Environmental Sciences Research Institute

Age constraining the Last Glacial Maximum deglaciation of northwest England using Optically Stimulated Luminescence (OSL) dating and Cosmogenic Isotope Surface Exposure (CISE) dating.

The timing of the Last Glacial Maximum deglaciation of northwest England is currently poorly constrained and it has not yet been established whether significant oscillations of the ice-sheet margin, similar to those documented in northeast Ireland, occurred in this region. Several age estimates are available but these do not directly date the wastage of the ice sheet. Rather they are minimum ages because they relate to events that were facilitated by the removal of ice cover. It is particularly important that dates are obtained that provide tighter constraints on ice wastage in this region in order to resolve the timing of deglaciation, to determine if ice-sheet readvance occurred, and to establish when the landscape became available for widespread colonisation by plants and animals.

To this end both Optically Stimulated Luminescence (OSL) dating and Cosmogenic Isotope Surface Exposure (CISE) dating are being applied to materials from a number of locations in northwest England in order to establish the timing and pattern(s) of ice wastage.

At Warton Crag and New Close aeolian silts (loess) dated by OSL indicate that these sites were ice free by 19.3-16.5 ka BP and are minimum ages for deglaciation. These ages offer better precision than many previously obtained dates from this region. At Norber, erratics of Silurian greywacke perched on pedestals of Carboniferous limestone returned CISE dates of 18 ka BP and represent the first direct age estimates for loss of ice cover. Together with data from the Greenland ice cores, our OSL and CISE ages are helping to redefine understanding of environmental changes in northwest England. Our ages along with radiocarbon ages from remains of large carnivorous mammals indicate that the region, although ice-free from 19-18 ka BP, remained persistently cold and dry until about 14.7 ka BP when climate changed abruptly and mammal colonisation occurred


The 8.2 ka event in the terrestrial record of northwest England.

The most pronounced centennial-scale anomaly in Holocene proxy-climate records in the North Atlantic region is the 8.2 ka BP event. The event is especially prominent in oxygen isotope records of the Greenland ice cores and in the lithic, petrologic and foraminiferal content of marine sediments, but has also been recorded in numerous terrestrial proxy records. The anomaly is characterised by an excursion towards cooler temperatures, peaking sometime between 8.5 and 8.0 ka BP, lasting 70-200 years, and often showing two distinct maxima. Some records also indicate that the 8.2 ka BP event punctuated a longer interval of climate downturn variously placed between 8.6 and 8.0 ka BP and 9.0-8.0 ka BP. In northwest England a phase of marked climate cooling identified in the sedimentary record of Hawes Water, north Lancashire occurred at 8.38 ka BP and lasted for 150 years. A high resolution (1-2 years) record based on oxygen isotope data along with speleothem ages from several caves in northwest England have confirmed the regional occurrence of the 8.2 ka BP event (at 8.2 ka) with a duration of 150 years. There is general consensus that the most likely cause of the 8.2 ka BP anomaly is disruption of the Meridional Overturning Circulation in the North Atlantic due to freshwater input from the final collapse of the North American ice-sheet, with the two-stage nature of the anomaly possibly correlating to the drainage of the proglacial lakes Ojibway and Agassiz.

Fifteen samples of loessic silts from widely-spaced locations on the karst uplands of northwest England have yielded Optically Stimulated Luminescence (OSL) dates that fall within or overlap with (within uncertainties) the early to mid-Holocene period (11.7-6.0 ka BP). Nine of the dates are coincident with the hypothesized climatic deterioration at 8.5-8.0 ka BP in the North Atlantic region and eight are coincident with the 8.2 ka BP event. These dates demonstrate that the silts are not primary air-fall loesses of deglacial/Lateglacial age ( 18.0-11.7 ka BP) but have been reworked and now consist of loess-derived colluvial deposits. There is no substantive archaeological or palynological evidence for Late Mesolithic hunter-gatherers having had a major impact on the landscape and it is considered highly unlikely that these people triggered colluviation. We estimate that during the 8.2 ka BP event there was a reduction in mean annual air temperature at these upland locations of 2.6-4.6ºC and proxy evidence from other sites indicates a shift to wetter conditions. It is inferred that there was greater snow accumulation in winter, that the snowpack survived for longer periods, and that there was an increase in the magnitude and frequency of frost-related processes and meltwater flooding. Together these changes in climate and their associated (sub)-surface processes were responsible for the reworking of the loess. The OSL dates indicate climatically-induced landscape dynamism in Great Britain during the latter half of the ninth millennium BP.