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CGC-12 The challenge of the Younger Dryas?
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Testing the synchroneity of events between Greenland and Scotland during GS-1 using annually-resolved glaciolacustrine sediments
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Adrian Palmer, Royal Holloway, University of London (United Kingdom)
John Lowe, Royal Holloway, University of London (United Kingdom)
James Rose, Royal Holloway, University of London (United Kingdom)
Alison Macleod, Royal Holloway, University of London (United Kingdom)
Simon Blockley, RLAHA, University of Oxford (United Kingdom)
Sune Olander Rasmussen, Niels Bohr Institute, University of Copenhagen (Denmark)
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The Greenland ice-core records provide the best resolved record of centennial to decadal-scale climate shifts during the Last Glacial Stage, and a robust dating scheme for the last 30 kyr in particular.The challenge that confronts those working with marine and lake records spanning the same period is to be able to resolve and correlate events with a comparable degree of precision.This is vital for understanding the spatial and temporal climate variability of the North Atlantic region, and hence testing competing theories about the mechanisms that drive abrupt climate change.Varved sediments provide this potential. Attempts have recently been made to compare large-scale variations in varve thickness records with ice-core δ18O isotopic variations. However, these approaches have 'tied' the records using pronounced match-points on the assumption that these represent synchronous events, which in turn assumes that both systems were responding to common forcing factors. This may be correct, but is circular in reasoning, as it does not provide a robust test of the assumptions employed. Here we provide interim results of a project that is designed to test the degree of synchroneity between δ18O isotopic variations in Greenland and varved records in Scotland which date to within GS-1. Glaciolacustrine sediments in Scotland accumulated in a number of proglacial lakes formed when glacier expansion during GS-1 blocked major drainage pathways. Micromorphological analysis of the individual laminations allows the identification of varved sediments, which were counted and measured using image analysis techniques.Examination of a number of sites shows that variations in varve thickness are due to changes in sedimentation rate related to the position of the local ice margin and these sites have provided the basis for a floating varve master chronology.A key characteristic within these data-sets is the number of discrete flux events within the summer layers, which reflects variations in local summer climatic conditions. Analysis of year-on-year variations in the number of summer flux events provides an index that visually matches long-term changes in δ18O isotopic values using the ss08sea GRIP timescale: increases in the numbers of summer flux events correspond closely with more enriched oxygen isotope values. Similar trends are observed in a visual match with the more highly resolved GRIP ss09 timescale, suggesting that sedimentation in Scotland and isotope variations in Greenland ice may have been responding to a common forcing element.The optimal match between the two records indicates that the glacially-dammed lake systems in Scotland existed between 12,119 and 11,506 GRIP years BP.Given the problems of using visual matching alone, we are the process of statistically testing the strength of the match, by examining both the frequency and magnitude of the changes and will also test the correspondence between the Scottish and ice-core data using tephrochronology.
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