Tertiary Oceans: the Cooling Planet 257
Changes in the sites of carbonate production also must have been important.
During times of low sea level, catbonate deposition would have been greatly decreased on the shelves, and this (together with erosion) would have increased the
carbonate ion content of the ocean, and hence the "alkalinity" of the ocean. In tum,
increased alkalinity allows the ocean to retain a larger share of the total C02 in the
system.
9.4.3 The Long-Range View. On long time scales, not just the ocean's exchange
with the atmosphere has to be considered, but also the weathering of silicates on land,
and the input of C02 from volcanism. The weathering process can be summarily
described by the formula
C02 + CaSi03 ~ CaC03 + Si02 ,
(9.1)
which shows the long-term uptake of C02 from trhe atmosphere. The (volcanogenic)
C02 on the left side, inasmuch as it evolved from the heating of subducted carbonates, is recycled on a very long-term scale (100 million to 1 billion years). Thus,
within the Earth below subduction zones, the equation reads backwards.
From the "Urey equation" (9.1) we can deduce how to effect a long-term decrease
of C02 in the atmosphere: by providing fresh CaSi03, through mountain building and
deep erosion. Mountain-building and sea level drop proceeded all through the late
Tertiary (Fig. 5.18), but especially in the last several million years, as shown in the
changing ratio of strontium isotopes within marine catbonates (see Fig. 8.19). Here is
one possible cause (of several) for the onset of the ice ages: a drop in atmospheric
C02 as a long-term trend within the late Tertiary.
9.5 Tertiary Oceans: the Cooling Planet
9.5.1 Trends and Events: Oxygen Isotopes. Of the host of tools available to paleoceanographers and sedimentologists for studying the sediments newly recovered by
deep-sea drilling, oxygen isotopes promised to yield the greatest return for investment, in the reconstruction of climatic change and the ocean's role in it. However,
nothing much could be achieved without biostratigraphy. It delivers the time frame
and also the all-important information on the response of plankton and benthos to
physical change. Paleobiological studies bring ancient oceans alive. They established,
for example, that "punctuations" in the history of evolution exist, and that they are a
response to climatic change. Only the continuous record of deep-sea sediments, with
its masses of microfossils, could establish such a pattern for sure. On land, hiatus-ridden records (that is, sequences with gaps) commonly do not allow a choice between
gradual and jumpy evolution - as Charles Darwin pointed out long ago in his book,
the Origin of Species (in 1859).
The central theme of climatic evolution in the Tertiary is an overall drop of sea
level, which is accompanied by a general cooling (Fig. 9.11A). The initial work on
the oxygen isotope record of the Tertiary deep ocean was done by C. Emiliani. In the
1950s he used samples of benthic foraminifera from the surface of the sea floor,
Changes in the sites of carbonate production also must have been important.
During times of low sea level, catbonate deposition would have been greatly decreased on the shelves, and this (together with erosion) would have increased the
carbonate ion content of the ocean, and hence the "alkalinity" of the ocean. In tum,
increased alkalinity allows the ocean to retain a larger share of the total C02 in the
system.
9.4.3 The Long-Range View. On long time scales, not just the ocean's exchange
with the atmosphere has to be considered, but also the weathering of silicates on land,
and the input of C02 from volcanism. The weathering process can be summarily
described by the formula
C02 + CaSi03 ~ CaC03 + Si02 ,
(9.1)
which shows the long-term uptake of C02 from trhe atmosphere. The (volcanogenic)
C02 on the left side, inasmuch as it evolved from the heating of subducted carbonates, is recycled on a very long-term scale (100 million to 1 billion years). Thus,
within the Earth below subduction zones, the equation reads backwards.
From the "Urey equation" (9.1) we can deduce how to effect a long-term decrease
of C02 in the atmosphere: by providing fresh CaSi03, through mountain building and
deep erosion. Mountain-building and sea level drop proceeded all through the late
Tertiary (Fig. 5.18), but especially in the last several million years, as shown in the
changing ratio of strontium isotopes within marine catbonates (see Fig. 8.19). Here is
one possible cause (of several) for the onset of the ice ages: a drop in atmospheric
C02 as a long-term trend within the late Tertiary.
9.5 Tertiary Oceans: the Cooling Planet
9.5.1 Trends and Events: Oxygen Isotopes. Of the host of tools available to paleoceanographers and sedimentologists for studying the sediments newly recovered by
deep-sea drilling, oxygen isotopes promised to yield the greatest return for investment, in the reconstruction of climatic change and the ocean's role in it. However,
nothing much could be achieved without biostratigraphy. It delivers the time frame
and also the all-important information on the response of plankton and benthos to
physical change. Paleobiological studies bring ancient oceans alive. They established,
for example, that "punctuations" in the history of evolution exist, and that they are a
response to climatic change. Only the continuous record of deep-sea sediments, with
its masses of microfossils, could establish such a pattern for sure. On land, hiatus-ridden records (that is, sequences with gaps) commonly do not allow a choice between
gradual and jumpy evolution - as Charles Darwin pointed out long ago in his book,
the Origin of Species (in 1859).
The central theme of climatic evolution in the Tertiary is an overall drop of sea
level, which is accompanied by a general cooling (Fig. 9.11A). The initial work on
the oxygen isotope record of the Tertiary deep ocean was done by C. Emiliani. In the
1950s he used samples of benthic foraminifera from the surface of the sea floor,