Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 191

high concentrations at the end of the Younger Dryas/beginning of post-glacial
conditions to the lowest values at the end of the Holocene. The record for this
same parameter in the GISP2 core shows the high concentrations at the end of
the Younger Dryas with only a slight decrease in concentrations through the
Holocene, except for the last few hundred years. In the case of the GISP2
record, sea salt Na þ concentrations increase noticeably during recent centu-
ries. Thus, the two ice cores show a difference in how they record the climatic
change. Because ice-core records from the Summit region of Greenland
reflect overall hemispheric conditions, the increased sea salt Na þ over the
last few hundred years probably reflects the cold conditions, more extensive
sea ice, and more vigorous NH circulation during the LIA. On the other hand,
the source area for sea salt reaching the Penny Ice Cap is probably more local,
thus the decrease in sea salt Na þ probably reflects greater sea ice in the Baffin
Bay and Labrador Sea area during the LIA. Consequently, records from these
smaller ice caps in the Arctic, such as the Penny Ice Cap (e.g. Fisher et al.
1998 ) better preserve regional climatic conditions than the Greenland
records. In the case of the Penny Ice Cap record, there appears to be an
overall general cooling through the Holocene, just as the Agassiz Ice Cap
melt record suggests.
Other records
Evidence for these cyclical changes in post-glacial climate are found in other
highly resolved records from the NH lending support to the conclusion that
the Greenland ice-core records reflect hemispheric conditions. As shown in
Figure 6.3 , the GISP2 ice-core record matches the known glacial record;
however, as noted in the figure, many of these glacial records are discontin-
uous as they were primarily based on morainal deposits. Consequently,
establishing that the periodicity of these Holocene climatic shifts is hemi-
spheric requires additional highly resolved and well-dated records. Several of
these records are now presented as summarized by Mayewski et al.( 2004 ).
In addition to the ice-core record, the most significant evidence for millen-
nial-scale cycles of Holocene climatic conditions comes from the marine
sediment record (Figure 6.4 , records g-j; Bond et al. 1997 ). The evidence for
such changes is found in radiocarbon-dated sediment cores from the North
Atlantic that reflect ice-rafting episodes (Figure 6.4 , record j). Ice-rafted
debris (IRD) consists of various types of sediment found on the ocean floor
that could have only reached their final depositional site, because of their
large particle size, via drifting icebergs. As icebergs rotate, overturn, and
eventually melt completely in their southward journey from glaciated areas
surrounding the northernmost Atlantic Ocean, they release this debris.
Specific indicators used to mark the presence of IRD are sections of core
containing high concentrations of lithic grains (Figure 6.4 , record g), high