The Ice Age Ocean 243
difficult to even attempt to answer from the land record: each succeeding glaciation
erased many of the traces of the previous one. The study of long cores from the
deep-sea floor opened the door to a new understanding. Such cores (Fig. 9.1), contain, in places, an uninterrupted record of the alternations of glacials and interglacials, within the "ice age".
Why is there an ice age at all? We do not know for sure. Clearly, it is colder during
an ice age than before or after, so that the Earth's heat budget is involved. This budget
is largely controlled by albedo (the amount of incoming light reflected into space)
and by the greenhouse effect (the trapping of infrared trying to leave Earth, by
greenhouse gases). Thus (unless we invoke a dimming of the sun), it seems reasonable to assume that albedo was increased and greenhouse gases were decreased,
during ice ages. We shall see, in the following, that this assumption is correct. Nevertheless, we still do not know the exact mechanisms responsible. Rather than approaching these very difficult questions of climatology, let us ask some quite simple
questions, such as:
1. What did the ocean look like during maximum glaciation?
2. What does the record show about the transition from the last glacial to the present?
3. What is the nature of the ice age cycles, that is, their frequency and amplitude, and
their extent in time?
9.2.2 Conditions in a Cold Ocean. How did the ice age ocean differ from the present
one? The question is by no means settled, but a consensus has been reached on
several aspects.
First, it is generally agreed that surface currents were stronger. It is obvious why
this should be so: surface currents are driven by winds, and winds depend on horizontal temperature gradients. With the ice rim and polar front much closer to the Equator,
the temperature difference between ice (0 DC or less) and the tropics (ca. 25 DC) was
compressed into a much shorter distance than now. Hence the temperature gradient
was greater, winds were stronger, and so were ocean currents.
Equatorial upwelling was intensified as a consequence, as well as coastal upwelling. Thus, at the same time that fertility decreased in high latitudes, due to ice cover,
it increased in mid-latitudes (because of intensified mixing) and in the subtropics
(due to upwelling).
Second, it is accepted that the ocean suiface was cooler, on the whole, than today.
With a substantial part of northern continents and seas under ice, the Earth reflected
the Sun's radiation more readily (had a higher albedo) than today, hence it absorbed
less of the radiation and its atmosphere was cooler. A cold atmosphere holds less
water than a warm one, and large areas on land therefore were drier than today. Water
vapor is the most important greenhouse gas, so that the trapping of infrared radiation
within the atmosphere was diminished. Dry areas (such as grasslands and deserts)
reflect more sunlight than wet ones (such as forests). Also, a more fertile ocean
would be slightly more reflective than a clear dark blue one with less algal growth.
All these factors favored reflection of the Sun's radiation back into space, and hence
favored cooling (Table 9.1).
difficult to even attempt to answer from the land record: each succeeding glaciation
erased many of the traces of the previous one. The study of long cores from the
deep-sea floor opened the door to a new understanding. Such cores (Fig. 9.1), contain, in places, an uninterrupted record of the alternations of glacials and interglacials, within the "ice age".
Why is there an ice age at all? We do not know for sure. Clearly, it is colder during
an ice age than before or after, so that the Earth's heat budget is involved. This budget
is largely controlled by albedo (the amount of incoming light reflected into space)
and by the greenhouse effect (the trapping of infrared trying to leave Earth, by
greenhouse gases). Thus (unless we invoke a dimming of the sun), it seems reasonable to assume that albedo was increased and greenhouse gases were decreased,
during ice ages. We shall see, in the following, that this assumption is correct. Nevertheless, we still do not know the exact mechanisms responsible. Rather than approaching these very difficult questions of climatology, let us ask some quite simple
questions, such as:
1. What did the ocean look like during maximum glaciation?
2. What does the record show about the transition from the last glacial to the present?
3. What is the nature of the ice age cycles, that is, their frequency and amplitude, and
their extent in time?
9.2.2 Conditions in a Cold Ocean. How did the ice age ocean differ from the present
one? The question is by no means settled, but a consensus has been reached on
several aspects.
First, it is generally agreed that surface currents were stronger. It is obvious why
this should be so: surface currents are driven by winds, and winds depend on horizontal temperature gradients. With the ice rim and polar front much closer to the Equator,
the temperature difference between ice (0 DC or less) and the tropics (ca. 25 DC) was
compressed into a much shorter distance than now. Hence the temperature gradient
was greater, winds were stronger, and so were ocean currents.
Equatorial upwelling was intensified as a consequence, as well as coastal upwelling. Thus, at the same time that fertility decreased in high latitudes, due to ice cover,
it increased in mid-latitudes (because of intensified mixing) and in the subtropics
(due to upwelling).
Second, it is accepted that the ocean suiface was cooler, on the whole, than today.
With a substantial part of northern continents and seas under ice, the Earth reflected
the Sun's radiation more readily (had a higher albedo) than today, hence it absorbed
less of the radiation and its atmosphere was cooler. A cold atmosphere holds less
water than a warm one, and large areas on land therefore were drier than today. Water
vapor is the most important greenhouse gas, so that the trapping of infrared radiation
within the atmosphere was diminished. Dry areas (such as grasslands and deserts)
reflect more sunlight than wet ones (such as forests). Also, a more fertile ocean
would be slightly more reflective than a clear dark blue one with less algal growth.
All these factors favored reflection of the Sun's radiation back into space, and hence
favored cooling (Table 9.1).