Geologic Climate Indicators
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that many guyots originated at that time. The reason(s) remain obscure, so far. In any
case, whether a volcano ends up as an atoll or a seamount may largely depend on the
climatic conditions which allowed or prevented the growth of coral reefs.
7.4.4 Great Barrier Reef. The 2000-km-long Great Barrier Reef, lying 30 to 250 km
off eastern Australia, is the most impressive reef structure in the world today, and
demonstrates how biological sedimentation increases the size of continents. The reef
grows on subsiding crust, and is most massive in the north (around 10° S), reaching
about 1500 m thickness. In the south (at 24° S), it is only 150 m thick. The difference
may be related to the migration of the Australian continent. Plate tectonics requires
that Australia moved some 12° to the north during the last 24 million years (the
"Neogene"). Coral growth could begin as soon as the northern margin crossed the
southern limit of growth, about 20 million years ago. The more southern portions
crossed this limit later, offering less opportunity for growth. The importance of plate
motion for the architecture of the Great Barrier Reef is diminished, however, by the
finding (from recent drilling) that the main reef is less than 1 million years old.
7.5 Geologic Climate Indicators
7.5.1 Chemical Indicators. We discussed some of the aspects of biological indicators of climate. Coral reefs, of course, can equally well be considered geologic
indicators: they are calcareous sediment bodies whose patterns can be mapped. In
fact, simple compilations of reef occurrences through time have been made from time
to time to indicate the position of the equatorial belt and, ultimately, the cross-latitudinal motion of the continents. Biogenous carbonate deposits in general yield similar clues (Fig. 7.9). Although the method is not very reliable, it does provide some
checks on geophysical results.
Salt and dolomite deposits also have been used to delineate climatic belts. They
are characteristic for the sUbtropics and result from excess evaporation as discussed
(Sects. 3.7.3 and 7.6). In tropical areas, laterite and the clay mineral kaolinite are
typical weathering products. They find their way into deep-sea deposits, where they
can be used to trace climatic changes on land (see Sect. 8.4.3).
7.5.2 Physical Geologic Indicators are especially obvious in high latitudes, due to
the effects of ice (Fig. 7.9).
On shelves entirely covered by ice, as in the Antarctic, erosion predominates. Bare
rock, polished and scratched, dominates; morainal debris fills depressions. Icebergs
calving from glaciers take enclosed debris with them, and release it as dropstones in
melting (Fig. 7.10). During the glacial periods of the Pleistocene, the area of the sea
floor affected by iceberg sedimentation was considerably expanded (Sect. 3.2.2).
Also, extensive shelf areas were exposed at the time due to the sealevel drop, and
glacial ice could deposit end moraines in their typical lobe-shaped morphology, as on
the East Coast north of New York and on the area of today's North Sea and Baltic.
These moraines, then, are glacial relicts which are being worked over by shelf currents at the present time (Sect. 5.4).
199
that many guyots originated at that time. The reason(s) remain obscure, so far. In any
case, whether a volcano ends up as an atoll or a seamount may largely depend on the
climatic conditions which allowed or prevented the growth of coral reefs.
7.4.4 Great Barrier Reef. The 2000-km-long Great Barrier Reef, lying 30 to 250 km
off eastern Australia, is the most impressive reef structure in the world today, and
demonstrates how biological sedimentation increases the size of continents. The reef
grows on subsiding crust, and is most massive in the north (around 10° S), reaching
about 1500 m thickness. In the south (at 24° S), it is only 150 m thick. The difference
may be related to the migration of the Australian continent. Plate tectonics requires
that Australia moved some 12° to the north during the last 24 million years (the
"Neogene"). Coral growth could begin as soon as the northern margin crossed the
southern limit of growth, about 20 million years ago. The more southern portions
crossed this limit later, offering less opportunity for growth. The importance of plate
motion for the architecture of the Great Barrier Reef is diminished, however, by the
finding (from recent drilling) that the main reef is less than 1 million years old.
7.5 Geologic Climate Indicators
7.5.1 Chemical Indicators. We discussed some of the aspects of biological indicators of climate. Coral reefs, of course, can equally well be considered geologic
indicators: they are calcareous sediment bodies whose patterns can be mapped. In
fact, simple compilations of reef occurrences through time have been made from time
to time to indicate the position of the equatorial belt and, ultimately, the cross-latitudinal motion of the continents. Biogenous carbonate deposits in general yield similar clues (Fig. 7.9). Although the method is not very reliable, it does provide some
checks on geophysical results.
Salt and dolomite deposits also have been used to delineate climatic belts. They
are characteristic for the sUbtropics and result from excess evaporation as discussed
(Sects. 3.7.3 and 7.6). In tropical areas, laterite and the clay mineral kaolinite are
typical weathering products. They find their way into deep-sea deposits, where they
can be used to trace climatic changes on land (see Sect. 8.4.3).
7.5.2 Physical Geologic Indicators are especially obvious in high latitudes, due to
the effects of ice (Fig. 7.9).
On shelves entirely covered by ice, as in the Antarctic, erosion predominates. Bare
rock, polished and scratched, dominates; morainal debris fills depressions. Icebergs
calving from glaciers take enclosed debris with them, and release it as dropstones in
melting (Fig. 7.10). During the glacial periods of the Pleistocene, the area of the sea
floor affected by iceberg sedimentation was considerably expanded (Sect. 3.2.2).
Also, extensive shelf areas were exposed at the time due to the sealevel drop, and
glacial ice could deposit end moraines in their typical lobe-shaped morphology, as on
the East Coast north of New York and on the area of today's North Sea and Baltic.
These moraines, then, are glacial relicts which are being worked over by shelf currents at the present time (Sect. 5.4).