Holocene tephrostratigraphic framework and monsoon evolution of East Asia: Key tephra beds for synchronising palaeoclimate records

Abstract

In East Asia our understanding of Holocene climate change, forcing mechanisms and propagation, require the precise chronological control of palaeoclimate records to allow robust integration of data sets. The existing chronologies, predominantly based on 14C method, however, are not sufficient to constrain key questions about abrupt climate shifts that occur within a century in the transitions between states. Widely dispersed tephra layers allow precise dating and synchronisation of sedimentary archives, providing a chronological framework for integrating records, especially where the visible tephra record is complemented by the addition of cryptotephra (non-visible ash). Despite significant tephra studies in this region, however, a comprehensive Holocene tephra framework is not available. To address this issue, we carry out a thorough review on Holocene tephra investigations undertaken in Japan. Using widespread tephra beds we present an integrated tephra framework and suggest the way forward for establishing this as a wider approach for East Asia. The framework is based on twenty-two ash layers that are mainly from Japan, and to a lesser extent China/N Korea, S Korea and Russia. Each tephra is assessed in terms of chronology, geochemistry and distribution. The framework is compared with high resolution palaeoclimate records from East Asia. Using this we demonstrate regional variations in monsoon evolution and more importantly, the potential of tephra isochrons in constraining such variations. Given the scarce identification of tephra layers in those well-resolved palaeoclimate records, we advocate a more systematic employment of the cryptotephra method, which would potentially lead to a significant advance in East Asian tephrochronology and the correlation of palaeoclimate archives in the region.