306
Carol T. STUART et al.
glacial advances. Throughout the time sequence, H
is
negatively correlated with oxygen isotope values and
positively correlated with Mg:Ca ratios. Even the magnitude of the change in diversity appears to match the
extent of glaciation. Severe glacial episodes signaled
by unusually heavy oxygen isotope values (e.g., those
at 2.42, 2.46, 2.5 and 2.7 million years) coincide with
some of the lowest values of diversity. Interestingly, the
variation in diversity is not a function of origination
and extinction of species. Species periodically lost
to the assemblage during glacial phases persist in
cores taken elsewhere in the Atlantic. and evidently
re-invade during interglacial phases. No new species
appeared. Cronin and Raymo (1997) suggested that
fluctuations in diversity do not result from changes in
bottom temperature per se or in raw nutrients (e.g.,
nitrogen and phosphorus). They proposed that changes
in primary production in surface waters and in the
consequent flux of organic material to the deep sea
that accompany the advance and retreat of ice sheets
provide the most plausible explanation. The data clearly
reveal that deep-sea diversity on local spatial scales
is not constant on millennial time scales, and that
diversity responded to cycles of Pliocene glaciation in
a highly repeatable way.
Glacial episodes also must have directly affected the
upper bathyal ecosystem. Studies on stable isotopes
in benthic foraminifera, fossil coral-reef terraces and
hydrographic models indicate five major low sea-level
stands during the last 500 000 years which were more
than 100 meters below present sea level (Rohling
et al., 1998). The last glacial maximum, 20 000 years
ago, depressed sea levels by about 120 meters.
Earlier glacial maxima during the Pleistocene may
have resulted in even lower levels. Such dramatically
lower sea levels would have eliminated much of the
productive neritic region, extended the euphotic zone
to the upper continental slope and increased the impact
of terrestrial and sea-surface climatic variation on the
upper reaches of the deep sea. All of these factors
probably acted to increase the temporal and spatial
heterogeneity of the upper slope. Active formation of
submarine canyons and downslope transport during
the glacial era must have greatly influenced deepsea ecosystems near continents (Emery and Uchupi,
1972; Gage and Tyler, 1991). The Late Pleistocene
was also a time of massive megaturbidite
1 formation
(Rothwell et al., 1998). These catastrophic sediment
slumps must have drastically altered the deep seascape
and obliterated natural communities over huge areas.
Thomas and Gooday (1996) have analysed changes
in the diversity of foraminiferans from seabed cores
over much larger scales of time and space. They
compared the number of species (rarefied to 100 individuals) in cores representing much of the Cenozoic
taken in the Weddell Sea off Antarctica and in the
equatorial Pacific. The most interesting pattern to
emerge, for the present discussion, is the divergence
of diversity between polar and equatorial sites that
arose in the Eocene at around 40 million years ago
(Fig. 10.8). This was the beginning of a period of
global cooling, when continental ice sheets developed
in Antarctica; the transition from the “greenhouse
world” of the Paleocene to the “icehouse world” that
persists to the present. The effects of this major
global change in surface climate affected the deep-sea
environment as well; indeed, there is growing evidence
of a strong interaction between surface climatology
and the deep thermohaline circulation (e.g., MacLeod
and Huber, 1996). Figure 10.8 shows the initial
establishment and persistence of a latitudinal gradient
in species diversity of deep-sea foraminiferans between
the tropics and polar regions. Since the formation
of ice cover in Antarctica in the late Eocene, polar
diversity has declined and tropical diversity remained
continuously higher. The low-diversity communities
at high latitudes became dominated by opportunistic
foraminiferan species exploiting sinking phytodetritus
as a food resource. Because of this, Thomas and
Gooday (1996) suggested that the seasonality in
organic enrichment attending global cooling and the
formation of an Antarctic ice sheet may have had
an important role in creating latitudinal gradients of
species diversity.
