Evolution And Mechanism Of East Asian Summer Monsoon Since Last Deglacial Recorded By Gonghai Lake, Shanxi Province

As a major component of the Asian summer monsoon system, East Asian summer monsoon(EASM) variability significantly affects economic activity and society within its area of influence. Thus it is important to investigate the variability of the EASM on various time-scales and to explore its underlying forcing mechanisms, in order to improve our ability to predict the long-term trends of regional and global climate. The variability of the EASM has been reconstructed from various types of paleoclimatic archives and proxies, such as loess-paleosol sequences, lake sediments, sandlands in northern China and speleothem oxygen isotope records. Amongst these various proxy records, stalagmites possess a significant advantage in that they can be precisely dated using U-series methods, and thus they have been widely used for reconstructing the EASM. Amongst the various indicators applied to stalagmites, δ18O records are regarded as yielding the most significant and reliable information about EASM changes on various time scales. However, whether or not the speleothem δ18O record is a clear palaeomonsoon signal is strongly debated. More and more studies suggested that the stalagmite δ18O records from the EASM region should not be regarded as a reliable indicator of the strength of the East Asian summer monsoon. In addition, monsoon precipitation over China exhibits large spatial differences. So, in which region will the rainfall be increased when the intensity of the East Asia summer monsoon is increasing? The modern climatic data show that stronger(weaker) EASM circulation carries more(less) water vapour from the tropical Pacific and Indian Oceans, resulting in higher(lower) precipitation over North China. The EASM intensity can be directly represented by precipitation in North China. Thus here we firstly evaluate the spatial pattern of EASM rainfall on different time scales during the Holocene within eastern China in order to confirm that rainfall in northern China is the best indicator of EASM intensity in paleo-monsoon studies and explore the potential mechanism(s) controlling the Chinese stalagmite δ18O, and then use a high-resolution, pollen-based quantitative precipitation reconstruction combined with other proxies from a well-dated sediment core retrieved from an alpine lake in North China to assess EASM variability and to characterize the underlying dynamical mechanisms during the last two millenniums, the Holocene and the late Glacial. The primary conclusions are as follows:1. We present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction(derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka(ka=thousands of years before present, where the “present” is defined as the year AD 1950). We found two millennial-scale monsoon declines, during the Younger Dryas stadial and at 9.5–8.5 ka, superimposed on a gradually intensified EASM from 14.7 to 7.0 ka. The EASM reached its maximum during the mid-Holocene from 7.8 to 5.3 ka within the context of generally high precipitation during ~8–3 ka. The EASM exhibited a sudden retreat at 3.3 ka. We attribute this EASM evolution to an ice-volume modulated Northern Hemisphere summer insolation(NHSI) forcing. The NHSI induced EASM intensification during the late deglaciation and the early Holocene was suppressed by the effect of North Atlantic freshwater input on the Atlantic Meridional Overturning Circulation and associated atmospheric circulation changes until the global ice volume almost approached its minimum at ~7 ka. Subsequently NHSI played a major role in driving the decreasing EASM, but ENSO variability may also have contributed to the late-Holocene EASM weakening. Our results demonstrate the significance of both highand low-latitude climatic factors in forcing EASM variability on a sub-orbital scale, and thus they enhance our understanding of monsoon dynamics and potentially improve the predictive abilities of climate simulations. In addition, our demonstration of a ~4 ka delay in the response of the maximum monsoon intensity to the Northern Hemisphere summer insolation maximum indicates that the prevailing view of an early Holocene EASM maximum inferred from speleothem oxygen isotope records in southern China should be significantly revised.2. Based on our precipitation record reconstructed from the lake sediments of Gonghai Lake and its comparisons with other EASM records over the past two millennia, we found that the EASM intensity has an unequivocal general weakened trend over the past two millennia and the orbital insolation changes have been shown to force this trend of EASM. The synchronicity of this long-term weakened trend, among all monsoon precipitation records in the EASM region, suggests the forcing was continental in scale. The summer monsoon broadly follows the Northern Hemisphere(NH) summer insolation, shows general weakened over the past two millennia, and correlates with solar variability. Several changes in the EASM was clearly expressed in our records including a gradual decline in intensity from 0-900 AD, a shift to high intensity during the Medieval Warm Period(MWP, 800-1300 AD) that was interrupted by an interval of megadroughts around 950–1050 AD, a weakening during the Little Ice Age(LIA), followed by an abrupt intensification from 1850 AD to the present. We attribute this EASM evolution to the land-sea temperature contrast between NH land and ocean which was driven by the orbital NH summer insolation. The solar activities might only play a role in driving the occurrences of climatic events such as the MWP and LIA recorded in the EASM. The Pacific-Indian ocean sea surface temperature(SST) induced EASM to decline during 950-1050 AD within the MWP. Our results demonstrate the significance of orbital factor in forcing EASM variability over the past two millennia on a long-term trend, whereas the solar activities might only play a subordinate role in affecting the EASM.3. Based on our precipitation record reconstructed from the lake sediments of Gonghai Lake and its comparisons with other EASM records, we present results from a diverse range of proxy paleoclimatic records from the monsoonal and temperate Asian region in order to evaluate the occurrence of such short-time-scale variability within the MWP. Within the context of an overall strong EASM during the MWP, a weakening of the monsoon was detected in many of the records during the period 1000-1100 AD. Comparison of the timing of this event with variations of sea surface temperature(SST) of the Indian Ocean-western Pacific and with proxy records of solar activity reveals a significant co-variation, suggesting that the driver of the event may