Open University Uranium-Series laboratory    
Earth and Environmental Sciences, The Open University, Milton Keynes, UK    

Hawes Water lake marls (Dr J Holmes, UCL, IP/672/0900 and Prof. J Marshall, Liverpool, IP/861/0505)
 

Abrupt changes in climate are features of both glacial and inter-glacial climate states (Alley et al., 2003). Studies of ice cores and sediments from the North Atlantic area demonstrate that the relative stability of early Holocene climate was interrupted by periods of short-lived cooling ( McDermott, et al., 2001; von Grafenstein, et al, 1999). The best studied of these Holocene cold events, that took place around 8.2 ka BP, has been tentatively linked to the catastrophic discharge of glacial meltwaters from the Laurentide ice sheets and the consequent short-term disruption of the Atlantic thermo-haline circulation (Alley et al., 1997; Clarke, et al., 2004). An earlier event, around 9.2 ka BP, has been recognised in European lake sediments (McDermott, et al., 2001; von Grafenstein, et al., 1999) and is also apparent in high-resolution ice core records from central Greenland (Bauer, et al., 2004). These events are important because they demonstrate that rapid cooling events have interrupted climatic states similar to today. The ‘8.2 ka BP event’ is a key focus for climate modellers interested in the stability of the North Atlantic circulation and the possible consequences of anthropogenic changes in greenhouse gases. Despite the importance of these events on terrestrial climates the underlying forcing remains poorly understood. New data provide quantification of the terrestrial changes in temperature and water composition in NW Europe and indicate that the events were linked to subtle background changes in climate.

 

Uranium-series dated records of the δ18O and water temperature (from chironomid assemblages) from Hawes Water, a small lake in NW England, document climatic variations between 11.2 ka and 5.5 ka years BP. Two episodes of marked cooling of lake waters, by about 1C, at 9.2 ka and 8.2 ka BP lasted approximately 50 and 150 years respectively. The significant decrease (> 1‰) in the δ18O of precipitation at Hawes Water is interpreted to be a direct downstream response to the freshening of the north east Atlantic surface water. The cooling is consistent with the climatic effects of a partial shutdown of the thermo-haline circulation caused by surface water freshening. Centennial-scale variability in the δ18O at Hawes Water (mean period ~500 years) is highly coherent with both the δ18O records from Greenland ice cores and 14C production.
 

 

 
 
 
The centennial-scale variability in the δ18O at Hawes Water, HWLC1 is based on U-series chronology.δ18O variation is highly coherent with the δ18O record from Greenland ice cores and the 14C production. Offset horizontal lines are based on the U-series chronology in HWLC1 and accepted chronologies for the NGRIP and 14C records, from Marshall et al., 2007.
 
   
The early Holocene centennial-scale climate variability is consistent with weak solar forcing with a lag of about 220 years. The short-lived events reflect unusual amplification of the centennial-scale cooling.
 
   

Alley et al. 1997 Geology 25 483–486
Alley et al. 2003 Science 299 2005–2011
Bauer et al. 2004 Paleoceanography 19 PA3014
Clarke et al. 2004 Quaternary Science Reviews 23 389–407
Marshall et al. 2007 Geology 35(7) 639-642
McDermott et al. 2001 Science 294 1328–1331
von Grafenstein et al. 1999 Science 284 1654–1658
 
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© Peter van Calsteren
Last updated: 23 December, 2011 10:55
 
23 December, 2011 10:55