period to the final Holocene epoch, with contrasting glacial boundary conditions. Abrupt climate changes of the three periods uniformly initiate and terminate within several years to a few hundreds years and have a worldwide influence. Investigating the climate changes of these periods systematically, therefore, help provide insight into the coupling between integral components of the earth climate system, such ice sheets, oceanic/atmospheric circulations and global CO2 concentration. Importanlty, this investigation also helps advance our understanding of whether greenhouse gases result in global warming of the recent several hundred years. A multi-proxy study of a stalagmite can understand fatihfully behaviors of the components of the earth climate system. Thus, the study of multiple proxies has a potential to confidently understand the occurrence of abrupt climate change, and to evaluate and examine the meaning of the controversial stalagmite δ18O proxy.Here we provide stable isotope and trace element records measured on stalagmites in Chengjiachun Cave, Guizhou and Hulu Cave, Nanjing. These records cover a time span of 21.5-2.0 ka and have averaging resolution of 5.2 a. The stalagmite (CJC-1) δ18O record in Chengjiachun Cave ranges from -6.5‰ to -14.5%o, with an amplitude of 8%o over the whole record and with an amplitude of 5%o during the Holocene period. The Holocene amplitude of 5%o is similar to that of modern meteoric precipitation. In addition, a good replication is observed between the CJC-1 and other Asian stalagmite records. The two scenarios indicate that CJC-1 grew under the condition of isotopic equilibrium. Therefore, the large-amplitude δ18O variation indicates the sensitivity to climate change outside the cave and could reflect change in Asian monsoon intensity. The speleothem δ18O records from the two caves show an exceptionally strong monsoon event during ～19.5-17.5 ka, comparable to the B(?)lling-Aller(?)d interstadial. This finding is different from some modeling results that the Asian monsoon decreased during the LGM. The strong monsoon event correlates with the enhanced Atlantic Meridional Overturning Circulation (AMOC), implying that the heat transported by the AMOC to Eurasia during the LGM induced the strong monsoon event.The’Mystery Interval’(MI,17.5-14.5 ka) was named after many mutually contradictory climatic characteristics across the globe during this period. Of the mysteries, the Big Dry’(～17.5-16.1 ka) to’Big Wet’(～16.1-14.5 ka) transition in Lake Estancia of New Mexico has not been fully explained now. By knife-shaving the annual layer-developed stalagmite YT, Hulu Cave, we find an abrupt 2%o increase in the record at ～16.1 ka, corresponding to a significant southward shift of the ITCZ at ～16.3 ka. Moreover, the duration of the shift is 19 a in the YT record, is roughly in accordance with that (17 a) of the southward shift of the ITCZ. These correlations indicate that the Heinrich 1a event occurred at 16.1 induced rearrangement of global hydrological circulation. At this time, the associated southward shift of polar jet stream over the Pacific Ocean likely produced the dry to wet transition in Southwest USA. This significant feature at 16.1 is also observed in the trace element and layer thickness records, supporting the occurrence of low-altitude hydrological reorganization. Furthermore, the multi-decadal to centennial correlation between these proxy records lends further support to the interpretation that the δ18O record in East Asia does reflect monsoon intensity, instead of moisture source change. Second puzzling phenomenon is a large, but unexplained, rapid 190%o decrease in atmospheric 14C during the MI. A larger spatial coupling is shown by a statistical analysis, marked by a roughly identical ratio of Younger Dryas (YD) to MI for relative changes in AMOC intensity, atmospheric CO2 concentration and △14C, opal upwelling of south of the Antarctic Polar Front and the Asian monsoon intensity. This close correlation suggests the importance of the AMOC in regulating the global climate change, and of Southern Ocean in influencing atmospheric CO2 concentration. Four centennial-scale oscillations are identified within YD in both the CJC-1 δ18O and δ13C records, and these variations show a strong resemblance to the NGRIP δ18O records. However, significant differences are observed between the CJC-1 δ18O and 813C records during the beginning of YD.The CJC-1 δ13C record mirrors the NGRIP δ18O record during the onset of YD, but the CJC-1 δ18O record, in accordance with previous stalagmite δ18O records, shows a longer duration relative to the NGRIP and CJC-1 δ13C records. Here the strong correlation between the CJC-1 δ13C and NGRIP δ18O records, avoiding influences of respective age uncertainty of both archives, confidently constrains the phase relationship between monsoon circulation (δ18O) and Greenland temperature. The lead of the Asian monsoon reduction over the Greenland temperature decrease during the onset of YD indicates that YD is not induced by AMOC change. The CJC-1 δ13C record shows a similar long-term trend to the NGRIP δ18O and sea level records. After removing the long-term trend of the CJC-1 δ13C record, significant centennial changes in CJC-1 δ13C agree well with ice-rafted debris (IRD) events in the North Atlantic, "8.2 ka" event, solar activity and Greenland temperature variations. These correlations lend further support to the conclusion that Holocene Asian monsoon is influenced by a combination of the North Atlantic climate and solar activity. In addition, the correlation indirectly indicates that Chinese stalagmite δ18O indeed reflect hydrological circulation. This study for the first time provides a continuous high-resolution stalagmite isotope record recovered from Chengjiachun Cave, Guizhou, which covers a period of 20 ka from the LGM to late Holocene. Together with the newly measured Hulu Cave record, we discussed the causes of the Big Dry to Big Wet transition of Southwest USA, CO2 increase and atmospheric 14C decrease during the MI. We finded that the Asian monsoon decrease led over the Greenland temperature reduction during the onset of YD, implying the occurrence of YD was not triggered by AMOC change. The Holocene CJC-1 δ13C record first registers the IRD events in the North Atlantic, corroborating the influence of the North Atlantic climate on the Asian monsoon.