Abrupt climate change in Ireland: Assessing the role of 'Seasonality' in the last glacial termination

Periods of abrupt climate change in the North Atlantic during glaciations have attracted considerable research over the last 40 years. Going by several names, including North Atlantic stadials, Greenland stadials, and Heinrich Stadials, they are characterised by cooling signatures in the North Atlantic basin; slowdown, displacement, or cessation of North Atlantic oceanic currents; and an anomalous southern spread of sea ice. In contrast, the Southern Hemisphere continues to warm during these stadials, leading to the concept of a ‘bipolar seesaw’. Despite this apparent antiphasing, similar records of deglaciation during these periods cover both the Northern and Southern Hemispheres, including areas adjacent to or downwind of the deeply cold North Atlantic. The ‘seasonality’ hypothesis was developed as a response, where winters are deeply cold but summers become warm enough to melt glaciers in the northern regions displaying this deeply cold climatic signature, while resolving the need for a global mechanism. As such, questions remain about stadial initiation, presentation, maintenance, and broader connection to global phenomena, including the size and response of local ice sheets to these stadials. The Connemara region on the central west coast of Ireland is an ideal location to investigate these questions: local ice sheet size and configuration has historically involved two competing hypotheses, and its plentiful relict glacial features and varied quartz-rich lithologies adjacent to large limestone formations make it an ideal location to test ice flow direction and perform cosmogenic nuclide surface-exposure dating to ascertain timing and pace of ice dissolution. The three studies presented here attempt to address these questions of cryospheric response, timing, and morphology. The first investigation looks at glacial erratic distributions throughout the Connemara region, presents 15 new 10Be cosmogenic nuclide dates, documents glacial meltwater features and striae throughout the region, establishes a radial flow pattern from central Connemara, and discusses potential mechanisms for the rapid deglaciation found at approximately 17 ka. The second manuscript presents a previously undocumented suite of moraines in a Connemara mountain valley, analyses the moraine till for depositional conditions, traces lithologies within the till to the bedrock locations, establishes a period of northward ice flow into the Connemara uplands during deglaciation, and finds rapid deglaciation within the statistical error of the sites from the first study, with an added vertical dimension. The third study investigates a previously documented, but largely unstudied, moraine suite in eastern Connemara finding evidence of the same brief readvance within rapid deglaciation suggested by the moraines in the second manuscript from ice flowing radially from Connemara, supported by glacio-geomorphological evidence, lithology tracing, and new 10Be dates analysed together with previously published 10Be dates and discusses the global and climatic connections this rapid deglaciation and brief readvance sequence represents. The project finds rapid deglaciation in Ireland during stadial conditions, rapid enough that the cause must include summer melt-season activity, supported by the tight statistical distributions of 10Be data, abundant meltwater features in the region, till characteristics, and European meltwater indices. The project also finds incontrovertible geomorphological evidence indicating an ice centre in Connemara that could not have supported an ice shelf post-Last Glacial Maximum, with ice retreat moving toward central Connemara rather than west to east toward the midlands.

Publisher

University of Galway

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CC BY-NC

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University of Galway Theses (PhD Theses)