38
Weferetal.
to reconstruct paleo-pH, Sanyal et al. (1996) have
demonstrated that the empirical fractionation between foraminiferal calcite and sea water as determined in controlled laboratory experiments, follows the predicted trend. Using this approach,
Sanyal et al. (1995) estimated that the glacial Pacific and Atlantic deep ocean was OJ pH units
higher than today.
There is evidence that the BalCa ratio in shells
of planktic and benthic foraminifera (not authigenic
minerals in the sediment) varies with (salinity normalized) alkalinity, so that paleo-Ba distributions can
be used, in principle, to infer the distribution of alkalinity in past oceans (Lea and Boyle 1990a,b;
Broecker 1992; Lea 1993). The use of foraminiferal
tracers such as BalCa is dependent on the assumption that the trace element is incorporated in shell
material in proportion to its sea water concentration. Lea and Spero (1992) have demonstrated that
0. universa records the ambient Ba concentration
of the seawater it grows in. Recent culture experiments with 0. universa and G. sacculifer, addressing the influence of external parameters such
as temperature and salinity on the partition coefficient, as well as inter- and intra-specific differences
(Lea and Spero 1994), have firmly bolstered the
contention that Ba/Ca ratios in planktic
foraminiferal shells can be used to reconstruct
paleo-Ba content.
One potential problem with using BalCa as a
proxy for alkalinity is that Ba (being of very low
concentration) can presumably vary substantially
in abundance between glacial and interglacial periods. If(as has been proposed) the glacial ocean
was less ventilated attimes, sulfate reduction would
have been more widespread, and this would have
tended to decrease the ease with which barite is
sequestered into sediments. The result would have
been an increased abundance ofBa throughout the
ocean during such conditions oflow ventilation.
Reconstruction of Wind Transport and
Conditions on Adjacent Land Areas
Wind-transported particles are a function of two
factors: intensity of wind (a function of temperature and pressure gradients in the atmosphere
and usually seasonal or with interannual peak
intensities) and availability in the hinterland (aridity). Thus, any sediment property chosen to act as
a "proxy" for wind transport will, in fact, reflect an
entire subsystem of erosion and deposition, of
which the wind is but one part.
Information about the atmospheric circulation
and climate, especially vegetation, is transported to
the ocean as minerals and as plant and animal remains by the wind transport or by the rivers (see
Dupont, this volume). Sarnthein and co-authors
have been able to reconstruct the wind system over
the Sahara and in the North Atlantic based on the
content of silt particles in North Atlantic sediments
(Sarnthein et al. 1981a, b). A clear interpretation
is commonly difficult to obtain because amounts of
terrigenous materials and mineral compositions are
dependent on many factors such as aridity, dust
mobilization and wind direction. The wind system
determines the direction and distance oftransport
(Schuetz 1980) and also affects the transport in the
ocean from the surface to the sea floor. Sediment
trap experiments have shown that organisms play
an important role through fecal pellets and formation of aggregates. For example, Ratmeyer et al.
(1999b) showed a strong seasonality of dust transport from the Sahara into the North Atlantic (Fig.
22). Ratmeyer et al. (1999a) could also demonstrate
that marine Al and lithogenic particle fluxes as well
as the grain-size distribution of the non-biogenic
fraction in the eastern subtropical Atlantic (off
northwest Africa) were directly linked to airmass
pathways and surface mineral aerosol concentrations.
Tiedemann et al. (1994) have published a new
time series of dust transport spanning more than
five million years in a core off west Africa. The
amount of dust input was used as a measure of the
climate of the Sahara: large amounts of dust are
tied to aridity and less dust is interpreted as a more
humid climate. The dust patterns are closely linked
to the irradiation-variable orbital precession. Windtransported material was found at great distances
in the ocean, as far as the equatorial regions. Pokras
and Mix (1985) used the fresh-water diatom
Melosira to document arid periods in tropical Africa, caused by reduction ofthe summer monsoons.
