Geoscience Reference
In-Depth Information
atmosphere from the mid-1940s through early 1960s and even from the Chernobyl
reactor accident. One of the few positive impacts of these testaments to human folly
is that the radioactive layers serve as useful stratigraphic markers.
10.2.4
Marine Sediment Cores
Ocean basins accumulate sediments - mixes of continentally derived material and
microscopic marine organisms that rain out, or live and die in the sediments and
accumulate year after year in layers. Oxygen isotope records are also preserved in
ocean cores. These come from the remains of calcareous organisms (planktonic and
benthic foraminifera) preserved in sediments.
The oxygen isotopic composition of the bodies of these organisms is determined
by two primary factors. The first is the isotopic composition of the ocean water.
During the buildup of continental ice sheets, water is transferred via the hydro-
logic cycle (evaporation, condensation and precipitation) from the oceans to land,
where it is stored for long periods as ice. Because the heavier
18
O evaporates less
readily than
16
O,
16
O is preferentially removed from the oceans. The oceans hence
become enriched in
18
O. The isotopic compositions of foraminifera preserved in
marine sediments are therefore primary interpreted as records of terrestrial ice vol-
ume. The second primary factor influencing foraminifera is the temperature of the
water. For every 1
o
C drop in ocean temperature, there is a relative enrichment in the
faunal debris of 0.02 o/oo
18
O. A correction for this has to be made so as to infer
information about terrestrial ice volume.
Like ice cores, ocean cores contain many different types of information about
the past. Different species of planktonic and benthic foraminifera thrive in differ-
ent types of environmental conditions, so changes in the abundance of different
taxa through time record environmental changes in the uppermost and deep ocean,
respectively. For example, the relative abundance (as a percentage of all forams
present) of left-coiling (sinistral)
N. Pachyderma
(a planktonic foram that lives in
cold water) yields information on SST. Ocean cores also yield information from
inorganic markers, such as detritus carried and dropped by icebergs.
10.3
Features of the Quaternary
10.3.1
General Timeline
How many major ice advances and retreats occurred over the Quaternary is unclear
and there are discrepancies to be resolved between continental and deep-sea chro-
nologies. Ice-rafted detritus (IRD) in marine sediments dated to around 2.6 Ma
marks the initiation of glaciation at sea level in the circum-North Atlantic region
(Ehlers and Gibbard,
2003
). S. Harris (
2001
) suggests that four major cold events
during the Pliocene and eleven during the Quaternary affected the Cordillera of
North America (including those of Alaska and the Yukon). One of the remarkable
