Transfer Method
191
Today, the polar front is just south of Greenland (Fig. 7.4,2 DC isotherm). During
the glacial maximum, the front ran from New York to be Iberian Peninsula. Norway,
and even England, were largely cut off from the warming influence of the Gulf
Stream, with dramatic climatologic consequences.
The motion of the polar front in the North Atlantic, as reflected in deep-sea cores,
is a prime example for climatic transgression. Such a transgression can be used to
determine the rate of change. How fast does a glacial period change into an interglacial one? How fast can the glaciers build up at the end of a warm period? These are
questions of considerable interest, because mankind is involved in climate modification, by industrial C02 emission and by deforestation, and because the present unusually warm period (since 10 000 years ago) has been about as long as comparable
ones in the past, throughout the Late Pleistocene. We shall return to these questions
in Chapter 9 when discussing the geologic record.
7.2.4 Limitations of the Transfer Method. The method outlined (or others like it)
allows an estimate to be made of any environmental variable which shows a correlation with foraminiferal abundances (or abundances of other plankton: coccoliths,
radiolarians, diatoms). However, the significance of such estimates is in doubt where
the environmental variable is not in control of growth. A prime example is salinity.
There is little evidence that salinity has any influence on plankton distribution within
the normal range of seawater salinity.
There are some other problems with the transfer method. First, exact calibration is
difficult. Sediment arriving on the sea floor becomes thoroughly mixed with older
sediment. Thus, the calibration set of sea-floor samples contains information spanning thousands of years. Yet, the calibration set of present sea surface conditions is
based on a very few years at most. Are these last few years - the ones whose data
enter the present-day temperature atlas, for example - representative for the last two
or three thousand years? Perhaps.
Second, there is the problem of differential dissolution, hence selective preservation of plankton shells on the sea floor. Most of the calcareous ooze on the deep-sea
floor is exposed to at least some dissolution. Thus, the assemblages from which we
wish to draw paleoclimatic information are being altered on the sea floor. The more
delicate shells are being removed, while the thick-walled forms are being concentrated (Fig. 7.5). This is also true for siliceous shells, although dissolution patterns are
different.
For older deposits the question arises: to what degree has evolution changed the
optima and ranges of the species? The question is ever-present in paleoecologic
research, and is always difficult to deal with.
7.3 Diversity and Shell Chemistry as Climatic Indicators
7.3.1 Diversity Gradients. One way to map changing climatic patterns is to focus
not on the species themselves but on the statistical patterns they make: their diversity
and their dominance. Diversity can be measured in various ways; the most realistic
191
Today, the polar front is just south of Greenland (Fig. 7.4,2 DC isotherm). During
the glacial maximum, the front ran from New York to be Iberian Peninsula. Norway,
and even England, were largely cut off from the warming influence of the Gulf
Stream, with dramatic climatologic consequences.
The motion of the polar front in the North Atlantic, as reflected in deep-sea cores,
is a prime example for climatic transgression. Such a transgression can be used to
determine the rate of change. How fast does a glacial period change into an interglacial one? How fast can the glaciers build up at the end of a warm period? These are
questions of considerable interest, because mankind is involved in climate modification, by industrial C02 emission and by deforestation, and because the present unusually warm period (since 10 000 years ago) has been about as long as comparable
ones in the past, throughout the Late Pleistocene. We shall return to these questions
in Chapter 9 when discussing the geologic record.
7.2.4 Limitations of the Transfer Method. The method outlined (or others like it)
allows an estimate to be made of any environmental variable which shows a correlation with foraminiferal abundances (or abundances of other plankton: coccoliths,
radiolarians, diatoms). However, the significance of such estimates is in doubt where
the environmental variable is not in control of growth. A prime example is salinity.
There is little evidence that salinity has any influence on plankton distribution within
the normal range of seawater salinity.
There are some other problems with the transfer method. First, exact calibration is
difficult. Sediment arriving on the sea floor becomes thoroughly mixed with older
sediment. Thus, the calibration set of sea-floor samples contains information spanning thousands of years. Yet, the calibration set of present sea surface conditions is
based on a very few years at most. Are these last few years - the ones whose data
enter the present-day temperature atlas, for example - representative for the last two
or three thousand years? Perhaps.
Second, there is the problem of differential dissolution, hence selective preservation of plankton shells on the sea floor. Most of the calcareous ooze on the deep-sea
floor is exposed to at least some dissolution. Thus, the assemblages from which we
wish to draw paleoclimatic information are being altered on the sea floor. The more
delicate shells are being removed, while the thick-walled forms are being concentrated (Fig. 7.5). This is also true for siliceous shells, although dissolution patterns are
different.
For older deposits the question arises: to what degree has evolution changed the
optima and ranges of the species? The question is ever-present in paleoecologic
research, and is always difficult to deal with.
7.3 Diversity and Shell Chemistry as Climatic Indicators
7.3.1 Diversity Gradients. One way to map changing climatic patterns is to focus
not on the species themselves but on the statistical patterns they make: their diversity
and their dominance. Diversity can be measured in various ways; the most realistic