Tertiary Oceans: the Great Partitioningt 259
where old sediments are laid bare by erosion, to identify an overall cooling trend
since the Cretaceous, from the increase of oxygen-I8 in their tests. His guess proved
correct.
Seven years after the Deep-Sea Drilling Project started, two paleontologist-geochemist teams had worked out the essential trends (Fig. 9.11B). The stratigraphies of
the oxygen isotopic composition of planktonic and benthic foraminifers shows separate trends in low latitudes, but similar trends in high latitudes. Thus, the overall
cooling in the Tertiary is largely a high-latitude (and deep-water) phenomenon. In
general, then, temperature gradients must have increased throughout the Tertiary,
since the middle of the Eocene, some 40 million years ago. Wind speed depends
strongly on temperature gradients. If so, winds and their offspring, the surface currents, greatly increased in the late Tertiary, as did coastal and mid-ocean upwelling.
Direct evidence that this is true is found in the increasing diatom supply, both in the
northern North Pacific and around the Antarctic, during the late Tertiary. Other evidence for fertility increases also exists. For example, radiolarian skeletons become
continuously more delicate after the Eocene. Apparently the content of dissolved
silica decreased through time, a sign that silica has been increasingly removed by
diatom production in upwelling areas.
The high-latitude cooling indicated in the oxygen isotope trends shows two major
steps: a more recent one in the middle of the Miocene, and an older one near the
Eocene-Oligocene boundary. We do not know the cause of these steps, which simply
enhance the overall trend of turning a warm ocean into a cold one. Perhaps, at some
critical treshold of cooling around Antarctica, deep water formation off its shores was
greatly enhanced and this set off a chain reaction producing a permanent reorganization in the ocean's circulation patterns, making the change irreversible. Or perhaps
the physical geography changed at that very time, barring or opening certain ocean
passages and thus redirecting the heat carried by ocean currents. The two types of
causes - internal feedback and change of geography - are not mutually exclusive. We
know that both are at work. The question is, how important they are in each given
situation.
9.5.2 The Great Partitioning. All through the Tertiary, changes in geography due to
plate motions profoundly affect the configuration of exchange between ocean basins.
A number of important valve points in the ocean's plumbing system (called "gateways") translate small motions of continents into large effects for ocean currents (Fig.
9.12). The dominant theme is a stepwise partitioning of the global ocean (originally
connected by large circumtropical passages) into the cul-de-sacs we have today. In a
sense, we trade northern and southern embayments along a broad world-encircling
tropical ocean for three separate super-large basins connected through a ring current
far in the south (the "Southern Exchange").
The gateways shown (Fig. 9.12) control access to the Arctic Ocean (east and west
of Greenland), connect the global ocean along the Equator ("Tethys Ocean" between
Africa and Eurasia, Panama Straits, Indonesian Seaway between Borneo and New
Guinea), and control the evolution of the Circumpolar Current (Tasmanian Passage,
Drake Passage). The difference from the present ocean to the Eocene ocean 45
where old sediments are laid bare by erosion, to identify an overall cooling trend
since the Cretaceous, from the increase of oxygen-I8 in their tests. His guess proved
correct.
Seven years after the Deep-Sea Drilling Project started, two paleontologist-geochemist teams had worked out the essential trends (Fig. 9.11B). The stratigraphies of
the oxygen isotopic composition of planktonic and benthic foraminifers shows separate trends in low latitudes, but similar trends in high latitudes. Thus, the overall
cooling in the Tertiary is largely a high-latitude (and deep-water) phenomenon. In
general, then, temperature gradients must have increased throughout the Tertiary,
since the middle of the Eocene, some 40 million years ago. Wind speed depends
strongly on temperature gradients. If so, winds and their offspring, the surface currents, greatly increased in the late Tertiary, as did coastal and mid-ocean upwelling.
Direct evidence that this is true is found in the increasing diatom supply, both in the
northern North Pacific and around the Antarctic, during the late Tertiary. Other evidence for fertility increases also exists. For example, radiolarian skeletons become
continuously more delicate after the Eocene. Apparently the content of dissolved
silica decreased through time, a sign that silica has been increasingly removed by
diatom production in upwelling areas.
The high-latitude cooling indicated in the oxygen isotope trends shows two major
steps: a more recent one in the middle of the Miocene, and an older one near the
Eocene-Oligocene boundary. We do not know the cause of these steps, which simply
enhance the overall trend of turning a warm ocean into a cold one. Perhaps, at some
critical treshold of cooling around Antarctica, deep water formation off its shores was
greatly enhanced and this set off a chain reaction producing a permanent reorganization in the ocean's circulation patterns, making the change irreversible. Or perhaps
the physical geography changed at that very time, barring or opening certain ocean
passages and thus redirecting the heat carried by ocean currents. The two types of
causes - internal feedback and change of geography - are not mutually exclusive. We
know that both are at work. The question is, how important they are in each given
situation.
9.5.2 The Great Partitioning. All through the Tertiary, changes in geography due to
plate motions profoundly affect the configuration of exchange between ocean basins.
A number of important valve points in the ocean's plumbing system (called "gateways") translate small motions of continents into large effects for ocean currents (Fig.
9.12). The dominant theme is a stepwise partitioning of the global ocean (originally
connected by large circumtropical passages) into the cul-de-sacs we have today. In a
sense, we trade northern and southern embayments along a broad world-encircling
tropical ocean for three separate super-large basins connected through a ring current
far in the south (the "Southern Exchange").
The gateways shown (Fig. 9.12) control access to the Arctic Ocean (east and west
of Greenland), connect the global ocean along the Equator ("Tethys Ocean" between
Africa and Eurasia, Panama Straits, Indonesian Seaway between Borneo and New
Guinea), and control the evolution of the Circumpolar Current (Tasmanian Passage,
Drake Passage). The difference from the present ocean to the Eocene ocean 45