Rapidly expanding research in paleoclimatology
and paleoceanography, particularly during the past
decade, is revolutionizing understanding of how major
ecosystems function, change and interact. While many
challenges remain (Charles, 1998), it seems clear
already that the deep-sea environment is linked to
climate cycles, both during long phases of gradual
temperature change and on surprisingly short time
scales during Quaternary glaciation. Environmental
changes on long and short time scales appear to affect
community structure in the deep sea, though it would
be premature to speculate on their general importance
1 See Glossary, p. 477.
Carol T. STUART et al.
glacial advances. Throughout the time sequence, H
is
negatively correlated with oxygen isotope values and
positively correlated with Mg:Ca ratios. Even the magnitude of the change in diversity appears to match the
extent of glaciation. Severe glacial episodes signaled
by unusually heavy oxygen isotope values (e.g., those
at 2.42, 2.46, 2.5 and 2.7 million years) coincide with
some of the lowest values of diversity. Interestingly, the
variation in diversity is not a function of origination
and extinction of species. Species periodically lost
to the assemblage during glacial phases persist in
cores taken elsewhere in the Atlantic. and evidently
re-invade during interglacial phases. No new species
appeared. Cronin and Raymo (1997) suggested that
fluctuations in diversity do not result from changes in
bottom temperature per se or in raw nutrients (e.g.,
nitrogen and phosphorus). They proposed that changes
in primary production in surface waters and in the
consequent flux of organic material to the deep sea
that accompany the advance and retreat of ice sheets
provide the most plausible explanation. The data clearly
reveal that deep-sea diversity on local spatial scales
is not constant on millennial time scales, and that
diversity responded to cycles of Pliocene glaciation in
a highly repeatable way.
Glacial episodes also must have directly affected the
upper bathyal ecosystem. Studies on stable isotopes
in benthic foraminifera, fossil coral-reef terraces and
hydrographic models indicate five major low sea-level
stands during the last 500 000 years which were more
than 100 meters below present sea level (Rohling
et al., 1998). The last glacial maximum, 20 000 years
ago, depressed sea levels by about 120 meters.
Earlier glacial maxima during the Pleistocene may
have resulted in even lower levels. Such dramatically
lower sea levels would have eliminated much of the
productive neritic region, extended the euphotic zone
to the upper continental slope and increased the impact
of terrestrial and sea-surface climatic variation on the
upper reaches of the deep sea. All of these factors
probably acted to increase the temporal and spatial
heterogeneity of the upper slope. Active formation of
submarine canyons and downslope transport during
the glacial era must have greatly influenced deepsea ecosystems near continents (Emery and Uchupi,
1972; Gage and Tyler, 1991). The Late Pleistocene
was also a time of massive megaturbidite
1 formation
(Rothwell et al., 1998). These catastrophic sediment
slumps must have drastically altered the deep seascape
and obliterated natural communities over huge areas.
Thomas and Gooday (1996) have analysed changes
in the diversity of foraminiferans from seabed cores
over much larger scales of time and space. They
compared the number of species (rarefied to 100 individuals) in cores representing much of the Cenozoic
taken in the Weddell Sea off Antarctica and in the
equatorial Pacific. The most interesting pattern to
emerge, for the present discussion, is the divergence
of diversity between polar and equatorial sites that
arose in the Eocene at around 40 million years ago
(Fig. 10.8). This was the beginning of a period of
global cooling, when continental ice sheets developed
in Antarctica; the transition from the “greenhouse
world” of the Paleocene to the “icehouse world” that
persists to the present. The effects of this major
global change in surface climate affected the deep-sea
environment as well; indeed, there is growing evidence
of a strong interaction between surface climatology
and the deep thermohaline circulation (e.g., MacLeod
and Huber, 1996). Figure 10.8 shows the initial
establishment and persistence of a latitudinal gradient
in species diversity of deep-sea foraminiferans between
the tropics and polar regions. Since the formation
of ice cover in Antarctica in the late Eocene, polar
diversity has declined and tropical diversity remained
continuously higher. The low-diversity communities
at high latitudes became dominated by opportunistic
foraminiferan species exploiting sinking phytodetritus
as a food resource. Because of this, Thomas and
Gooday (1996) suggested that the seasonality in
organic enrichment attending global cooling and the
formation of an Antarctic ice sheet may have had
an important role in creating latitudinal gradients of
species diversity.
Rapidly expanding research in paleoclimatology
and paleoceanography, particularly during the past
decade, is revolutionizing understanding of how major
ecosystems function, change and interact. While many
challenges remain (Charles, 1998), it seems clear
already that the deep-sea environment is linked to
climate cycles, both during long phases of gradual
temperature change and on surprisingly short time
scales during Quaternary glaciation. Environmental
changes on long and short time scales appear to affect
community structure in the deep sea, though it would
be premature to speculate on their general importance
1 See Glossary, p. 477.