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510
Chapter 14 I Biostratigraphy
Figure 14.15
and other factors that can cause them to be diachronous (occur at different times
in different areas). A different approach to the use of abundance zones yields cor
relations that have time-stratigraphic significance; this approach is correlation
based on the maximum abundance of a taxon that results from geographical shifts
of an environmentally sensitive fossil assemblage (Haq and Wo rsley, 1982). Be
cause of latitudinally related temperature dierences in the ocean, some species or
other taxa are restricted to biogeographic provinces that are defined by latitude.
Thus, low-latitude taxa are ecologically excluded from high latudes, and vice
versa; however, changes in climate can allow these taxa to shift into a different bio
geographic province. During major glacial stages, for example, high-latitude taxa
can expand into lower latitudes, and during warming trends between major glacial
stages, low-latitude taxa can expand into higher latitudes. From a geochronological
point of view, the spreading out of certain planktonic species in response to major
climatic fluctuations is essentially isochronous.
Climate-related shifts in planktonic taxa at specific times thus provide bio
geographical abundance events that can be correlated from one area to another.
For each core or outcrop section studied, climatic curves are constructed on the
basis of percentages of warm-climate to cool-climate taxa or relative abundance of
a particular taxon. These curves can then be used to identify episodes of warming
and cooling that can be correlated from one section to another. Figure 14.15, con
structed from this type of information, illustrates how climatically controlled lati
tudinal shifts in calcareous nannoplankton assemblages in the North Atlantic
during Miocene time has been used for chronostratigraphic correlation in Deep
Sea Drilling Program (DSDP) cores.
A related approach is time-stratigraphic correlation based on the coiling ra
tios of planktonic foraminifera, as described Eicher (1976). The multicham
bered shells of some foraminifera are known to coil in one direction when the
species lives in areas of warm water, and in the opposite direction when it lives in
areas of cold water. The foraminifer Globorotalia truncatulinoides, for example, has
dominantly right-handed coils in warm water and left-handed coils in cold water.
TIME
IN
Ma
Use of biogeographical abundance
zones as a means of time correlation.
Cycles of latitudinal shifts of calcare
ous nannoplankton assemblages in
the North Atlantic Ocean during the
Miocene are interpreted as responses
to major fluctuations in climate. The
major shifts of relatively warmer,
midlatitude assemblages into higher
latitudes can be used for the refine
ment of the biochronological scale in
the higher latitudes from which
marker, low-latitude taxa are normal
ly excluded. [From Haq, B. V., and
R. Worsley, 1982, Biochronology
Biologic events in time resolution,
their potential and limitations, in
Odin, G. S. (ed.), Numerical dating
in stratigraphy: john Wiley & Sons,
Ltd., Fig. 4, p. 27. Reprinted by
permission.]
C. pelagicus
Assemblage
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20"
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PALEOLATITUDE
C. floridanus
Assem b�age
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