194 Imprint of Climatic Zonation on Marine Sediments
sink and is oxidized at depth, releasing fixed carbon back to the water as C02. Thus,
deeper water is richer in 12C than surface water, and benthic shells acquire correspondingly lower () 13C values than planktonic ones. At the same time, the deeper
water also has more C02 by the same process. In principle, therefore, we can say that
the difference in () 13C values of co-occurring benthic and planktonic foraminifers is
a measure of the efficiency of extracting carbon out of surface waters. This fundamental connection between the carbon isotope compositions of benthic and planktonic shells on the one hand, and the pumping of carbon from the surface to depth on
the other, was first pointed out by the geochemist and oceanographer W. S. Broecker.
It has important implications for the reconstruction of the C02 pressure in surface
waters (and hence the atmosphere) from the () l3C values in fossil foraminifera on the
sea floor.
7.3.4 Other Chemical Markers. Other useful markers include both metals and certain organic compounds, which are incorporated differently into shells or into organic
matter, depending on productivity or temperature. For example, cadmium has been
used for the reconstruction of nutrient patterns; its concentration parallels that of
phosphate, and it is incorporated into foraminiferal shells in accordance with its
abundance in seawater.
Magnesium in calcareous shells tends to increase with temperature. This effect
appears to be more pronounced in the lower organisms, such as calcareous algae and
foraminifera, than in the higher ones, for example the bamacles. The relationship has
not been exploited for paleoclimate work to any significant degree.
Mineralogy follows temperature to some degree. There is a tendency for relatively
more aragonite to be precipitated by benthic organisms in warm waters than in cold
waters, where calcite is distinctly favored. Aragonite is the less stable form of calcium carbonate and changes to calcite during diagenesis. Thus, aragonite content of
ancient calcareous sediments is largely a result of post-depositional alteration, rather
than of conditions of formation.
7.4 Coral Reefs, Markers of Tropical Climate
7.4.1 Global Distribution. The classic geologic method of mapping climatic belts
independently of individual species is to take typical shelf assemblages, such as coral
reef associations. Even if the species within the associations have changed their
preferences, as a whole, the reef association is believed to indicate tropical shallow
waters. Today, the tropical ocean is the core region for coral reefs (Fig. 7.6). Reefs
extend into the sUbtropics on the western sides of the ocean basins, but are greatly
restricted on the eastern sides. They cover about 4 % of the total shelf area at the
present time. The warm western boundary currents are responsible for carrying the
reef limits to their latitudinal extremes, while eastern boundary currents bring cold
water from high latitudes and are responsible for the restricted distribution of reefs on
the eastern side of ocean basins. The reefs of the west Atlantic island Bermuda reach
to about 30° N thanks to the Gulf Stream. On the eastern margin, in the sphere of
sink and is oxidized at depth, releasing fixed carbon back to the water as C02. Thus,
deeper water is richer in 12C than surface water, and benthic shells acquire correspondingly lower () 13C values than planktonic ones. At the same time, the deeper
water also has more C02 by the same process. In principle, therefore, we can say that
the difference in () 13C values of co-occurring benthic and planktonic foraminifers is
a measure of the efficiency of extracting carbon out of surface waters. This fundamental connection between the carbon isotope compositions of benthic and planktonic shells on the one hand, and the pumping of carbon from the surface to depth on
the other, was first pointed out by the geochemist and oceanographer W. S. Broecker.
It has important implications for the reconstruction of the C02 pressure in surface
waters (and hence the atmosphere) from the () l3C values in fossil foraminifera on the
sea floor.
7.3.4 Other Chemical Markers. Other useful markers include both metals and certain organic compounds, which are incorporated differently into shells or into organic
matter, depending on productivity or temperature. For example, cadmium has been
used for the reconstruction of nutrient patterns; its concentration parallels that of
phosphate, and it is incorporated into foraminiferal shells in accordance with its
abundance in seawater.
Magnesium in calcareous shells tends to increase with temperature. This effect
appears to be more pronounced in the lower organisms, such as calcareous algae and
foraminifera, than in the higher ones, for example the bamacles. The relationship has
not been exploited for paleoclimate work to any significant degree.
Mineralogy follows temperature to some degree. There is a tendency for relatively
more aragonite to be precipitated by benthic organisms in warm waters than in cold
waters, where calcite is distinctly favored. Aragonite is the less stable form of calcium carbonate and changes to calcite during diagenesis. Thus, aragonite content of
ancient calcareous sediments is largely a result of post-depositional alteration, rather
than of conditions of formation.
7.4 Coral Reefs, Markers of Tropical Climate
7.4.1 Global Distribution. The classic geologic method of mapping climatic belts
independently of individual species is to take typical shelf assemblages, such as coral
reef associations. Even if the species within the associations have changed their
preferences, as a whole, the reef association is believed to indicate tropical shallow
waters. Today, the tropical ocean is the core region for coral reefs (Fig. 7.6). Reefs
extend into the sUbtropics on the western sides of the ocean basins, but are greatly
restricted on the eastern sides. They cover about 4 % of the total shelf area at the
present time. The warm western boundary currents are responsible for carrying the
reef limits to their latitudinal extremes, while eastern boundary currents bring cold
water from high latitudes and are responsible for the restricted distribution of reefs on
the eastern side of ocean basins. The reefs of the west Atlantic island Bermuda reach
to about 30° N thanks to the Gulf Stream. On the eastern margin, in the sphere of