10
Weferetal.
LGM-SST decreased much stronger than estimated by CLIMAP. But it will be difficult to estimate annual average temperatures, as most temperature proxies provide mixed seasonal information, where the mix is uncertain.
The transfer function techniques have found
wide application with various types of fossils, and
have greatly stimulated the systematic mapping of
paleotemperatures. The concepts of paleotemperature reconstruction, as first developed for
planktic foraminifers, apply analogously to the other
groups of microfossils (Sachs 1973; Morley 1979,
etc.). Coccolithophorids, like foraminifers, are susceptible to carbonate dissolution, and the dissolution is selective, removing delicate fossils first and
leaving robust forms behind. In addition, in high
latitudes, and also in high productivity regions, only
a few species are found, so that temperature estimates become less reliable.
Diatoms and radiolarians, consisting of opal, are
subject to a different set of dissolution parameters
than the calcareous fossils, resulting in a different
distribution pattern at the sea floor. The abundance
ofthese fossils is related to silicate supply (which
is a limiting factor, unlike carbonate in the case of
calcareous fossils). Although silicate supply is related to temperature (through the upwelling process), correlations differ from region to region and
probably through time as well. In both cases, differential preservation favors heavily silicified species within the assemblage (Romero et al. this volume). Radiolarian species are distributed throughout the water column with the deeper-living forms
commonly having heavier skeletons.
Diatoms and radiolarians have been used for
temperature estimates in the Pacific and in the
Antarctic Oceans, especially where calcareous
fossils are less abundant or even absent. Pisias et
al. (1997) calculated mean annual sea-surface temperatures and seasonal temperature ranges for the
northern and southeastern Pacific from radiolarian
assemblages, using an approach similar to that of
Imbrie and Kipp (1971). They used 170 surface
samples to develop the transfer functions. Standard error estimates were 1.6°C, both for SST and
seasonal temperature range. Temperatures derived
from radiolarian-based transfer functions compared well with temperature calculations from U'J7
index in a core taken from the northeastern Pacific
which covers the last 20,000 years. New calibrations of a diatom transer function for the Southern
Ocean are provided by Zielinski et al. (1998) using
218 surface sediment samples recovered in the
Atlantic and western Indian sector. The defined
transfer functions were applied on a sediment core
from the northern Antarctic Zone (Fig. 4).
How can the temperature proxies be improved?
A primary requirement is a better understanding of
the processes which transform a biocoenosis into
a taphocoenosis. Sediment-trap experiments can be
very helpful in examining these processes. In general, the results show a pronounced seasonality
related to productivity (Lange et al. 1994; Treppke
et al. 1996b). A large portion of the community is
destroyed by zooplankton grazing (Sancetta 1989).
As a rule, selective dissolution causes an enrichment of robust shells (Treppke et al. 1996a). Another important consideration is that, under conditions of strong seasonality, the community found on
the sea floor represents just a brief time period
within each year (Gersonde and Wefer 1987). The
distribution and changes in species composition
with increased water depth continues to provide a
focus for research in this context (e.g. Abelmann
and Gersonde 1992; Boltovskoy 1996a, b).
Temperature Estimates from Oxygen
Isotope Composition
As mentioned in the introduction, the oxygen isotope composition of calcareous fossils depends on
the temperature of calcification. The other most
important factor is the composition of the water
itself (Equation 2), which varies through time and
from place to place, and can substantially affect
temperature estimates. The oxygen isotope composition of a given shell is reported as 8 18 0 which
is defined as the deviation ofthe ratio ofl80 to 16 0
measured in the carbonate of the shell from the
ratio in a standard, in tenths of a percent (= perm ii,
0/0)·
The glacial to postglacial change in8 18 0 in the
ocean is slightly greater than 1 0 / 00 , reflecting preferential deposition ofl60 on continental ice sheets
during glacial periods. One permil corresponds to
an apparent change in temperature of SoC (taking
Weferetal.
