The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 140

Meanwhile, back in 1993, the year after Wallace Broecker presented his hypothesis
of the importance of thermohaline circulation change to glacials, fellow Columbia
University researchers, with Gerard Bond and colleagues, presented evidence and
a picture of what might have been happening during the last glacial. It had been
known since the late 1980s that at several distinct times during the last glacial
icebergs had carried moraine (rocks and geological detritus) into the North Atlantic.
There the icebergs melted, releasing the moraine; this can be seen in ocean sed-
iment cores. These events were summarised in 1988 by Hartmut Heinrich. What
Bond and colleagues did was to tie these Heinrich events into a class of glacial
temperature changes within the last glacial (known as Dansgaard-Oeschger cycles)
as revealed by Greenland ice cores. They used a palaeoclimatic bio-indicator, the
abundance of planktic foram Neogloboquadrina pachyderma , in North Atlantic
sediment cores. The emerging theory was that in the northern hemisphere, as the
glacial progressed, the ice sheets grew. As the one over North America (the Lauren-
tide sheet) became thicker it pressed down on the underlying geology and provided
increasing thermal insulation. Geothermal heat and the ice's weight caused basal
melting and the sheet slipped, fracturing to release icebergs into the North Atlantic.
This iceberg armada freshened the Atlantic sea water, so shutting down the Broecker
conveyor and in turn cooling the North Atlantic. (We now know, from the work of
Negre et al. [2010], that the southerly abyssal current actually reversed during glacial
times in addition to - if not as part of - changes in North Atlantic circulation, hence
considerable change in the North Atlantic down-welling (or meridional overturn-
ing circulation [MOC], as it is sometimes called in the academic literature). This
weakening of the MOC then facilitated ice-sheet build-up (and with iceberg flow
temporarily reduced) until the process repeated, while the thermohaline circulation
slowly recovered. There is also evidence to suggest that the presence of a large ice
sheet over northern North America affected air circulation but that ice-sheet collapse
may have allowed the air circulation to return to a pattern similar to today's, which
would have helped the restoration of the conveyor. Meanwhile, with each cycle the
ice sheet grew bigger and although its oceanic margins could see iceberg loss, its
terrestrial margins would not. Similarly the Fennoscandian ice sheet bordering the
sea over northern Europe also discharged icebergs while its terrestrial-bounded edges
obviously did not.
Whereas the Heinrich events and associated climate changes are largely instigated
by northern hemisphere factors, only the larger of such events of the last glacial are
reflected in the southern hemisphere's Antarctic ice-core record; the weaker ones
are not.
Nonetheless, whereas Heinrich events are a major part of northern hemisphere
climatic events and only weakly reflected at the South Pole, they do have a pro-
nounced effect in the tropics in terms of both climate and biology. The Intertropical
Convergence Zone (ITCZ) is a band of rainfall that spans the Atlantic. It migrates
north-south with the seasons. The effect of the ITCZ in our current interglacial is that
it makes a band across Brazil wet and a parallel area to the south dry. This arid zone
of eastern South America currently separates two tropical rainforests, the Amazonian
and the Atlantic forests. In 2004 researchers, led by Xianfeng Wang, reported on a
geological climate record of speleothem (secondary calcium carbonate deposits in
caves formed by running water) and travertine deposits (again, calcium carbonate