28
Weferetal.
scopic barite crystals. Preservation of barite in
sediments is a crucial factor (e.g. Pay tan and
Kastner 1996). It has not been studied in detail as,
for instance, has the organic carbon degradation
during burial.
Reconstruction of Productivity from
Microfossil Assemblages
Within each group of planktic shelled organisms,
some species occur preferentially in high-productivity regions while others avoid these, or cannot
compete in bloom situations. Thus, the relative
abundances of certain species should contain clues
to the intensity of production at the time of sedimentation. Anum ber of species have been identified as indicators of high productivity (e.g. Prell and
Curry 1980; for a review see Vincent and Berger
1981). In low latitudes, for example, these
include the foraminifersl species G. b ullo ides,
N. dutertrei, and G. tum ida. In temperate latitudes,
G. quinqueloba is a good candidate, while
N. pachyderma (sin.) indicates cold upwelling water. Thus, simply recording the changes in abundance of such species relative to the more ubiquitous tropical (or temperate) species should provide
valuable clues to productivity fluctuations.
Treating productivity as just another parameter
like temperature, one can also apply the Imbrie and
Kipp technique (or similar statistical methods) for
reconstruction. Prerequisite for this procedure is a
sufficiently large calibration set that includes the
range of variation of the downcore assemblages to
be assessed. Mix (1989a,b), using the CLIMAP
data set and the productivity map of Berger (1989),
performed such a calibration and applied it to the
glacial distributions of planktic foraminifers in the
Atlantic Ocean. As expected from previous results
(e.g. Sarnthein et al. 1987), strong increases in
productivity were indicated in those regions that
have high productivity today, suggesting increased
nutrient content in subsurface waters, or increased
mixing, or both. However, in certain regions the
faunal-based estimates differ considerably from
organic matter-based estimates, indicating that one
or the other of the methods (or both) produced incorrect reconstructions. Such discrepancies are
interesting because their resolution produces new
insights regarding how well proxies record actual
changes in the ocean.
Benthic foraminifers live on the organic material falling to the sea floor. Thus, their abundance
should vary with food supply from above. This is
indeed the case, and the accumulation rate of
benthic foraminifers can be used to reconstruct
productivity (Herguera and Berger 1991; Herguera
1992). Also, as the food supply changes, the bacterial flora changes and with it the conditions oflife
on the sea floor. Thus, the species assemblage of
benthic organisms also responds to long term
changes in the supply of organic matter. This is
obvious when comparing benthic assemblages
from the open ocean (dominated by Cibicidoides,
Eponides, Melonis, Oridorsalis etc.) with those
at the margin (dominated by Uvigerina, Bolivina,
and Bulimina), but it is also true for more subtle
changes (Burke et al. 1993; Loubere 1994;
Schmiedl and Mackensen 1997).
Diatom assemblages are also used in reconstructions of paleoproductivity. For example,
Pokras (1987) found increased productivity in the
eastern equatorial Atlantic for glacial times, based
on changes in the diatom assemblage. Jansen and
van Iperen (1991) likewise drew conclusions about
variations in the intensity of upwelling and organic
productivity in the Angola Basin using diatoms.
"High-productivity" species were more common
during cold periods, taken to be times of intensified
coastal and oceanic upwelling. Using a diatom
transfer function Schrader and Skorknes (1991)
determined variations in coastal upwelling off Peru.
They found that productivity at II ° and 13 .5°S was
very variable during the last 400,000 years. The
variations in productivity were not in phase with
glacial-interglacial fluctuations. Paleoproductivity
reconstructions off northwest Africa based on diatom assemblages show increased upwelling during
glacial periods relative to warm episodes (Abrantes
1991). When using microfossils to reconstruct productivity, one has to keep in mind that "cold-water" species and "upwelling" species are the same,
in many instances. This coupling complicates matters.
Weferetal.
scopic barite crystals. Preservation of barite in
sediments is a crucial factor (e.g. Pay tan and
Kastner 1996). It has not been studied in detail as,
for instance, has the organic carbon degradation
during burial.
Reconstruction of Productivity from
Microfossil Assemblages
Within each group of planktic shelled organisms,
some species occur preferentially in high-productivity regions while others avoid these, or cannot
compete in bloom situations. Thus, the relative
abundances of certain species should contain clues
to the intensity of production at the time of sedimentation. Anum ber of species have been identified as indicators of high productivity (e.g. Prell and
Curry 1980; for a review see Vincent and Berger
1981). In low latitudes, for example, these
include the foraminifersl species G. b ullo ides,
N. dutertrei, and G. tum ida. In temperate latitudes,
G. quinqueloba is a good candidate, while
N. pachyderma (sin.) indicates cold upwelling water. Thus, simply recording the changes in abundance of such species relative to the more ubiquitous tropical (or temperate) species should provide
valuable clues to productivity fluctuations.
Treating productivity as just another parameter
like temperature, one can also apply the Imbrie and
Kipp technique (or similar statistical methods) for
reconstruction. Prerequisite for this procedure is a
sufficiently large calibration set that includes the
range of variation of the downcore assemblages to
be assessed. Mix (1989a,b), using the CLIMAP
data set and the productivity map of Berger (1989),
performed such a calibration and applied it to the
glacial distributions of planktic foraminifers in the
Atlantic Ocean. As expected from previous results
(e.g. Sarnthein et al. 1987), strong increases in
productivity were indicated in those regions that
have high productivity today, suggesting increased
nutrient content in subsurface waters, or increased
mixing, or both. However, in certain regions the
faunal-based estimates differ considerably from
organic matter-based estimates, indicating that one
or the other of the methods (or both) produced incorrect reconstructions. Such discrepancies are
interesting because their resolution produces new
insights regarding how well proxies record actual
changes in the ocean.
Benthic foraminifers live on the organic material falling to the sea floor. Thus, their abundance
should vary with food supply from above. This is
indeed the case, and the accumulation rate of
benthic foraminifers can be used to reconstruct
productivity (Herguera and Berger 1991; Herguera
1992). Also, as the food supply changes, the bacterial flora changes and with it the conditions oflife
on the sea floor. Thus, the species assemblage of
benthic organisms also responds to long term
changes in the supply of organic matter. This is
obvious when comparing benthic assemblages
from the open ocean (dominated by Cibicidoides,
Eponides, Melonis, Oridorsalis etc.) with those
at the margin (dominated by Uvigerina, Bolivina,
and Bulimina), but it is also true for more subtle
changes (Burke et al. 1993; Loubere 1994;
Schmiedl and Mackensen 1997).
Diatom assemblages are also used in reconstructions of paleoproductivity. For example,
Pokras (1987) found increased productivity in the
eastern equatorial Atlantic for glacial times, based
on changes in the diatom assemblage. Jansen and
van Iperen (1991) likewise drew conclusions about
variations in the intensity of upwelling and organic
productivity in the Angola Basin using diatoms.
"High-productivity" species were more common
during cold periods, taken to be times of intensified
coastal and oceanic upwelling. Using a diatom
transfer function Schrader and Skorknes (1991)
determined variations in coastal upwelling off Peru.
They found that productivity at II ° and 13 .5°S was
very variable during the last 400,000 years. The
variations in productivity were not in phase with
glacial-interglacial fluctuations. Paleoproductivity
reconstructions off northwest Africa based on diatom assemblages show increased upwelling during
glacial periods relative to warm episodes (Abrantes
1991). When using microfossils to reconstruct productivity, one has to keep in mind that "cold-water" species and "upwelling" species are the same,
in many instances. This coupling complicates matters.