Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 193

percentages of fresh volcanic glass from Iceland or Jan Mayen (Figure 6.4 ,
record j), and high percentages of hematitic-stained grains (mostly quartz and
feldspar) that originate from sedimentary deposits containing red beds, such
as on Svalbard and in the eastern Canadian Arctic (Figure 6.4 , record i).
Although the presence of IRD in North Atlantic sediment cores has been used
in the evaluation of rapid climatic change events during the last glacial, the
work of Bond et al.( 1997 ) shows that such events occurred throughout the
Holocene, albeit of a lesser magnitude than equivalent-type events during the
last glacial, as is the case for the ice-core records (Figure 6.1 ).
Bond et al.( 1997 ) suggests that there were abrupt reorganizations in
climate and ocean circulation in the North Atlantic to the point of bringing
large numbers of icebergs into areas of the North Atlantic that do not see them
today. Peaks in these periods of presumably cool conditions occur at 11 100,
10 300, 9400, 8100, 5900, 4200, 2800 and 1400 years ago. Any younger
events would not be recorded because that part of the record was not collected
in the sediment cores used in their study. The findings of Bond et al. suggest
that there are periods of ocean surface cooling at an approximately millennial
cyclicity (Figure 6.4 , record h). Using the presence of planktonic foraminifera
in the same layers that IRD was found suggests that the amount of cooling
during these colder periods of the Holocene did not exceed 2 8C or 15 to 20%
of the full Holocene to glacial temperature change. Nevertheless, the south-
ward migration of ice-bearing surface waters would be on the order of 58 of
latitude further south than present, thus penetrating into what is now the warm
North Atlantic Current. The good agreement in the timing of peaks in cooler
ocean surface temperatures with the time-series and periodicity of peaks in
various glaciochemical species from the GISP2 ice core (Figure 6.4 ) suggest
that Holocene changes in atmosphere and oceanic systems in the North
Atlantic were coupled. Because this approximately 1500-year periodicity of
climate change is found in other Holocene records, there is apparently a
millennial-scale control within our climate system. Changes in NH climate
during post-glacial times should, therefore, be highly influenced by this cycle
of climatic change. However, the cause of this cyclicity is not known.
Given the ice-core and marine sediment records just discussed, there are
general time periods in the Holocene that appear to reflect millennial-scale
changes between overall cooler conditions with more vigorous circulation
patterns and overall warmer conditions with less vigorous circulation pat-
terns. During the cooler periods, atmospheric conditions were generally
dustier, whereas the NH atmosphere was less dusty during the warmer
periods. Timing of this cyclicity appears to be between about 1500 and
2500 years (i.e. millennial-scale changes).
Further evidence of the periodic fluctuation of climatic conditions during
the Holocene comes from several other time series from terrestrial sources.