- Figure 1 shows the core locations of the 461 Holocene Atlantic
and Indian Ocean samples analysed by Bé and Hutson (1977),
Hutson and Prell (1980), and Kipp (1976), whose data are used
in this study. The average number of counted specimens of planktic
foraminifera in these samples is 494 + 180 (standard deviation),
with about 300 as a minimum, except in some highly dominated
high latitude samples, and a record of 35 species and 6 variants
from the size fraction >150 µm. Most species are rare
in the assemblages. The number of counted specimens allows for
statistically significant analyses of relative abundances greater
than about 1 %. All lower relative abundance counts are subject
of statistical uncertainty. The data is available from the National
Geophysical Data Center (NGDC). According to Kipp (1976) the
taxonomy follows Parker (1962, 1967), with modifications by Bé
(1967) and Bé and Hamlin (1967). Saito et al. (1981) provided
additional descriptions of species concepts used by CLIMAP project
members (1976, 1981). Generic assignments in this paper follow
Hemleben et al. (1989).
Summer and winter temperature, salinity, and water density
at the sea surface and at 200 m depth were extracted for each
core location from the Levitus (1982) database made available
by the National Oceanographic and Atmospheric Administration
(NOAA). From this data I computed the vertical temperature and
salinity gradient between the sea surface and 200 m depth and
of the water density as a measure of stratification, separately
for summer and winter according to the Levitus (1982) definitions
(see Fig. 2). Vertical gradients are expressed as "surface
value minus value at 200 m depth". Seasonal variation (seasonality)
is expressed as "summer value minus winter value".
The depth of 200 m was chosen as a reference horizon below the
seasonally variable mixed layer. Such a reference is necessary
because some species descend below the seasonal thermocline during
their ontogenetic cycle and may react to conditions below the
mixed layer. The oceanographically more meaningful conditions
at the base of the mixed layer present problems with the definition
of the seasonal thermocline, the steep gradients at this level,
and in some ocean areas a seasonal thermocline does not exist
for part of the year. Figure 3 shows little seasonal change at
200 m depth on a global scale. Oceanographic conditions at that
depth can be used to characterise gradients between the mixed
layer and the central waters.
Biogeographic distribution maps of individual species have
been published by Bé and Hutson (1977), and Kipp (1976).
They will not be duplicated in this paper but will be applied
in the interpretations. It is recommended to use these maps and
references to other biogeographic work in these publications,
with results presented here.
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