have resulted from changes in the Indian Ocean-western Pacific, related to changes in solar activity. To further address the issue of a terrestrial-oceanic linkage, we used the ECHO-G coupled climate model to simulate the variation of EASM precipitation over the last millennium. The model results suggest an interval of weak East Asian summer monsoon at 1000-1100 AD, and they also reveal a significant positive correlation with the SST of the Indian Ocean-western Pacific.4. In order to further confirm the robustness of our EASM precipitation record reconstructed from the lake sediments of Gonghai Lake. Environmental magnetic studies were conducted on a 9.42 m-long sediment core from Gonghai Lake, North China. Radiocarbon dating indicates that the record spans the last 15,000 cal yr. BP. The principal magnetic mineral in the sediments is pseudo-single domain magnetite of detrital origin with minimal post-depositional alteration. Although the variations in the concentration of detrital magnetic minerals and their grain size throughout the core reflect inputs from both soil erosion and eolian dust, it is shown that their climatic and environmental significance changes with time. In the lowermost part of the core, ~15,000-11,500 cal yr BP, the magnetic minerals were supplied mainly by bedrock erosion, soil erosion and dust input when climate ameliorated after the cold and dusty last glacial maximum. The increasing magnetic susceptibility(χ) in this interval may indicate a combination of changes in the lake environment together with catchment-surface stabilization and a decreasing proportion of dust input. In the central part of the core, ~11,500-1000 cal yr BP, the detrital magnetic minerals mainly originated from dust inputs from outside the catchment when the lake catchment was covered by forest, and catchment-derived sediment supply(and thus the lake sediment accumulation rate) were minimal. The generally low concentration of magnetic minerals in this part of the core reflects the highest degree of soil stability and the strongest summer monsoon during the Holocene. In the uppermost part of the core, the last ~1000 years, detrital magnetic minerals mainly originated from erosion of catchment soils when the vegetation cover was sparse and the sediment accumulation rates were high. Within this part of the core the high magnetic susceptibility reflects strong pedogenesis in the lake catchment, and thus a strong summer monsoon. This scenario is similar to that recorded in loess profiles. Overall, the results document three main stages of summer monsoon history with abrupt shifts from one stage to another: an increasing and variable summer monsoon during the last deglacial, a generally strong summer monsoon in the early and middle Holocene and a weak summer monsoon in the late Holocene. Specially, during the late Holocene, the variations in the χ and S-300 parameters of the core clearly suggests generally abundant precipitation and a strong summer monsoon during the Medieval Warm Period and a weak summer monsoon during during the Little Ice Age, and also indicates a dry event around AD1050, interrupting the generally humid MWP. Therefore, all of these results further demonstrate the robustness of our EASM precipitation record reconstructed from the lake sediments of Gonghai Lake.5. Our results show that a significantly enhanced East Asian summer monsoon(EASM) is characterized by increased rainfall in northern China and by reduced rainfall in southern China, and this relationship occurs on different time scales during the Holocene. This conclusion is robust because this characteristic pattern of EASM monsoon rainfall is consistent with the dynamics of the EASM, a subtropical monsoon that differs from a tropical monsoon such as the Indian summer monsoon. As a subtropical monsoon, the EASM is usually characterized by prevailing lower-tropospheric southerly winds. In addition, the monsoon precipitation over eastern China occurs in front of the maximum southerly wind centers, i.e., the rain belt known as the Meiyu Front in China, which lies on the northwestern flank of the North Pacific Subtropical High. Thus, the prevailing southerly winds/Subtropical High system are stronger(weaker) over eastern China and advance northwards to a higher latitude(are confined to southern China) when the EASM circulation is stronger(weaker). This results in a more northward-(southward) located monsoon rain belt over eastern China, which finally leads to more(less) rainfall in northern China and to less(more) rainfall in southern China. For example, in most parts of northern China, a decreasing trend of the summer precipitation over the last 50 years is positively correlated with weakening of the monsoonal wind intensity; in southern China, however, summer precipitation has increased gradually over the last 50 years, and is negatively correlated with a weakening of the monsoonal wind intensity. Therefore from a paleomonsoon perspective, rainfall in northern China is the reliable indicator of EASM intensity in paleo-monsoon studies, rather than rainfall in southern China, and thus is traditionally considered as the appropriate EASM proxy.6. Based on the results from a diverse range of proxy paleoclimatic records from northern China, we found the pattern of variability revealed in the Holocene EASM records from northern China in this study are distinctly different from that exhibited by the stalagmite δ18O records from southern China. The main difference lies in the timing of the EASM maximum. Thus the stalagmite δ18O record from southern China is quite different from the EASM records from northern China where precipitation variability is traditionally considered as an EASM proxy. Thus it is clear that the stalagmite δ18O records from southern China should not henceforth be regarded as a reliable indicator of the EASM. In addition, all of the well-dated Holocene stalagmite δ18O records, covering a broad geographical region, exhibit a remarkably similar trend of variation and are statistically well-correlated on different time scales. This indicates that they are driven by a common mechanism, i.e., the coherent δ18O signal over East Asia. However, in contrast with the clear consistency in the δ18O values in all of the cave records, both instrumental and paleoclimatic records exhibit significant spatial variations in rainfall on a decadal-to- centennial time scale over eastern China, and both paleoclimatic records and models suggest that Holocene East Asian summer monsoon precipitation reached a maximum at different periods in different regions of China. Thus this further indicates that the stalagmite δ18O records from the EASM region is not a reliable indicator of the strength of the East Asian summer monsoon.