Circulation of the ocean
Several proxies are available to decipher the history of
ocean circulation. The ratio of the stable carbon isotopes
(
12
C/
13
C) in benthic foraminifera is a measure of the age
of the bottom water. During descent of oxygenated surface
water to the deep sea, organic matter enriched with the stable isotope
12 C is degraded. Through this process, water in
the deep ocean becomes more enriched in
12
C, which is
recorded in the shells of benthic foraminifera. The more
organic matter that was decomposed using oxygen, the
more
12
C is present in the shells, and the isotopic ratio is
lighter. A similar mechanism operates for the cadmium/
calcium isotope ratios. The Cd/Ca isotope ratios and the
phosphate content of the water show a linear relationship.
With the decomposition of organic matter, cadmium and
phosphate are released, and therefore, the Cd/Ca ratios in
the shells of benthic foraminifera provide information on
the phosphate content, which is related to the age of the
bottom water and ventilation of the ocean in the past.
A relatively new proxy is the isotopic ratio of protactinium
231 Pa to thorium
230
Th. Both isotopes are formed by
the radioactive decay of uranium dissolved in the water
column, and
231 Pa has a longer residence time in the water
column due to its greater half-life. Changes in the
231
Pa/
230
Th ratio are related to the intensity of the thermohaline circulation. Another proxy is the ratio of the neodymium isotopes
143
Nd and
144
Nd. This ratio
characterizes specific bodies of water. The neodymium
isotopes are incorporated into manganese crusts and nodules, and, because of the low growth rates of the nodules,
the temporal resolution is low, but the records can go far
back into the Earth’s history. There are also a number of
sedimentological proxies, e.g., the proportion of sortable
silt (10–63 m) of deep-sea sediments, which provides
information about bottom currents.
Not only currents or water masses are reconstructed;
diatoms and biomarkers also allow the reconstruction of
past sea-ice boundaries (Gersonde et al., 2014). Promising
new biomarkers include sea-ice diatom-specific highly
branched isoprenoids with 25 carbon isotopes. A review
is given by Stein et al. (2012) describing efficient kinds
of proxies that can be used to reconstruct sea-ice cover
and temperatures in polar environments.
Sea-level
One specific issue is the past sea-level stand in relation to
climate. There are many regional sea-level reconstructions
that need to be related to the eustatic sea-level. The problem is the dynamics of ice sheets. During the melting of
ice shelves and the associated rise of the land, the ice body
is stabilized. For the correction of sea-level reconstructions, it is important to know where the ice was deposited
or melted. For an example of a sea-level reconstruction
that takes into account the melting of the Greenland Ice
Sheet and West Antarctic Ice Sheet, see Raymo and
Mitrovica (2012). While for the present sea-level rise of
between 1.5 and 2 mm/year thermal expansion plays a
major role, ice dynamics are dominant over longer
periods.
CO 2 content of the atmosphere
Boron isotope ratios (
11
B/
10
B) can be used to reconstruct
the past CO 2 content of the atmosphere. The boron isotope
ratio depends on the acidity, pH, and alkalinity of the
water, which are determined by the CO 2 concentration of
the atmosphere and the concentration of bicarbonate ions
in the ocean. The boron isotope ratios are incorporated
into the shells of planktonic foraminifera and are a proxy
for CO 2 levels during earlier times. The theory is understood (e.g., Hönisch and Hemming, 2004), but the measurements are complicated, and hence the data contain
large uncertainties.
What role the ocean plays in the CO 2 content of the
atmosphere depends on the physical and biological
pumps. An important location for studying the effectiveness of the biological pump is the Southern Ocean, as
there are unused nutrients available. Therefore, many
investigations of the effectiveness of the biological pump
during past times are carried out with respect to the position of fronts, sea ice, and dust flux (e.g., Ziegler et al.,
2013).
Land vegetation and rainfall
The pollen of land plants is very resistant to degradation. It
is transported by rivers or through the atmosphere into the
ocean, settles to the seafloor, and is embedded in the sediment. With pollen communities, the history of vegetation
and precipitation patterns of land masses can be
reconstructed when the transport routes are known.
Recently, pollen distributions have been combined with
organic proxies to reconstruct vegetation history and precipitation patterns using the signatures of plant-waxspecific hydrogen (dD) and carbon isotopes (d
13 C)
(Kuechler et al., 2013) (Figure 3).
Stratigraphy
A prerequisite for climate reconstructions is a sound chronological classification of the sedimentary record.
Depending on the time period, different methods are available. Approximately the last 60,000 years can be dated
with the radioactive isotope
14 C, which has a half-life of
5730 years. New possibilities of dating were created by
the development of accelerator mass spectrometry
(AMS) because significantly smaller sample sizes were
sufficient for the measurements. In contrast to 14C, which
has a relatively long half-life of 5730 years and thus does
not allow high temporal resolution, 210Pb produced in the
atmosphere and deposited in sediments over the past
100–150 years can be dated. This dating method is used
in marine and lacustrine sediments, peat and ice deposits.
