A major breakthrough in Quaternary paleoclimatology came in the late 1960s with the drilling, extraction, and analysis of the Camp Century ice-core from north-west Greenland (Dansgaard et al. 1969, 1971). There are now many polar and alpine ice-cores that have been analyzed in detail for a wide range of paleo-climatic proxies. These include stable isotopes, borehole temperatures, and melt (proxies for paleotemperatures and humidity), 10Be (solar activity), CO2, CH4, and N2O content (atmospheric composition), conductivity, acidity, and sulfate (volcanic activity), microparticle content, trace elements, and electrical conductivity (tropospheric turbidity), mineral dust and particle size and concentration (wind speeds), major ions (atmospheric circulation), salt content (sea-ice extent, marine storminess), and seasonal signals and 10Be (net accumulation rates) (Bradley 1999; Fisher and Koerner 2003). Alley (2000) and Bowen (2005) give fascinating and exciting accounts of work on Greenland ice-cores and on low-latitude alpine ice-cores, respectively. Fischer et al. (2006) reviews current ice-core projects worldwide.
The most important contribution from ice-core research to understanding Holocene climate history was the demonstration that there are fundamental differences between the climate of the Holocene and of the preceding 100 000 years. When viewed at this broad time-scale, Holocene records from Greenland ice-cores indicate that the Holocene generally has been a period of overall relative stability with small fluctuations in many paleoclimate proxies, in contrast to the preceding 100 000 years where there were rapid changes between two or more climatic modes, so-called Dansgaard-Oeschger events or oscillations. There appears to have been 24 such oscillations between 15 000 and 60 000 years ago with an amplitude of warming and cooling during each event of about 10°C in central Greenland (Bradley 1999; Alley 2000; Oldfield 2005).
When viewed in the context of the Holocene only, the major paleoclimatic contributions from ice-core research are (i) providing clear evidence for unprecedented rapid climate variations in the Greenland and Ellesmere Island ice-cores at centennial and even decadal scales (e.g. O'Brian et al. 1995), (ii) quantitative measurements of greenhouse-gas concentrations (CO2, CH4, etc.) from air bubbles enclosed in Antarctic ice (Raynaud et al. 2003), and (iii) providing near continuous records of ice-accumulation rates, volcanic activity, and dust deposition from both polar and low-latitude alpine areas. Fisher and Koerner (2003) discuss in detail Holocene ice-core research and its major findings, whereas Cecil et al. (2004) review alpine ice-core research from mid- and low-latitudes for the past 500 years.
The discovery of the abrupt "8.2 ka event" in a range of Greenland ice-core proxies (Alley et al. 1997) rekindled interest in Holocene climate instability and in the underlying mechanisms for rapid climate changes. The event of about 200 years duration appears to represent a major climatic instability event with a 5°C magnitude in Greenland and a considerable geographic extent in the North Atlantic regions and possibly elsewhere in the Northern Hemisphere (Alley and Agustsdottir 2005; Rohling and Palike 2005). The event does not appear to be unique in the early Holocene as there appear to be other rapid but less extreme events at, for example, 9200 years ago and between 10 200 and 10 400 years ago, 10 800 and 10 900 years ago, and 11 300 and 11 500 years ago, the so-called Pre-Boreal oscillation (e.g. Schwander et al. 2000; Bjorck et al. 2001). The most likely cause for the 8.2 ka event and other early Holocene rapid climate events is the discharge of large amounts of glacial meltwater from glacial lakes Agassiz and Ojibway via the Hudson Strait into the north-west Atlantic (Clarke et al. 2003). This freshwater pulse reduced surface salinity in the north-west Atlantic, thereby reducing the formation of intermediate water in the Labrador Sea and of North Atlantic deep water. This in turn may have led to a reduction in the northward transport of heat associated with meridional overturning circulation in the North Atlantic (Clark et al. 2001; Labeyrie et al. 2003; Oldfield 2005). Considerable attention is being paid by Holocene researchers to the 8.2 ka event and other early Holocene abrupt events because they show that a sufficiently large freshwater pulse can rapidly disrupt ocean circulation and climate over a large area when the Earth is in an interglacial climate mode (Alley et al. 2003; Oldfield 2005; Schmidt and LeGrande 2005).
In terms of paradigm shifts, Holocene ice-core research has led to the addition of episodic meltwater input into the North Atlantic in the early Holocene to about 8000 years ago as a major forcing function to the COHMAP internal boundary conditions (e.g. Rensen et al. 2001; Rind et al. 2001).
Air bubbles preserved in the Antarctic Dome and Vostock ice cores provide a means for investigating the concentrations of atmospheric trace-gases (CO2, CH4, N2O) in the Holocene (Raynaud et al. 2003). These indicate an increase of CO2 concentrations about 8000 years ago and an increase in methane concentrations about 5000 years ago. These discoveries have led to the so-called Anthropocene hypothesis proposed by Ruddiman (2003) and Ruddiman and Thomson (2001).
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