10
David THISTLE
few shells that are produced and fall to the seafloor are
dissolved away. As a result, the sediment (abyssal red
clay) consists of terrigenous particles. Accumulation
rates are low, c. 0.5 mm per thousand years.
Where productivity is high, the production rate of
both siliceous and calcium carbonate shells is high.
If the water is deep, the calcium carbonate shells
that reach the seafloor dissolve. The sediment will
be composed of terrigenous and siliceous particles, a
diatomaceous or a radiolarian ooze. For example, a
radiolarian ooze occurs under the band of high productivity along the equator in the Pacific. Some productive
regions occur where the underlying water is relatively
shallow. In these regions, the rate of calcium carbonate
dissolution is much reduced, and foraminiferan and
coccolithophorid oozes occur (e.g., along most of
the Mid-Atlantic Ridge) because production by these
plankters is greater than that by those producing silica
shells. Biological oozes accumulate at a relatively rapid
rate of centimeters per thousand years. Near continents,
the supply of terrestrial particles overwhelms that of
biological particles, and biological oozes do not form.
Accumulation rates vary, but they are higher than for
biological oozes.
A substantial portion of the surface area of soft
bottoms can be occupied by pebble- to cobble-sized
manganese nodules. Manganese nodules are accretions
of metals (mostly iron and manganese) that grow
slowly (~1 mm per 10 000 y). They occur in a few
regions of the deep Atlantic, but widely in the deep
Pacific, particularly beneath the central gyres. At their
most abundant, nodules can almost completely cover
the surface of the seabed.
Large-scale processes control sediment composition,
so it tends to be uniform over hundreds of square kilometers. At the spatial scale at which most individual
organisms experience their environment (millimeters
to meters), the seafloor is made heterogeneous by
two processes. The organisms themselves structure the
seafloor by building tubes, tests, and mudballs in which
to live (Fig. 2.6). These structures are used by other
organisms as habitat (Thistle and Eckman, 1990). The
second process is small-scale disturbance that creates
patchiness in the deep-sea floor – in, for example,
species composition, sediment texture, and food content (Grassle and Sanders, 1973; Grassle and MorsePorteous, 1987). Where they occur, manganese nodules
Fig. 2.6. Some representative organism-constructed structures from
deep-sea soft-bottom habitats. A. Empty test of the foraminiferan
Oryctoderma sp., which is inhabited by a polychaete. B. and
C. Foraminifers (the dashed line indicates the surface of the
sediment). Scale lines equal 1.0 mm. Modified from Thistle (1979).
Reproduced with permission of Plenum Press.
impose a third type of small-scale heterogeneity on the
surrounding soft bottom.
Environmental variation in geologic time
The preceding description of physical conditions in
the deep sea applies to the modern ocean, but an
understanding of modern deep-sea communities cannot
be achieved without the incorporation of a historical
perspective, because environmental changes at many
time scales have helped to shape the present fauna. For
example, since the early Eocene (~54 Ma BP)
2 , deepwater temperatures have decreased from about 12ºC
to their present values (Flower and Kennett, 1994) in
four major cooling phases, in the early Middle Eocene,
Late Eocene, Late Miocene, and Plio-Pleistocene
(Lear et al., 2000). These abrupt temperature changes
have been correlated with changes in the deep-sea
fauna. For example, the sharp drop at the Eocene–
Oligocene boundary (~38 Ma BP) is correlated with
large changes in the benthic foraminifer (Kennett,
1982) and ostracod (Benson et al., 1984) assemblages.
Within the Pliocene (2.85–2.40 Ma BP), bottom-water
temperatures varied by 2ºC on a 40 000-yr time
scale in the North Atlantic, as glaciers advanced and
2 1 Ma = 10 6 years.
David THISTLE
few shells that are produced and fall to the seafloor are
dissolved away. As a result, the sediment (abyssal red
clay) consists of terrigenous particles. Accumulation
rates are low, c. 0.5 mm per thousand years.
Where productivity is high, the production rate of
both siliceous and calcium carbonate shells is high.
If the water is deep, the calcium carbonate shells
that reach the seafloor dissolve. The sediment will
be composed of terrigenous and siliceous particles, a
diatomaceous or a radiolarian ooze. For example, a
radiolarian ooze occurs under the band of high productivity along the equator in the Pacific. Some productive
regions occur where the underlying water is relatively
shallow. In these regions, the rate of calcium carbonate
dissolution is much reduced, and foraminiferan and
coccolithophorid oozes occur (e.g., along most of
the Mid-Atlantic Ridge) because production by these
plankters is greater than that by those producing silica
shells. Biological oozes accumulate at a relatively rapid
rate of centimeters per thousand years. Near continents,
the supply of terrestrial particles overwhelms that of
biological particles, and biological oozes do not form.
Accumulation rates vary, but they are higher than for
biological oozes.
A substantial portion of the surface area of soft
bottoms can be occupied by pebble- to cobble-sized
manganese nodules. Manganese nodules are accretions
of metals (mostly iron and manganese) that grow
slowly (~1 mm per 10 000 y). They occur in a few
regions of the deep Atlantic, but widely in the deep
Pacific, particularly beneath the central gyres. At their
most abundant, nodules can almost completely cover
the surface of the seabed.
Large-scale processes control sediment composition,
so it tends to be uniform over hundreds of square kilometers. At the spatial scale at which most individual
organisms experience their environment (millimeters
to meters), the seafloor is made heterogeneous by
two processes. The organisms themselves structure the
seafloor by building tubes, tests, and mudballs in which
to live (Fig. 2.6). These structures are used by other
organisms as habitat (Thistle and Eckman, 1990). The
second process is small-scale disturbance that creates
patchiness in the deep-sea floor – in, for example,
species composition, sediment texture, and food content (Grassle and Sanders, 1973; Grassle and MorsePorteous, 1987). Where they occur, manganese nodules
Fig. 2.6. Some representative organism-constructed structures from
deep-sea soft-bottom habitats. A. Empty test of the foraminiferan
Oryctoderma sp., which is inhabited by a polychaete. B. and
C. Foraminifers (the dashed line indicates the surface of the
sediment). Scale lines equal 1.0 mm. Modified from Thistle (1979).
Reproduced with permission of Plenum Press.
impose a third type of small-scale heterogeneity on the
surrounding soft bottom.
Environmental variation in geologic time
The preceding description of physical conditions in
the deep sea applies to the modern ocean, but an
understanding of modern deep-sea communities cannot
be achieved without the incorporation of a historical
perspective, because environmental changes at many
time scales have helped to shape the present fauna. For
example, since the early Eocene (~54 Ma BP)
2 , deepwater temperatures have decreased from about 12ºC
to their present values (Flower and Kennett, 1994) in
four major cooling phases, in the early Middle Eocene,
Late Eocene, Late Miocene, and Plio-Pleistocene
(Lear et al., 2000). These abrupt temperature changes
have been correlated with changes in the deep-sea
fauna. For example, the sharp drop at the Eocene–
Oligocene boundary (~38 Ma BP) is correlated with
large changes in the benthic foraminifer (Kennett,
1982) and ostracod (Benson et al., 1984) assemblages.
Within the Pliocene (2.85–2.40 Ma BP), bottom-water
temperatures varied by 2ºC on a 40 000-yr time
scale in the North Atlantic, as glaciers advanced and
2 1 Ma = 10 6 years.