264 Paleoceanography - the Deep-Sea Record
ocean would have lowered the catbon dioxide content of the atmosphere, preparing
the way for the subsequent build-up of ice.
Besides internal feedback mechanisms favoring cooling and ice growth on Antarctica (albedo increase and a lowering of atmospheric C02), we again have to consider
rearrangements in geography. In the case of the middle Miocene, it is the Indonesian
Seaway that closes some time between 20 and 10 million years ago. It is not clear,
however, how the closing of this passage would have contributed to the cooling.
A much discussed question is to what extent the shift in 0 18 0 toward heavier
values (Fig. 9.llb, MM) reflects cooling, and how much is due to ice buildup. Ice
rafting off Antarctica (as seen in "ice-rafted debris") apparently only sets in "in
earnest" towards the end of the middle Miocene. At that time, one surmises, significant glacial ice growth began on Antarctica. Such buildup could have been responsible for roughly 50 m drop in sea level, beyond the general regression from uplift in
Asia and elsewhere, which is reflected in strontium isotope stratigraphy.
9.5.6 The End-of-Eocene Cooling. Going back in time, we skip over the enigmatic
Oligocene, with its low plankton diversity and its strange Braarudosphaera blooms,
to take a closer look at the late Eocene and the Eocene-Oligocene boundary. On the
very first leg of the Glomar Challenger, the Eocene posed a major puzzle: the drill bit
hit chert in the western North Atlantic, at the level of a prominent seismic reflector
(see Sects. 3.6.2 and 8.6.4). Later, similar beds were found in many parts of the
ocean. In fact, only since the late Eocene (that is, the last 40 million years) do pelagic
sediments look "modem". From then on they have a facies distribution similar to that
outlined in Chapter 8.1.
In essence, Eocene sedimentary facies are less clearly differentiated from each
other than post-Eocene ones. The chemical fractionation machine of the ocean was
less efficient, both because ocean basins communicated more easily than now, and
because there was less of a temperature gradient driving the machine (that is, the
winds were weak, and hence the upwelling that concentrates organic-rich sediments,
silica, and phosphate in the margins). This situation changed towards the end of the
Eocene: the machine turned on. The polar regions cooled, presumably due to thermal
isolation of the Arctic and Antarctic and due to positive feedback from snow cover
and changing vegetation (increase of albedo, Sect. 9.2.2). The polar front migrated
equator-ward, pushing a late Eocene rain belt (and thus the temperature regions)
away from Antarctic shores. We see the effect on the oxygen isotope values in high
latitudes: they quickly move to higher values during the late Eocene, with an especially dramatic change at the Eocene-Oligocene boundary (Fig. 9.11b, EO). This is
the point at which the water in high latitude shelves is cold enough, and saline
enough, to sink and fill the deep ocean basins.
Whatever the cause - most likely internal feedback processes set in motion by
overall regression - the late Eocene cooling irreversible initiated a new type of ocean,
one where strong asymmetries began to develop between north and south, between
Atlantic and Pacific, and between margins and deep sea.
ocean would have lowered the catbon dioxide content of the atmosphere, preparing
the way for the subsequent build-up of ice.
Besides internal feedback mechanisms favoring cooling and ice growth on Antarctica (albedo increase and a lowering of atmospheric C02), we again have to consider
rearrangements in geography. In the case of the middle Miocene, it is the Indonesian
Seaway that closes some time between 20 and 10 million years ago. It is not clear,
however, how the closing of this passage would have contributed to the cooling.
A much discussed question is to what extent the shift in 0 18 0 toward heavier
values (Fig. 9.llb, MM) reflects cooling, and how much is due to ice buildup. Ice
rafting off Antarctica (as seen in "ice-rafted debris") apparently only sets in "in
earnest" towards the end of the middle Miocene. At that time, one surmises, significant glacial ice growth began on Antarctica. Such buildup could have been responsible for roughly 50 m drop in sea level, beyond the general regression from uplift in
Asia and elsewhere, which is reflected in strontium isotope stratigraphy.
9.5.6 The End-of-Eocene Cooling. Going back in time, we skip over the enigmatic
Oligocene, with its low plankton diversity and its strange Braarudosphaera blooms,
to take a closer look at the late Eocene and the Eocene-Oligocene boundary. On the
very first leg of the Glomar Challenger, the Eocene posed a major puzzle: the drill bit
hit chert in the western North Atlantic, at the level of a prominent seismic reflector
(see Sects. 3.6.2 and 8.6.4). Later, similar beds were found in many parts of the
ocean. In fact, only since the late Eocene (that is, the last 40 million years) do pelagic
sediments look "modem". From then on they have a facies distribution similar to that
outlined in Chapter 8.1.
In essence, Eocene sedimentary facies are less clearly differentiated from each
other than post-Eocene ones. The chemical fractionation machine of the ocean was
less efficient, both because ocean basins communicated more easily than now, and
because there was less of a temperature gradient driving the machine (that is, the
winds were weak, and hence the upwelling that concentrates organic-rich sediments,
silica, and phosphate in the margins). This situation changed towards the end of the
Eocene: the machine turned on. The polar regions cooled, presumably due to thermal
isolation of the Arctic and Antarctic and due to positive feedback from snow cover
and changing vegetation (increase of albedo, Sect. 9.2.2). The polar front migrated
equator-ward, pushing a late Eocene rain belt (and thus the temperature regions)
away from Antarctic shores. We see the effect on the oxygen isotope values in high
latitudes: they quickly move to higher values during the late Eocene, with an especially dramatic change at the Eocene-Oligocene boundary (Fig. 9.11b, EO). This is
the point at which the water in high latitude shelves is cold enough, and saline
enough, to sink and fill the deep ocean basins.
Whatever the cause - most likely internal feedback processes set in motion by
overall regression - the late Eocene cooling irreversible initiated a new type of ocean,
one where strong asymmetries began to develop between north and south, between
Atlantic and Pacific, and between margins and deep sea.