LGM-SST decreased much stronger than estimated by CLIMAP. But it will be difficult to estimate annual average temperatures, as most temperature proxies provide mixed seasonal information, where the mix is uncertain.
The transfer function techniques have found
wide application with various types of fossils, and
have greatly stimulated the systematic mapping of
paleotemperatures. The concepts of paleotemperature reconstruction, as first developed for
planktic foraminifers, apply analogously to the other
groups of microfossils (Sachs 1973; Morley 1979,
etc.). Coccolithophorids, like foraminifers, are susceptible to carbonate dissolution, and the dissolution is selective, removing delicate fossils first and
leaving robust forms behind. In addition, in high
latitudes, and also in high productivity regions, only
a few species are found, so that temperature estimates become less reliable.
Diatoms and radiolarians, consisting of opal, are
subject to a different set of dissolution parameters
than the calcareous fossils, resulting in a different
distribution pattern at the sea floor. The abundance
ofthese fossils is related to silicate supply (which
is a limiting factor, unlike carbonate in the case of
calcareous fossils). Although silicate supply is related to temperature (through the upwelling process), correlations differ from region to region and
probably through time as well. In both cases, differential preservation favors heavily silicified species within the assemblage (Romero et al. this volume). Radiolarian species are distributed throughout the water column with the deeper-living forms
commonly having heavier skeletons.
Diatoms and radiolarians have been used for
temperature estimates in the Pacific and in the
Antarctic Oceans, especially where calcareous
fossils are less abundant or even absent. Pisias et
al. (1997) calculated mean annual sea-surface temperatures and seasonal temperature ranges for the
northern and southeastern Pacific from radiolarian
assemblages, using an approach similar to that of
Imbrie and Kipp (1971). They used 170 surface
samples to develop the transfer functions. Standard error estimates were 1.6°C, both for SST and
seasonal temperature range. Temperatures derived
from radiolarian-based transfer functions compared well with temperature calculations from U'J7
index in a core taken from the northeastern Pacific
which covers the last 20,000 years. New calibrations of a diatom transer function for the Southern
Ocean are provided by Zielinski et al. (1998) using
218 surface sediment samples recovered in the
Atlantic and western Indian sector. The defined
transfer functions were applied on a sediment core
from the northern Antarctic Zone (Fig. 4).
How can the temperature proxies be improved?
A primary requirement is a better understanding of
the processes which transform a biocoenosis into
a taphocoenosis. Sediment-trap experiments can be
very helpful in examining these processes. In general, the results show a pronounced seasonality
related to productivity (Lange et al. 1994; Treppke
et al. 1996b). A large portion of the community is
destroyed by zooplankton grazing (Sancetta 1989).
As a rule, selective dissolution causes an enrichment of robust shells (Treppke et al. 1996a). Another important consideration is that, under conditions of strong seasonality, the community found on
the sea floor represents just a brief time period
within each year (Gersonde and Wefer 1987). The
distribution and changes in species composition
with increased water depth continues to provide a
focus for research in this context (e.g. Abelmann
and Gersonde 1992; Boltovskoy 1996a, b).
Temperature Estimates from Oxygen
Isotope Composition
As mentioned in the introduction, the oxygen isotope composition of calcareous fossils depends on
the temperature of calcification. The other most
important factor is the composition of the water
itself (Equation 2), which varies through time and
from place to place, and can substantially affect
temperature estimates. The oxygen isotope composition of a given shell is reported as 8 18 0 which
is defined as the deviation ofthe ratio ofl80 to 16 0
measured in the carbonate of the shell from the
ratio in a standard, in tenths of a percent (= perm ii,
0/0)·
The glacial to postglacial change in8 18 0 in the
ocean is slightly greater than 1 0 / 00 , reflecting preferential deposition ofl60 on continental ice sheets
during glacial periods. One permil corresponds to
an apparent change in temperature of SoC (taking