International Geologiical Congress - Oslo 2008

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CGC-13 Fjords: climate and environmental change

 

Holocene Rapid Climate Change: Pervasive millennial-scale climate variability in the NE Atlantic

 

William Austin, University of St Andrews (United Kingdom)
John Howe, Scottish Association for Marine Science (United Kingdom)
Alix Cage, University of St Andrews (United Kingdom)
Tracy Shimmield, Scottish Association for Marine Science (United Kingdom)
 

 

Rapid climate change (RCC) during the Holocene, particularly post-dating the demise of large Northern Hemisphere ice sheets after 8000 cal. yr BP, is a global phenomenon and is almost certainly driven by long-term changes in insolation, upon which solar variability, although a weak direct forcing mechanism, is superimposed. At least five significant intervals are identified in numerous palaeoclimate records since the major 9000-8000 cal. yr RCC, within which the intensively studied 8200 cal. yr 'event' is embedded; these are: 6000-5000, 4200-3800, 3500-2500, 1200-1000 and 600-150 cal. yr BP.

Most of the Holocene RCCs are associated with bipolar cooling, an expansion-intensification of high latitude circulation systems and drying-aridity at low latitudes.
Giant piston core (MD04-2832) from the main basin of Loch Sunart, Argyll, NW Scotland, is 22m long and appears to contain a record of continuous sedimentation back to nearly 8000 cal. yr BP. Based upon the age-depth model for core MD04-2832, isotopic shifts recorded in the benthic foraminifera Ammonia beccarii coincide with both the rate and magnitude of the Holocene RCCs. Indeed, the renewal history of bottom waters in the fjord basins appear to be driven by large-scale atmospheric circulation changes.

Numerical models of fjordic circulation were developed for Loch Sunart and sensitivity analyses, based on recent extremes in the North Atlantic Oscillation (NAO) indices, reveal that its circulation is highly sensitive to the strength of the prevailing Westerlies. Data from the Greenland Summit Ice Core (GISP2) reveal patterns of changing high latitude atmospheric circulation during the Holocene which are interpreted as influential upon the location and strength of the Westerlies and should therefore be recorded in circulation proxies from Loch Sunart.
Strengthening Westerlies, in particular, force more frequent deep-water renewal events (DWRE), which change the balance of δ13CDIC in the main basin in favour of coastal waters, a process reflected in the δ13Cforam record.

It appears that δ18Oforam provides a record of bottom water temperature and salinity changes which are coincident with the timing of ice-rafting, surface freshening and rates of ventilation in the open ocean North Atlantic. We highlight the significance of a sediment record that integrates both atmospheric- and marine-driven climate proxies within a common, high-resolution stratigraphy.

 

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