Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 184

planktic foraminifera from many sediment cores at various locations at the time of
the Last Glacial Maximum (LGM). When temperatures were calculated using the
Imbrie and Kipp correlations, it was found (to the surprise of many) that tropical
ocean temperatures during the LGM were only about 1.5 C cooler than those
during interglacial times.
As Broecker (2002) pointed out, the full range of ocean temperatures should
be considered. The coldest temperatures in polar regions are constrained by the
freezing point of seawater. Once the temperature drops to 1.8 C, sea ice will
form. That did not change during the LGM. Presuming that the CLIMAP
findings are correct, it would seem that tropical ocean temperatures also did not
change during the LGM. But this does not necessarily mean that mid-latitude sea
surface temperatures did not change during the LGM. In fact, the results indicated
that sea surface temperatures were 2-4 C colder at the LGM for latitudes greater
than about 45 in both the NH and the SH. Broecker concluded: ''taken together
with the temperature change at high elevation, this seems to be telling us that the
Earth's cold sphere moved in on the Earth's warm sphere both from above and
from the poles.''
However, the reported small decrease in tropical sea surface temperatures
during the LGM has been challenged. One study found that the isotope make-up
of planktic species is also dependent on the pH of the ocean, which increases
during glacial times. This would suggest that the actual decrease in the tempera-
ture of tropical seawater was more like 3 C than 1.5 C. Other methods for
paleothermometry have been proposed that also lead to lower tropical sea surface
temperatures, some of them suggesting that they were 2 Cto5 C colder than
today.

5.7 ICE-RAFTED DEBRIS

Bischof (2000) wrote a book on ice-rafted debris as a source of data on past
climates, ocean currents, and prevailing winds. Ice rafting is the drift of floating
ice in the ocean from one place to another:

''Wherever ice forms in contact with the land, whether as icebergs calving
from a glacier that overrode and incorporated bedrock fragments into the ice, or
as sea ice that forms when seawater freezes in contact with unconsolidated
sediments in coastal environments, terrigenous debris from the ice's place of
origin becomes embedded in the ice.''

As floating ice moves with ocean currents and is propelled by winds, the
incorporated particles drop to the sea floor when the ice melts or when icebergs
break up or turn over. Eventually, all the incorporated debris gets deposited on
the ocean floor during the lifetime of the floating ice:

''Over the course of decades, centuries and thousands of years, a continuous
archive of iceberg and sea ice drift has formed in the deep-sea sediments. The