Page 278 - Earth's Climate Past and Future

254 PART IV • Deglacial Climate Changes

Northern Atlantic Greenland Greenland ice (Figure 14–2). Times of colder air (more
sediment core ice core negative δ O) over Greenland correlate with times of
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% polar species δ 18 O (‰) cold ocean temperatures (larger percentages of the polar
100 60 20 –42 –38 species) in the North Atlantic Ocean. Dating of the
10,000 youngest of the glacial ice core cycles by annual layer
counts and of the ocean cores by the radiocarbon
method (adjusted to calendar years) confirmed that the
20,000 two sequences correlate closely for the part of the record
younger than 30,000 years. The North Atlantic Ocean
surface was cold when the air over Greenland was cold.
30,000
Both records show a similar pattern: repeated slow
drifts toward colder, more glacial conditions followed by
relatively abrupt shifts back to warmer conditions. The
40,000 Major best dated of the ice-rafting events occurred at times
ice-rafting when the climate had been cooling for several millennia,
events and each ice-rafting episode was followed by a rapid
Years ago 50,000 return to warmer temperatures. Some of the cooling
sequences did not culminate in major ice-rafting episodes.
60,000 An initial question was whether the relative increases
in the amount of ice-rafted debris compared to the
foraminifera were caused by faster delivery of ice-rafted
70,000 debris, slower deposition of foraminifera, or both.
Radiocarbon dating (adjusted to calendar years) of the
CaCO shells of foraminifera contained in the younger
3
80,000 ice-rafting layers indicated tenfold or larger increases in
the rate of deposition of ice-rafted debris as well as
smaller decreases (generally by less than half) in the rate
90,000 of deposition of foraminifera.
Another major question was the source or sources of
the ice-rafted debris. Although most of the ice sheets
surrounding the North Atlantic contributed to the
influxes, a large fraction of the grains deposited in the
primary ice-rafting zone at 45°–50°N latitude came
from the northeastern margin of the Laurentide ice
sheet covering North America (Figure 14–3).
Initial investigations of limestone fragments during
the major ice-rafting (Heinrich) events showed that
source rocks in and north of Hudson Bay were the major
source of this debris. Further evidence supporting this
conclusion came from geochemical (isotopic) analysis of
ice-rafted mineral grains that pointed to bedrock sources
FIGURE 14-2 Millennial oscillations in the North Atlantic north and east of Hudson Bay. Detailed sampling of
Ocean Millenial-scale fluctuations in the composition of
North Atlantic foraminifera and in ice-rafting influxes (left) these large ice-rafting events showed, however, that the
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match δ O changes in Greenland ice cores (right). (Modified first debris deposited often came from smaller ice sheets
from S. Stanley, Earth System History, ©1999 by W. H. Freeman and around the North Atlantic Ocean. Only later did the
Company, after G. Bond et al., “Correlations Between Climatic distinctive limestone debris from North America arrive.
Records from North Atlantic Sediments and Greenland Ice,” Much smaller amounts of ice-rafted debris were
Nature 365 [1993]: 143–47.) deposited during the smaller fluctuations. This debris
came from a range of source regions, two of which left
distinctive evidence (Figures 14–3 and 14–4). Fragments
indication of the presence of colder North Atlantic of clear and dark volcanic glass originated mainly
waters carrying larger numbers of icebergs. from eruptions on Iceland. Iron-stained quartz grains
Changes in the percentage of polar foraminifera in came from several regions where outcrops of Pangaean-
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North Atlantic cores generally match δ O changes in age sandstone contain quartz grains stained red by