Paleoenvironment And East Asian Summer Monsoon Rainfall Evolution In Southern Taiwan During Last Deglaciation And Early Holocene

The climate variability during the last deglaciation is one of the most intriguing areas of paleoclimate research due to a serial abrupt climatic oscillation(Heinrich stadial 1-H1,B(?)lling-Aller(?)d-BA,and Younger Dryas event-YD).A better understanding of these events may advance our understanding of global climate system and its forcing.The East Asian summer monsoon(EASM)is an important component of the global climate system and plays a significant role in global hydrolog:ic and energy cycles.The strength of the EASM is a important factor that has influenced the past environment changes and culture evolution in East Asia and the prosperity of livelihood of over a billion people who live in monsoonal regions in East Asia.Taiwan is located along the front edge of the EASM range and its climate is mainly influenced by EASM.The annual precipitation in Taiwan is more than 2000 mm,approximately 90%of which occurs during the summer seasons.In addition,Taiwan' climate was significantly influenced by typhoon and almost half of the rainfall of Taiwan during summer comes from episodic typhoons.However,well-dated and high-resolution climatic records during the last deglaciation are rarely been reported in Taiwan.In this study,a well-dated and high-resolution lacustrine record from Dongyuan Lake at southern Taiwan was used to reconstruct local environmental variation and multi-timescale oscillations of EASM during last deglaciation and early Holocene(17-9 ka BP).Based on a comprehensive analysis of the proxy data from Dongyuan Lake and comparison with other climatic records,we got the following conclusions:The oscillation of the EASM intensity during last deglaciation registered in our record generally coincided with the variation of North Atlantic climate at millennial and centennial time scales suggesting the strength of EASM is modulated by climate anomalies in the North Atlantic region through changes in the intensity of the Atlantic Meridional Overturning Circulation(AMOC)and migration of the annual mean position of the Intertropical Convergence Zone(ITCZ).However,some significant discrepancies between our records and Greenland ice core record within these events are observed.Our records shows a clear two phase characteristic during H1 with a significant decrease in EASM rainfall during the later H1 suggesting further southward shift of the ITCZ while no obvious change was observed in Greenland ice core record.During the BA,Greenland ice core temperature record shows a clear decreasing trend;however,the EASM rainfall in our records shows an opposite trend suggesting other climate forcing beyond North Atlantic climate jointly modulated the EASM rainfall variation.The EASM rainfall significantly decreased during the YD with a time lag of 500-800 yr when compared to Greenland ice core record at both the onset and termination of the YD event.We found a series of centennial-scale distinct wet events were in concert with Sea surface temperature(SST)anomaly events in the western tropical Pacific,which indicates an inherent connection between the western tropical Pacific SST and the variations of EASM rainfall in Sothern Taiwan.Increased SST in the western tropical Pacific likely enhanced the meridional heat transport from tropical ocean to high latitudes,which futher increase the monsoon convection.Two pronounced high sedimentation events at 10.5 and 9.4 ka BP accompanied by an amount of wood fragment burial are observed in the lake sediments during the Early Holocene.These events were synchronous with the largest landslides events in past 16 ka recorded in alluvial terraces over Taiwan Island.Such coherence possibly resulted from significantly increased both intensity and frequency of typhoon activities during these period.Lignin,an abundant biopolymer found almost exclusively in terrestrial vascular plants,has been regarded as resistant to microbial degradation in comparison to other plant components.Thus,lignin in lake sediments and peat deposits offers considerable information in paleoclimate research,as compositions of lignin oxidation products can reveal information about vegetation source,terrestrial organic matter degradation processes,and thus climate.We analyzed the lignin oxidation products of the core sediments from Dongyuan Lake.The lignin phenol ratios show a clear shift from gymnosperm-dominant to angiosperm-dominant vegetation type in the lake catchment at 12.2 ka BP,which probably suggesting a warm climate that resulted from persistent increase in solar insolation and SST in tropical Pacific.Significantly decreased lignin concentrations and apparently elevated lignin degradation parameters in combination with other geochemical proxies during 12.2-10.7 ka BP suggest a catastrophic drought had occurred in southern Taiwan during this specific period.We propose that deepened groundwater flow routes,caused by significantly decreased precipitation during the warm and dry summers,may leach the older carbon with higher degradation state and lignin depleted in the catchment Moreover,lignin is subject to increased degradation before it is transported and buried in sediments because of a longer residence time in soil during dry periods.The climate changes registered in our records lagged the temperature records in North Atlantic region with 500-800 years.Generally,the tropical hydroclimate records required 300-500 years to reach and recover from the full YD conditions.We note that the drastic changes in proxies from Dongyuan Lake occurred at the time of the maximum oxygen isotope values of stalagmites during the YD,and the termination of catostropic dry event in our record occurred at the time of stalagmite oxygen isotope values recovered to its pre-YD condition.This might imply the climate change reached to a threshold which lead a significant environmental change in southern Taiwan.The time lag between our record and the climate change in North Atlantic region might suggest a delayed response of vegetation and soil processes to rapid climate changes.We investigated the sulfur geochemistry including isotopic composition of the same core from Dongyuan Lake.The variation of sulfur isotope during last deglaciation and early Holocene ranged from + 9.5 to + 17.1‰.Two significant positive shifts in ?34S were observed at 10.5 and 9.4 ka BP with an extraordinary sulfur peak(up to 4.8%)accompanying pyrite presence at 10.5 ka BP.Carbon-sulfur-iron relationships suggested that the bacterial sulfate reduction had occurred in sediments instead of water column,and the Dongyuan Lake sediment is a closed system to efficiently trap H2S generated via sedimentary sulfate reduction.The sources sulfur input to the lake sediments and potential processes involving the sulfur isotope variation are discussed(e.g.bacterial sulfate reduction,volcanic emissions,sulfide oxidation from catchment and marine aerosols).All processes except marine aerosol input cannot explain the peaks of sulfur contert and ?34S.The positive 834S shifts appeared concurrently with the recorded maximum mass-wasting events over Taiwan which resulted from enhanced typhoon activities.The synchronicity among records suggests the positive shift in ?34S was likely caused by greatly increased influx of marine sourced sulfate with more positive ?34S(+20.3‰)due to enhanced typhoon activities in the early Holocene.Our sulfur geochemistry data provides new insights of the sea spray history or marine influence onto the terrestrial environment at coastal regions.