Page 279 - Earth's Climate Past and Future

CHAPTER 14 • Millennial Oscillations of Climate 255

iron oxidation during the ancient monsoon climates
(Chapter 4). Other regions around the Atlantic margins
also delivered debris during these smaller oscillations.
Long sequences of ocean sediments recovered by
the Ocean Drilling Project show that millennial fluctu-
ations also occurred during previous glaciations, during
both the 41,000-year cycles prior to 0.9 Myr ago and
the subsequent oscillations at ~100,000 years. The
largest millennial oscillations occurred during times
when ice sheets were large; fluctuations were negligible
during interglacial climates.
Main ice-rafted deposition oped in North Atlantic surface waters, scientists
Because the millennial oscillations are so well devel-
searched for evidence that this signal had penetrated
into deep water formed in this region. The method
Volcanic rocks Red sandstones exploited was the same one used to measure similar
Limestones Chemically distinctive rocks changes at orbital scales (Chapter 10): more negative
δ C values in the CaCO shells of bottom-dwelling
13
3
FIGURE 14-3 Sources and deposition of ice-rafted debris (benthic) foraminifera mark times when deep water
Highest rates of deposition of ice-rafted debris occur in the from North Atlantic sources was replaced by bottom
North Atlantic Ocean between 45°and 50°N. During smaller water formed in the Southern Ocean. Because these
ice-rafting episodes, sources of debris include volcanic rocks deep-water δ C signals are measured in the same cores
13
on Iceland and red sandstone rocks on several coastal containing the planktic foraminifera used to monitor
margins. During large ice-rafting events, massive amounts of changes in the surface waters, the relative timing of the
material come from eastern North America, including two kinds of changes can be determined, even without
limestone from Hudson Bay and fragments from other accurate knowledge of absolute ages. Initial explorations
regions with distinctive chemical signatures. (Adapted from
13
of these δ C trends detected millennial oscillations, but
G. Bond et al., “Evidence of Massive Discharges of Icebergs into
the North Atlantic During the Last Glacial Period,” Nature 360 the timing did not match the surface-ocean fluctuations
in a convincing way. We will return to this issue later.
[1992]: 245–49.)

A B C

FIGURE 14-4 Sand-sized grains ice-rafted into the North Atlantic (A) Volcanic debris from
Iceland and (B) red-stained quartz grains from sandstone rocks around the Atlantic margins.
(C) Sources of red-stained quartz grains include red sandstones from the Orkney Islands, off
northern Scotland. (A and B: courtesy of G. Bond, Lamont-Doherty Earth Observatory of Columbia
University. C: John Forbes/PEP.)