The Pleistocene Cycles
249
Pacific, through "basin-basin fractionation" with the Atlantic (Sect. 7.6.5). Whenever
North Atlantic Deep Water (NADW) production is increased, the Atlantic traps the
carbonate, leaving less for the Pacific.
On much of the Atlantic sea floor, carbonate cycles tend to swing counter to those
in the central Pacific. The Atlantic carbonate cycles are largely dilution cycles, although effects from varying dissolution and production must also be considered.
During glacials, the supply of terrigenous materials from the continents surrounding
the Atlantic is greatly increased. In high latitudes, the moving ice grinds up enormous
masses of rock. The plant cover, which protects soil from erosion, is less dense. In the
sUbtropics, deserts are widespread, delivering dust. Flash floods in semi-arid regions
are efficient conveyors of huge amounts of material. The tropical rainforest is much
reduced, and semi-arid regions are expanded. The shelves are exposed, and subject to
erosion. All these factors contribute to the glacial increase of terrigenous deposition
rates. As the supply of terrigenous material increases, of course, the proportion of
carbonate in pelagic sediments decreases accordingly - the carbonate is diluted. By
changing the degree of dilution, carbonate cycles are produced.
9.3.3 The Faunal (and Floral) Cycles can be expressed in various ways, most
commonly as warm-cold cycles. Examples are the quantitative plots of Parker (1958)
and Imbrie and Kipp (1971). Parker's method contrasts the relative abundance of
warm-water and cold-water planktonic foraminifers as a function of core depth. The
Imbrie-Kipp method calibrates the warm- and cold-water percentages against surface
water temperature, by a statistical technique called factor regression. Based on this
calibration, the most probable temperatures of surface waters are then calculated for
samples deeper in the core. This technique was also used for constructing the 18 K
map (Fig. 9.2). We see that there were considerable temperature variations in the
Caribbean. We also see that the last change from cold to warm occurred about 11 000
years ago, with the beginning of the Holocene. The faunal cycles rather closely
follow the associated oxygen isotopes, without, however, duplicating them. In principle, comparison of the two curves allows the separation of temperature effect from
ice effect, in the oxygen isotope record, to be discussed next.
There is one other interesting fact that emerges from the faunal analysis: the
present time is rather unusual in being so warm. For the last one half million years or
so the climate was mostly much colder.
9.3.4 Oxygen Isotope Cycles. The fluctuations in the oxygen isotope composition of
foraminiferal shells were first described by C. Emiliani in his classic paper Pleistocene temperatures, published in 1955. He analyzed the planktonic foraminifera
from several long cores taken in the Caribbean and North Atlantic. He concentrated
on those species with the lowest oxygen-18 (Globigerinoides ruber and Globigerinoides sacculifer), reasoning that these species must live in shallow water and therefore reflect surface water temperature. Since the temperture of growth affects the
180/160 ratio (Sect. 7.3.2), the isotopic fluctuations reflect warm-cold cycles. In
addition, the composition of seawater controls the 18 0/ 16 0 ratios in the shells. The
seawater composition fluctuates with the waxing and waning of the continental ice
249
Pacific, through "basin-basin fractionation" with the Atlantic (Sect. 7.6.5). Whenever
North Atlantic Deep Water (NADW) production is increased, the Atlantic traps the
carbonate, leaving less for the Pacific.
On much of the Atlantic sea floor, carbonate cycles tend to swing counter to those
in the central Pacific. The Atlantic carbonate cycles are largely dilution cycles, although effects from varying dissolution and production must also be considered.
During glacials, the supply of terrigenous materials from the continents surrounding
the Atlantic is greatly increased. In high latitudes, the moving ice grinds up enormous
masses of rock. The plant cover, which protects soil from erosion, is less dense. In the
sUbtropics, deserts are widespread, delivering dust. Flash floods in semi-arid regions
are efficient conveyors of huge amounts of material. The tropical rainforest is much
reduced, and semi-arid regions are expanded. The shelves are exposed, and subject to
erosion. All these factors contribute to the glacial increase of terrigenous deposition
rates. As the supply of terrigenous material increases, of course, the proportion of
carbonate in pelagic sediments decreases accordingly - the carbonate is diluted. By
changing the degree of dilution, carbonate cycles are produced.
9.3.3 The Faunal (and Floral) Cycles can be expressed in various ways, most
commonly as warm-cold cycles. Examples are the quantitative plots of Parker (1958)
and Imbrie and Kipp (1971). Parker's method contrasts the relative abundance of
warm-water and cold-water planktonic foraminifers as a function of core depth. The
Imbrie-Kipp method calibrates the warm- and cold-water percentages against surface
water temperature, by a statistical technique called factor regression. Based on this
calibration, the most probable temperatures of surface waters are then calculated for
samples deeper in the core. This technique was also used for constructing the 18 K
map (Fig. 9.2). We see that there were considerable temperature variations in the
Caribbean. We also see that the last change from cold to warm occurred about 11 000
years ago, with the beginning of the Holocene. The faunal cycles rather closely
follow the associated oxygen isotopes, without, however, duplicating them. In principle, comparison of the two curves allows the separation of temperature effect from
ice effect, in the oxygen isotope record, to be discussed next.
There is one other interesting fact that emerges from the faunal analysis: the
present time is rather unusual in being so warm. For the last one half million years or
so the climate was mostly much colder.
9.3.4 Oxygen Isotope Cycles. The fluctuations in the oxygen isotope composition of
foraminiferal shells were first described by C. Emiliani in his classic paper Pleistocene temperatures, published in 1955. He analyzed the planktonic foraminifera
from several long cores taken in the Caribbean and North Atlantic. He concentrated
on those species with the lowest oxygen-18 (Globigerinoides ruber and Globigerinoides sacculifer), reasoning that these species must live in shallow water and therefore reflect surface water temperature. Since the temperture of growth affects the
180/160 ratio (Sect. 7.3.2), the isotopic fluctuations reflect warm-cold cycles. In
addition, the composition of seawater controls the 18 0/ 16 0 ratios in the shells. The
seawater composition fluctuates with the waxing and waning of the continental ice