Tephrochronology Of The Changbaishan Volcano And A Lacustrine Archive In Northern Japan

Tephrochronology is the study of the stratigraphy,age,glass chemistry and distribution of volcanic ash(i.e.tephra)layers and the use of widely dispersed tephra isochrons for precise dating and correlation of geological,paleoclimate and archaeological records.Palaeoclimate records in East Asia,a region where environmental changes are primarily controlled by the Asian monsoon,are essential for our understanding of monsoon dynamics.Importantly,East Asia is a volcanically active region,which possesses great potential for offering widespread tephra layers.As such the region is an ideal area for optimising the use of tephra layers to synchronise palaeoclimate records.However,before this can be achieved,a comprehensive regional tephra framework is required,in order to guide future correlations and minimise miscorrelation.In this PhD project,the proximal volcanic stratigraphy of Changbaishan volcano China/N Korea and a cryptotephra deposit from Lake Kushu northern Japan were investigated.Based on glass chemistry and independent chronology,three proximal pyroclastic fall units have been correlated to a distal lacustrine tephra.Proximal-distal correlations of the Millennium eruption tephra(~1ka)allowed the clarification of controversial volcanic stratigraphy of the Changbaishan volcano.In addition,highresolution ice-core tephra age has been used to constrain the radiocarbon chronology of the distal lacustrine record.Using the 16-m-long Kushu Holocene sedimentary sequence(RK12 core),this study reported the first cryptotephra stratigraphy for northern Japan.The presented RK12 Holocene tephrostratigraphy integrated local and far-travelled tephras originating from Japan,China/North Korea,Russia and most likely Indonesia.The identification of Shiveluch SH#12 tephra in the lake,documented the first example of a Russian tephra onshore in Japan.This study extended the ash dispersal of several key tephra markers in the region and identified a number of potential eruption events.A refined age-depth model has been proposed for the RK12 sedimentary record.Using the high-resolution pollen record from Lake Kushu,this study showed that the large Millennium Eruption of Changbaishan volcano had limited or no long term impact on northern Japan environments.Given that tephra layers recorded in the lake were very widely dispersed,linking archives spanning from the polar region through high northern latitudes to the tropics,this tephra study has made the Lake Kushu an important archive for future climate research.A prominent cryptotephra layer in the sequence exhibited chemical fingerprint that did not match with known Asian Northern Hemisphere eruptions.With further investigation of Southern Hemisphere tephra sources,this study proposed the identification of a cryptotephra from Krakatau volcano Indonesia,in northern Japan over 6700 km from the vent.This finding provided the first empirical evidence of transhemispheric latitudinal distribution of volcanic ash,which revealed a previously unknown eruption event and opened up the possibility for establishing an inter-hemispheric cryptotephra lattice in the region.Finally,this study established the criteria for selection of tephra isochrons to be incorporated in an updated East Asian Holocene tephrostratigraphic framework.In addition,the chronostratigraphic relationships between tephras in the framework and climate change events were fully evaluated,in particular the extent to which these isochrons could be used to constrain regional variations of short-lived climate oscillations.A list of potential tephra markers in the region were also evaluated,and future work as well as opportunities were pointed out.Given the fruitful results from recent pioneering cryptotephra studies in East Asia,it is now essential that a more systematic approach to the application of cryptotephra studies is applied to terrestrial and marine records from the region.