Abstract/Summary

Monsoon climates have varied on multiple timescales through the late Quaternary; Monsoons expanded and contracted in response to orbital forcing, interannual-interdecadal variability responded to the changing importance of climate modes, and abrupt events were triggered by rapid shifts in oceanic circulation. Individual speleothem oxygen isotope (δ18Ospel) records have documented this variability, often with high temporal resolutions and good age control. However, few comprehensive regional and global-scale analyses of these speleothem records exist. In this thesis, the Speleothem Isotopes Synthesis and AnaLysis database is used to reconstruct past monsoon climate at a range of timescales, including multi-millennial to interannual-interdecadal timescales, and across centennial-scale abrupt events. The use of a large-scale database enabled novel statistical techniques to be used on these records, including: regression modelling, to determine the climate processes that drive Holocene δ18Ospel evolution; compositing, to construct regional signals and assess uncertainty; ordination techniques, to determine the relationship between record similarity and geographic location; new techniques to reconstruct interannual-interdecadal variability; and breakpoint analysis and significance testing to identify abrupt events. Reconstructions were compared to climate models, to provide insight into the processes driving the observed changes. Comparison of regional Holocene δ18Ospel composites with isotope-enabled climate models suggests that monsoon δ18Ospel is primarily driven by changes in regional precipitation and atmospheric circulation. Holocene trends emphasise the importance of orbital forcing, with modification by internal feedbacks. Reconstructions and climate-model simulations of regional monsoon interannual-interdecadal variability through the Holocene suggest a non-linear relationship with orbital forcing, likely because of the complex interplay between insolation and climate modes. Finally, speleothem records show the 8.2 ka event is the most significant abrupt event in the Holocene. This thesis provides the most extensive global 8.2 ka synthesis yet, showing a globally coherent monsoon response and rapid global transmission.