The Pleistocene Cycles
247
ice surges are one possible mechanism), but it is very likely that the ocean's circulation was involved in a drastic change in the heat budget in some fashion. Somehow,
the influx of tropical heat from the south, which today warms the northern North
Atlantic, was greatly diminished. A cascade of feedback processes then took over
(beginning, for example, with a great expansion of sea ice), plunging the world back
into the ice age, from which it had just escaped. This miniature ice age only lasted
about one millenium and ended abruptly, perhaps within decades.
In the northern Atlantic, a series of distinct layers of the last glacial period rich in
ice-rafted debris and poor in foraminifera suggests the occurrence of other abrupt
climatic events, perhaps triggered by short pulses of massive discharges of icebergs.
Our inability to explain the mysterious Younger Dryas cold spell points out some
fundamental limitations to paleoceanography (and paleoclimatic) research. The climate systems is extremely complex (see Epilog). Many of the important elements of
the system are not well constrained. The ocean's functions (such as redistribution of
heat by currents) cannot be isolated and determined as long as the functions of the
other elements of the system remain uncertain. We know that the ocean plays a
dominant role in the heat budget, not only because of currents, but also because of its
control on albedo (e. g., via sea ice cover) and water vapor (the dominant greenhouse
gas), and also because it controls short-term changes in atmospheric C02 (via its
carbon cycle and the biological pump, (discussed in Sect. 9.4.2).
9.3 The Pleistocene Cycles
9.3.1 The Evidence. The marine geologists who studied the cores recovered by the
Albatross Expedition soon recognized that the Pleistocene record shows a long series
of alternating climatic states. This finding has attained great importance in the earth
sciences, especially in the study of climate dynamics. The cycles express themselves
as fluctuations in faunal and floral composition, in the abundance of carbonate, and
in the ratio of oxygen-IS to oxygen-16 in foraminiferal shells, as well as other
properties (see Fig. 9.4).
9.3.2 The Carbonate Cycles of the equatorial Pacific (first described by G. Arrhenius) are commonly interpreted as dissolution cycles, with high dissolution in
interglacials (low CaC03 values) and low dissolution in glacials (high CaC03
values). Productivity variations (which were once thought to be solely responsible for
the carbonate fluctuations) apparently playa secondary role in producing the cycles.
What is the evidence for this interpretation? The argument is based on the vertical
fluctuation of levels of equal preservation, measured as the degree of shell damage,
above the lysocline. A vertical range of 0.5 to 1 km (as observed) is sufficient to
produce very pronounced carbonate cycles, in depths below 3500 m.
The causes for the dissolution cycles are not clear. Fluctuation of the sea level
must be important. During low sea level stands, shelves - the preferred sites for
carbonate deposition - are exposed. Thus, they cannot receive carbonate. The balance
piles up on the deep-sea floor from deposition of plankton shells. Conversely, during
247
ice surges are one possible mechanism), but it is very likely that the ocean's circulation was involved in a drastic change in the heat budget in some fashion. Somehow,
the influx of tropical heat from the south, which today warms the northern North
Atlantic, was greatly diminished. A cascade of feedback processes then took over
(beginning, for example, with a great expansion of sea ice), plunging the world back
into the ice age, from which it had just escaped. This miniature ice age only lasted
about one millenium and ended abruptly, perhaps within decades.
In the northern Atlantic, a series of distinct layers of the last glacial period rich in
ice-rafted debris and poor in foraminifera suggests the occurrence of other abrupt
climatic events, perhaps triggered by short pulses of massive discharges of icebergs.
Our inability to explain the mysterious Younger Dryas cold spell points out some
fundamental limitations to paleoceanography (and paleoclimatic) research. The climate systems is extremely complex (see Epilog). Many of the important elements of
the system are not well constrained. The ocean's functions (such as redistribution of
heat by currents) cannot be isolated and determined as long as the functions of the
other elements of the system remain uncertain. We know that the ocean plays a
dominant role in the heat budget, not only because of currents, but also because of its
control on albedo (e. g., via sea ice cover) and water vapor (the dominant greenhouse
gas), and also because it controls short-term changes in atmospheric C02 (via its
carbon cycle and the biological pump, (discussed in Sect. 9.4.2).
9.3 The Pleistocene Cycles
9.3.1 The Evidence. The marine geologists who studied the cores recovered by the
Albatross Expedition soon recognized that the Pleistocene record shows a long series
of alternating climatic states. This finding has attained great importance in the earth
sciences, especially in the study of climate dynamics. The cycles express themselves
as fluctuations in faunal and floral composition, in the abundance of carbonate, and
in the ratio of oxygen-IS to oxygen-16 in foraminiferal shells, as well as other
properties (see Fig. 9.4).
9.3.2 The Carbonate Cycles of the equatorial Pacific (first described by G. Arrhenius) are commonly interpreted as dissolution cycles, with high dissolution in
interglacials (low CaC03 values) and low dissolution in glacials (high CaC03
values). Productivity variations (which were once thought to be solely responsible for
the carbonate fluctuations) apparently playa secondary role in producing the cycles.
What is the evidence for this interpretation? The argument is based on the vertical
fluctuation of levels of equal preservation, measured as the degree of shell damage,
above the lysocline. A vertical range of 0.5 to 1 km (as observed) is sufficient to
produce very pronounced carbonate cycles, in depths below 3500 m.
The causes for the dissolution cycles are not clear. Fluctuation of the sea level
must be important. During low sea level stands, shelves - the preferred sites for
carbonate deposition - are exposed. Thus, they cannot receive carbonate. The balance
piles up on the deep-sea floor from deposition of plankton shells. Conversely, during