Fennoscandian ice sheet) at the maximum of the glaciation
caused an enrichment in
18 O of the global ocean of +0.4‰.
We now know that the enrichment in
18 O in the glacial
ocean was in fact close to +1.0‰ (Schrag et al. 2002).
Despite these inaccuracies, the work of Emiliani was the first
to demonstrate from paleoclimate observations that glacial—
interglacial periods indeed oscillated with cyclicities predicted by the Milankovitch theory several decades earlier
(see Chap. 28). Emiliani also proposed the ‘Marine Isotopic
Stage’ (MIS) nomenclature, now universally adopted, to
characterize the alternation of warm and cold Pleistocene
phases, with odd numbers for interglacial periods and even
numbers for glacial ones (1 for the Holocene, 2 for the last
glacial period, and so on). He also discovered that the last
interglacial, or MIS 5 (Fig. 21.1), was interrupted by two
colder periods, which led him to divide it into three warm
subperiods (designated 5a, 5c and 5e from the most recent to
the oldest) and two cold ones (5b and 5d). The term ‘5e’ is
still frequently used, as it has been incorporated into the
European continental reconstructions as the equivalent to the
Eemian warm period. The isotopic stratigraphy formalism
has since been generalized, with subdivisions either numbered as decimals between alternating warmer (e.g., 5.1 for
5a, 5.3 for 5c and 5.5 for 5e) and colder (5.2 and 5.4) periods
(Pisias et al. 1984) or as letters (Railsback et al. 2015).
With the assumption that past variations in foraminiferal
18 O/
16 O ratios in cores from different ocean basins had to be
approximately synchronous across global climate changes,
Emiliani paved the way for a global marine isotopic
stratigraphy. The demonstration that the volume of ice caps
was indeed the dominant component of the isotopic signal
recorded in marine cores reinforced the stratigraphic value of
the marine isotopic stage age scale, which became a major
reference tool for past climate change studies. The routine
use of drilling ships as part of the International Ocean
Drilling Program has allowed the recovery of sediment cores
that cover the last tens of millions of years, extending the
isotopic sequences not only to the Quaternary (Fig. 21.2),
but as far back as the Paleocene, 60 Ma ago.
By 1970–1980, the paleoclimate community had arrived
at the conclusion that variations in the oxygen isotopic
composition provided a remarkable stratigraphic tool to
establish long-term correlations. However, new tools still
needed to be developed to precisely reconstruct past SST, as
well as variations in other oceanic features such as salinity,
or the direction and intensity of deep-water currents.
This chapter will focus on the development of classic and
new paleoceanographic tracers over the last decades. We
will mainly, but not exclusively, focus on tracers that are
based on foraminifera, since these abundant microfossils
have been extensively used because of their ubiquity in the
oceans and their great preservation potential. Their faunal
associations and isotopic composition have been widely
used as paleoclimate indicators for decades, and the advent
of relatively new indicators implies an undiminished interest.
Other indicators such as corals will be briefly discussed in
association with some of the tracers.
Sea Surface Temperature
The surface temperature of the ocean is an essential climate
parameter that governs heat exchange with the atmosphere.
SST also modulates the solubility of gases, oxygen and CO 2
in particular, and their exchange rates with the atmosphere.
The amplitude of the spatial variability of SST is well
known: it ranges between −1.96 °C, freezing point for seawater at 35 psu, and 30–35 °C, maximum temperature
recorded for the open ocean. However, its temporal variability is more difficult to constrain because it not only varies
on a daily basis, but also seasonally and annually. In situ
measurement sensors are precise to ±0.001 °C at a given
location, water depth and time. Satellite data provide global
coverage and allow long-term monitoring of the evolution of
SST, but their accuracy is, at best, close to 0.1 °C, and
surface values are averaged over tens or hundreds of square
kilometers. Paleoceanographers cannot aim to reconstruct
SST variations with this level of precision. Nonetheless,
given the magnitude of changes in the past, relevant information may be acquired when SST changes are estimated to
the nearest degree. Paleoceanographers also aim to estimate,
whenever possible, the amplitude of the seasonal cycle, and
the temperature distribution of the upper water column.
For these studies, two major types of paleotemperature
indicators are used: (i) changes in the distribution of fossil
planktonic flora or fauna (foraminifera, diatoms, dinoflagellates, radiolarians), and (ii) geochemical tracers produced
by these organisms or recorded in their fossil skeletons.
The Distribution of Marine Fauna and Flora
The distribution of the various groups that make up the marine
planktonic ecosystem was extensively studied during the
major exploration campaigns that marked the nineteenth and
early twentieth centuries. Foraminifera, single-celled protozoans that secrete a calcareous shell, were the most generally
recorded group. They are very diverse and inhabit all the
oceans, from the coldest to the warmest. However, each
individual species has a limited tolerance to environmental
changes, particularly temperature, which allowed biologists at
the end of the nineteenth century to highlight the zonal distribution of many species. They also established existing
relationships between climate and the abundance of certain
21 Climate and the Evolution of the Ocean: The Paleoceanographic …
227
caused an enrichment in
18 O of the global ocean of +0.4‰.
We now know that the enrichment in
18 O in the glacial
ocean was in fact close to +1.0‰ (Schrag et al. 2002).