Since the
14 C content of the atmosphere is subject to
fluctuations caused by the cycles of sunspots and fluctuations of the Earth’s magnetic field, calibration curves are
needed based on dendrochronology and other
PALEOCEANOGRAPHIC PROXIES
625
Several proxies are available to decipher the history of
ocean circulation. The ratio of the stable carbon isotopes
(
12
C/
13
C) in benthic foraminifera is a measure of the age
of the bottom water. During descent of oxygenated surface
water to the deep sea, organic matter enriched with the stable isotope
12 C is degraded. Through this process, water in
the deep ocean becomes more enriched in
12
C, which is
recorded in the shells of benthic foraminifera. The more
organic matter that was decomposed using oxygen, the
more
12
C is present in the shells, and the isotopic ratio is
lighter. A similar mechanism operates for the cadmium/
calcium isotope ratios. The Cd/Ca isotope ratios and the
phosphate content of the water show a linear relationship.
With the decomposition of organic matter, cadmium and
phosphate are released, and therefore, the Cd/Ca ratios in
the shells of benthic foraminifera provide information on
the phosphate content, which is related to the age of the
bottom water and ventilation of the ocean in the past.
A relatively new proxy is the isotopic ratio of protactinium
231 Pa to thorium
230
Th. Both isotopes are formed by
the radioactive decay of uranium dissolved in the water
column, and
231 Pa has a longer residence time in the water
column due to its greater half-life. Changes in the
231
Pa/
230
Th ratio are related to the intensity of the thermohaline circulation. Another proxy is the ratio of the neodymium isotopes
143
Nd and
144
Nd. This ratio
characterizes specific bodies of water. The neodymium
isotopes are incorporated into manganese crusts and nodules, and, because of the low growth rates of the nodules,
the temporal resolution is low, but the records can go far
back into the Earth’s history. There are also a number of
sedimentological proxies, e.g., the proportion of sortable
silt (10–63 m) of deep-sea sediments, which provides
information about bottom currents.
Not only currents or water masses are reconstructed;
diatoms and biomarkers also allow the reconstruction of
past sea-ice boundaries (Gersonde et al., 2014). Promising
new biomarkers include sea-ice diatom-specific highly
branched isoprenoids with 25 carbon isotopes. A review
is given by Stein et al. (2012) describing efficient kinds
of proxies that can be used to reconstruct sea-ice cover
and temperatures in polar environments.
Sea-level
One specific issue is the past sea-level stand in relation to
climate. There are many regional sea-level reconstructions
that need to be related to the eustatic sea-level. The problem is the dynamics of ice sheets. During the melting of
ice shelves and the associated rise of the land, the ice body
is stabilized. For the correction of sea-level reconstructions, it is important to know where the ice was deposited
or melted. For an example of a sea-level reconstruction
that takes into account the melting of the Greenland Ice
Sheet and West Antarctic Ice Sheet, see Raymo and
Mitrovica (2012). While for the present sea-level rise of
between 1.5 and 2 mm/year thermal expansion plays a
major role, ice dynamics are dominant over longer
periods.
CO 2 content of the atmosphere
Boron isotope ratios (
11
B/
10
B) can be used to reconstruct
the past CO 2 content of the atmosphere. The boron isotope
ratio depends on the acidity, pH, and alkalinity of the
water, which are determined by the CO 2 concentration of
the atmosphere and the concentration of bicarbonate ions
in the ocean. The boron isotope ratios are incorporated
into the shells of planktonic foraminifera and are a proxy
for CO 2 levels during earlier times. The theory is understood (e.g., Hönisch and Hemming, 2004), but the measurements are complicated, and hence the data contain
large uncertainties.
What role the ocean plays in the CO 2 content of the
atmosphere depends on the physical and biological
pumps. An important location for studying the effectiveness of the biological pump is the Southern Ocean, as
there are unused nutrients available. Therefore, many
investigations of the effectiveness of the biological pump
during past times are carried out with respect to the position of fronts, sea ice, and dust flux (e.g., Ziegler et al.,
2013).
Land vegetation and rainfall
The pollen of land plants is very resistant to degradation. It
is transported by rivers or through the atmosphere into the
ocean, settles to the seafloor, and is embedded in the sediment. With pollen communities, the history of vegetation
and precipitation patterns of land masses can be
reconstructed when the transport routes are known.
Recently, pollen distributions have been combined with
organic proxies to reconstruct vegetation history and precipitation patterns using the signatures of plant-waxspecific hydrogen (dD) and carbon isotopes (d
13 C)
(Kuechler et al., 2013) (Figure 3).
Stratigraphy
A prerequisite for climate reconstructions is a sound chronological classification of the sedimentary record.
Depending on the time period, different methods are available. Approximately the last 60,000 years can be dated
with the radioactive isotope
14 C, which has a half-life of
5730 years. New possibilities of dating were created by
the development of accelerator mass spectrometry
(AMS) because significantly smaller sample sizes were
sufficient for the measurements. In contrast to 14C, which
has a relatively long half-life of 5730 years and thus does
not allow high temporal resolution, 210Pb produced in the
atmosphere and deposited in sediments over the past
100–150 years can be dated. This dating method is used
in marine and lacustrine sediments, peat and ice deposits.
Since the
14 C content of the atmosphere is subject to
fluctuations caused by the cycles of sunspots and fluctuations of the Earth’s magnetic field, calibration curves are
needed based on dendrochronology and other
PALEOCEANOGRAPHIC PROXIES
625