Planktic foraminifera are the only extant group in which
all species can be preserved as fossils in the geologic record.
Their shell remains contribute to the oceanic carbonate sediment
on about 70% of the ocean area which is, therefore, a source
for global studies of planktic foraminifera (Belyaeva, 1976).
Sediment assemblages of planktic foraminifera reveal distinct
biogeographic pattern which reflects patterns in the plankton
(Hutson, 1977; Bé and Tolderlund, 1971; Bé and
Hutson, 1977). The relative abundances of species in the sediment
indicate ecologic preferences and maximum relative abundances
correlate with ecologic optima (Hecht, 1976). Consequently sediment
assemblages can be used in studies of the ecology of planktic
foraminifera with certain limitations and advantages of the sediment
record.
The species composition in sediment assemblages reflects the
relative intensity of shell production and preservation of the
individual species under conditions of the environment and taphonomic
effects. Environmental conditions include physical, chemical,
and biological factors. In this paper I will analyse the importance
of physical conditions for relative abundances of planktic foraminifera
based on counts in sediment surface (Holocene) samples by Bé
and Hutson (1977) and Hutson and Prell (1980) for the Indian
Ocean and Kipp (1976) for the North Atlantic, which have been
revised for CLIMAP (1976, 1981). Some species are primarily related
to chemical and biological conditions. For these species the
analysis will be incomplete. Some information on their ecologic
preferences can be extracted from geographic pattern in their
relative abundances.
Sinking velocities of empty shells are in the order of hundreds
of meters per day leading to deposition within one to three weeks
at any depth in the ocean (see data and references by Takahashi
and Bé , 1985). In this time a lateral transport of the
sinking tests occurs which leads to differences between living
and sedimented assemblages at a given point in the ocean. Living
assemblages of planktic foraminifera, however, are known to characterise
water masses and are subject to the limited mixing with other
water masses due to physical contrasts in the water. This applies
for living and dead shells and will tend to preserve the relations
of surface water masses with a characteristic assemblage of planktic
foraminifera. Current directions in the intermediate and deep
water often do not correspond to those in the surface waters.
This effect produces complicated trajectories until a shell settles
on the sea bottom and these trajectories may vary with the seasonal
dynamics in the water masses. A site of deposition, therefore,
collects faunas from a certain area and the combined effect of
current directions, current speeds, and residence times of shells
in the different water masses introduces noise to the data. This
noise affects the analysis of those species more seriously that
live in geographically confined areas because relations with
small water masses or distinct oceanographic features may be
obscured.
Selective dissolution and other effects of preservation alter
the species composition of sediment assemblages. Bé and
Hutson (1977), Hutson (1977), Hutson and Prell (1980), and Kipp
(1976) were aware of this problem. CLIMAP Project Members (1976,
1981) eliminated problematic samples but effects of preservation
on faunal assemblages are not excluded.
Bioturbation leads to mixing of assemblages. The mixed layer
in the Atlantic is about 5 cm thick, sedimentation rates of oceanic
carbonate are typically in the order of 1 cm/Ka (e.g. Seibold
and Berger, 1993). These relations implie a long-term averaging
of assemblages that were produced over some thousands of years
under variable environmental conditions. Comparisons between
oceanographic measurements and sediment assemblages of microfossils
will, therefore, never produce ideal correlations even if such
relationships exist in some species.
The above discussion may suggest serious limitations in the
analysis of sediment microfossil assemblages for the assessment
of ecological data. Depth-controlled plankton tows and sediment
traps are probably more suitable for such analyses. Most plankton
net studies of planktic foraminifera, however, suffer from spot
sampling at an arbitrary time and do not allow to assess changes
in the assemblages during the bi-monthly to annual reproduction
cycles of the various species at a given location (see maps by
Bé and Hutson, 1977). The seasonal dynamics is often not
represented in plankton net samples. Sediment trap samples reveal
substantial changes in assemblages on seasonal time scales and
allow correlation of these changes with oceanographic parameters.
Few sediment traps, however, have been deployed over longer time
and in fact a multi-year record for the Atlantic and Indian Ocean
is presently available only from Bermuda (Deuser and Ross, 1989;
Deuser et al., 1981). I will demonstrate for some species how
such data supports the interpretation of sediment assemblages
and subsequently how sediment trap assemblages collected from
small ocean areas demonstrate reliability of data derived from
sediment assemblages on biogeographic scales. Geologic applications,
however, rely on the calibration of relations between micropaleontologic
tracers and environmental conditions based on sediment samples.
CLIMAP (1976, 1981) is an example of the advantages and disadvantages
of this approach. The goal of this paper is two-fold: to add
to ecologic knowledge on planktic foraminifera, and to facilitate
their applicability for paleoenvironmental analyses.