Despite these inaccuracies, the work of Emiliani was the first
to demonstrate from paleoclimate observations that glacial—
interglacial periods indeed oscillated with cyclicities predicted by the Milankovitch theory several decades earlier
(see Chap. 28). Emiliani also proposed the ‘Marine Isotopic
Stage’ (MIS) nomenclature, now universally adopted, to
characterize the alternation of warm and cold Pleistocene
phases, with odd numbers for interglacial periods and even
numbers for glacial ones (1 for the Holocene, 2 for the last
glacial period, and so on). He also discovered that the last
interglacial, or MIS 5 (Fig. 21.1), was interrupted by two
colder periods, which led him to divide it into three warm
subperiods (designated 5a, 5c and 5e from the most recent to
the oldest) and two cold ones (5b and 5d). The term ‘5e’ is
still frequently used, as it has been incorporated into the
European continental reconstructions as the equivalent to the
Eemian warm period. The isotopic stratigraphy formalism
has since been generalized, with subdivisions either numbered as decimals between alternating warmer (e.g., 5.1 for
5a, 5.3 for 5c and 5.5 for 5e) and colder (5.2 and 5.4) periods
(Pisias et al. 1984) or as letters (Railsback et al. 2015).
With the assumption that past variations in foraminiferal
18 O/
16 O ratios in cores from different ocean basins had to be
approximately synchronous across global climate changes,
Emiliani paved the way for a global marine isotopic
stratigraphy. The demonstration that the volume of ice caps
was indeed the dominant component of the isotopic signal
recorded in marine cores reinforced the stratigraphic value of
the marine isotopic stage age scale, which became a major
reference tool for past climate change studies. The routine
use of drilling ships as part of the International Ocean
Drilling Program has allowed the recovery of sediment cores
that cover the last tens of millions of years, extending the
isotopic sequences not only to the Quaternary (Fig. 21.2),
but as far back as the Paleocene, 60 Ma ago.
By 1970–1980, the paleoclimate community had arrived
at the conclusion that variations in the oxygen isotopic
composition provided a remarkable stratigraphic tool to
establish long-term correlations. However, new tools still
needed to be developed to precisely reconstruct past SST, as
well as variations in other oceanic features such as salinity,
or the direction and intensity of deep-water currents.
This chapter will focus on the development of classic and
new paleoceanographic tracers over the last decades. We
will mainly, but not exclusively, focus on tracers that are
based on foraminifera, since these abundant microfossils
have been extensively used because of their ubiquity in the
oceans and their great preservation potential. Their faunal
associations and isotopic composition have been widely
used as paleoclimate indicators for decades, and the advent
of relatively new indicators implies an undiminished interest.
Other indicators such as corals will be briefly discussed in
association with some of the tracers.
Sea Surface Temperature
The surface temperature of the ocean is an essential climate
parameter that governs heat exchange with the atmosphere.
SST also modulates the solubility of gases, oxygen and CO 2
in particular, and their exchange rates with the atmosphere.
The amplitude of the spatial variability of SST is well
known: it ranges between −1.96 °C, freezing point for seawater at 35 psu, and 30–35 °C, maximum temperature
recorded for the open ocean. However, its temporal variability is more difficult to constrain because it not only varies
on a daily basis, but also seasonally and annually. In situ
measurement sensors are precise to ±0.001 °C at a given
location, water depth and time. Satellite data provide global
coverage and allow long-term monitoring of the evolution of
SST, but their accuracy is, at best, close to 0.1 °C, and
surface values are averaged over tens or hundreds of square
kilometers. Paleoceanographers cannot aim to reconstruct
SST variations with this level of precision. Nonetheless,
given the magnitude of changes in the past, relevant information may be acquired when SST changes are estimated to
the nearest degree. Paleoceanographers also aim to estimate,
whenever possible, the amplitude of the seasonal cycle, and
the temperature distribution of the upper water column.
For these studies, two major types of paleotemperature
indicators are used: (i) changes in the distribution of fossil
planktonic flora or fauna (foraminifera, diatoms, dinoflagellates, radiolarians), and (ii) geochemical tracers produced
by these organisms or recorded in their fossil skeletons.
The Distribution of Marine Fauna and Flora
The distribution of the various groups that make up the marine
planktonic ecosystem was extensively studied during the
major exploration campaigns that marked the nineteenth and
early twentieth centuries. Foraminifera, single-celled protozoans that secrete a calcareous shell, were the most generally
recorded group. They are very diverse and inhabit all the
oceans, from the coldest to the warmest. However, each
individual species has a limited tolerance to environmental
changes, particularly temperature, which allowed biologists at
the end of the nineteenth century to highlight the zonal distribution of many species. They also established existing
relationships between climate and the abundance of certain
21 Climate and the Evolution of the Ocean: The Paleoceanographic …
227