The studies of DeMenocal et al. (1993) also confirm the importance of the trade winds for atmos-
Weferetal.
to reconstruct paleo-pH, Sanyal et al. (1996) have
demonstrated that the empirical fractionation between foraminiferal calcite and sea water as determined in controlled laboratory experiments, follows the predicted trend. Using this approach,
Sanyal et al. (1995) estimated that the glacial Pacific and Atlantic deep ocean was OJ pH units
higher than today.
There is evidence that the BalCa ratio in shells
of planktic and benthic foraminifera (not authigenic
minerals in the sediment) varies with (salinity normalized) alkalinity, so that paleo-Ba distributions can
be used, in principle, to infer the distribution of alkalinity in past oceans (Lea and Boyle 1990a,b;
Broecker 1992; Lea 1993). The use of foraminiferal
tracers such as BalCa is dependent on the assumption that the trace element is incorporated in shell
material in proportion to its sea water concentration. Lea and Spero (1992) have demonstrated that
0. universa records the ambient Ba concentration
of the seawater it grows in. Recent culture experiments with 0. universa and G. sacculifer, addressing the influence of external parameters such
as temperature and salinity on the partition coefficient, as well as inter- and intra-specific differences
(Lea and Spero 1994), have firmly bolstered the
contention that Ba/Ca ratios in planktic
foraminiferal shells can be used to reconstruct
paleo-Ba content.
One potential problem with using BalCa as a
proxy for alkalinity is that Ba (being of very low
concentration) can presumably vary substantially
in abundance between glacial and interglacial periods. If(as has been proposed) the glacial ocean
was less ventilated attimes, sulfate reduction would
have been more widespread, and this would have
tended to decrease the ease with which barite is
sequestered into sediments. The result would have
been an increased abundance ofBa throughout the
ocean during such conditions oflow ventilation.
Reconstruction of Wind Transport and
Conditions on Adjacent Land Areas
Wind-transported particles are a function of two
factors: intensity of wind (a function of temperature and pressure gradients in the atmosphere
and usually seasonal or with interannual peak
intensities) and availability in the hinterland (aridity). Thus, any sediment property chosen to act as
a "proxy" for wind transport will, in fact, reflect an
entire subsystem of erosion and deposition, of
which the wind is but one part.
Information about the atmospheric circulation
and climate, especially vegetation, is transported to
the ocean as minerals and as plant and animal remains by the wind transport or by the rivers (see
Dupont, this volume). Sarnthein and co-authors
have been able to reconstruct the wind system over
the Sahara and in the North Atlantic based on the
content of silt particles in North Atlantic sediments
(Sarnthein et al. 1981a, b). A clear interpretation
is commonly difficult to obtain because amounts of
terrigenous materials and mineral compositions are
dependent on many factors such as aridity, dust
mobilization and wind direction. The wind system
determines the direction and distance oftransport
(Schuetz 1980) and also affects the transport in the
ocean from the surface to the sea floor. Sediment
trap experiments have shown that organisms play
an important role through fecal pellets and formation of aggregates. For example, Ratmeyer et al.
(1999b) showed a strong seasonality of dust transport from the Sahara into the North Atlantic (Fig.
22). Ratmeyer et al. (1999a) could also demonstrate
that marine Al and lithogenic particle fluxes as well
as the grain-size distribution of the non-biogenic
fraction in the eastern subtropical Atlantic (off
northwest Africa) were directly linked to airmass
pathways and surface mineral aerosol concentrations.
Tiedemann et al. (1994) have published a new
time series of dust transport spanning more than
five million years in a core off west Africa. The
amount of dust input was used as a measure of the
climate of the Sahara: large amounts of dust are
tied to aridity and less dust is interpreted as a more
humid climate. The dust patterns are closely linked
to the irradiation-variable orbital precession. Windtransported material was found at great distances
in the ocean, as far as the equatorial regions. Pokras
and Mix (1985) used the fresh-water diatom
Melosira to document arid periods in tropical Africa, caused by reduction ofthe summer monsoons.
The studies of DeMenocal et al. (1993) also confirm the importance of the trade winds for atmos-