On the Reconstruction of Paleosalinities
T. Wolff, B. Grieger, WHale, A. Durkoop, S. Mulitza, 1. Pdtzold
andG. Wefer
Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 0440,
D-28334 Bremen, Germany
*corresponding author (e-mail) . . twoljj@uni-bremen.de
Abstract: Methods potentially useful for paleosalinity reconstructions are summarized and applied
to surface sediments and two cores from the Atlantic. The first approach is based on the oxygen
isotope ratio in calcite tests of planktonic foraminifera in conjunction with an independent seasurface temperature estimate. A two-step procedure from foraminiferal/iI'O to the isotopic composition
of sea-water and from there to an estimate of paleosalinity is proposed. The quality of the estimation
of /i1·0 for sea-water depends heavily on the reliability of the independent temperature method. The
fmal salinities are obtained through an assumed /i 18 0-salinity relationship for sea-water. Propagation
of errors yields large deviations, especially in the tropics where slopes of the /i1'O-salinity relationship
are low. An uncertainty in temperature of ± I °C leads to errors in the salinity reconstruction of ±O. 5 to
± 1.2 %0. Downcore application at a site in the western equatorial Atlantic indicates salinity increases
of roughly 2.5 %0 in the glacials, but also demonstrates sensitivity of the results to the temperature
estimates and the /i 18 0-salinity relationship. Two additional methods of paleosalinity estimation
were investigated both of which employ foraminiferal abundance data. An Artificial Neural Network
was used for the first time to reconstruct paleosalinities. The obtained results were then compared to
results from a Modern Analog Technique. Application to surface sediments yields comparable
results for both methods, with standard deviations between 0.49 and 0.63 %0. Salinity calculations
performed on downcore data in the tropical and subtropical South Atlantic indicate that the results
are controlled by today's temperature-salinity relationship. This leads to the conclusion that
paleosalinity reconstructions from species composition of foraminiferal assemblages are unrealistic,
because of a predominant response of the fauna to temperature.
Introduction
Temperature and salinity determine the density of
a water mass, thus, they are important keys to the
thermohaline circulation and as boundary conditions
they are of special interest for modelling purposes
(Washington and Parkinson 1986). The formation
of deep water is extremely sensitive to salinity
(Broecker and Denton 1989) and so, therefore, are
general circulation models of the ocean
(Lautenschlager et al. 1992). These models are
executed with Newtonian or flux boundary conditions. For sea-surface temperatures (SSTs),
Newtonian boundary conditions are usually employed, i.e. direct SST estimates (e.g. from
CLIMAP 1976, CLIMAP 1981) are taken.
Paleosalinities have to be roughly estimated, otherwise flux boundary conditions are used instead. This
implicates that salinities are replaced by freshwater fluxes taken from atmospheric model outputs.
To improve ocean modelling, paleosalinity "measurements" with high accuracies are needed.
No direct methods exist so far. In other words,
there is no geological parameter that can be related
directly to paleosalinity in the open ocean. Instead,
salinities are estimated by performing a number of
successive computations on compound proxies, each
of which requires approximations.
From FISCHER G, WEFER G (eds) 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 207-228
T. Wolff, B. Grieger, WHale, A. Durkoop, S. Mulitza, 1. Pdtzold
andG. Wefer
Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 0440,
D-28334 Bremen, Germany
*corresponding author (e-mail) . . twoljj@uni-bremen.de
Abstract: Methods potentially useful for paleosalinity reconstructions are summarized and applied
to surface sediments and two cores from the Atlantic. The first approach is based on the oxygen
isotope ratio in calcite tests of planktonic foraminifera in conjunction with an independent seasurface temperature estimate. A two-step procedure from foraminiferal/iI'O to the isotopic composition
of sea-water and from there to an estimate of paleosalinity is proposed. The quality of the estimation
of /i1·0 for sea-water depends heavily on the reliability of the independent temperature method. The
fmal salinities are obtained through an assumed /i 18 0-salinity relationship for sea-water. Propagation
of errors yields large deviations, especially in the tropics where slopes of the /i1'O-salinity relationship
are low. An uncertainty in temperature of ± I °C leads to errors in the salinity reconstruction of ±O. 5 to
± 1.2 %0. Downcore application at a site in the western equatorial Atlantic indicates salinity increases
of roughly 2.5 %0 in the glacials, but also demonstrates sensitivity of the results to the temperature
estimates and the /i 18 0-salinity relationship. Two additional methods of paleosalinity estimation
were investigated both of which employ foraminiferal abundance data. An Artificial Neural Network
was used for the first time to reconstruct paleosalinities. The obtained results were then compared to
results from a Modern Analog Technique. Application to surface sediments yields comparable
results for both methods, with standard deviations between 0.49 and 0.63 %0. Salinity calculations
performed on downcore data in the tropical and subtropical South Atlantic indicate that the results
are controlled by today's temperature-salinity relationship. This leads to the conclusion that
paleosalinity reconstructions from species composition of foraminiferal assemblages are unrealistic,
because of a predominant response of the fauna to temperature.
Introduction
Temperature and salinity determine the density of
a water mass, thus, they are important keys to the
thermohaline circulation and as boundary conditions
they are of special interest for modelling purposes
(Washington and Parkinson 1986). The formation
of deep water is extremely sensitive to salinity
(Broecker and Denton 1989) and so, therefore, are
general circulation models of the ocean
(Lautenschlager et al. 1992). These models are
executed with Newtonian or flux boundary conditions. For sea-surface temperatures (SSTs),
Newtonian boundary conditions are usually employed, i.e. direct SST estimates (e.g. from
CLIMAP 1976, CLIMAP 1981) are taken.
Paleosalinities have to be roughly estimated, otherwise flux boundary conditions are used instead. This
implicates that salinities are replaced by freshwater fluxes taken from atmospheric model outputs.
To improve ocean modelling, paleosalinity "measurements" with high accuracies are needed.
No direct methods exist so far. In other words,
there is no geological parameter that can be related
directly to paleosalinity in the open ocean. Instead,
salinities are estimated by performing a number of
successive computations on compound proxies, each
of which requires approximations.
From FISCHER G, WEFER G (eds) 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 207-228