Clues to Ocean History: a Brief Overview of Proxies
3
water temperatures for the Last Glacial Maximum
(LGM). Large glacial-to-postglacial temperature
differences were found in high latitudes, while differences in the tropics are small. The systematic
exploration of oxygen isotopes in foraminifera, taken
to be a proxy for the waxing and waning of global
ice mass, led to the recognition of orbital forcing of
ice-age climate, that is, Quaternary sedimentation
is dominated by Milankovitch cycles. Thanks to
orbital tuning, the sequence of warm and cold periods over the past few million years can be reconstructed with high temporal resolution from the deepsea sediments, and then related to Quaternary deposits on land. Pre-Quaternary sediments are accessible through deep-ocean drilling. Major research themes include the warm climate of the
Cretaceous, the CretaceouslTertiary boundary, and
climate change throughout the Tertiary, especially
in connection with the opening and closing of ocean
passages and the concomitant modification of circulation systems in the individual ocean basins.
The sediment record is not limited to information relating to the history of the oceans; it also
contains messages from adjacent land masses providing insight into the history of climate and of vegetation cover on the continents. In fact, marine
sediments usually yield better information on the
climate of the land masses than outcrops on land
because the ocean deposits are more nearly continuous, and age assignment is generally more reliable. Examples for this approach include evidence
for a worldwide desertification during the Last Glacial Maximum about 20,000 years ago (Sarnthein
1978), the Late Quaternary vegetation history of
northern Africa (Hoogchiemstra 1988; Dupont
1993) showing the influence of precession-controlled monsoons (Rossignol-Strick 1983) and pulses
of aridity in tropical Africa (Janssen and van Iperen
1991 ).
The accuracy of reconstructions of past environmental conditions, including circulation and productivity in the ocean, is dependent on the quality
of the database. To make optimal use of sedimentderived information, it is necessary to calibrate sediment properties to present-day oceanographic and
biological (productivity) conditions mapping modem
mass fluxes of carbonate, opal, organic carbon,
terrigenous material, (wind- and river-transported,
as well as ice-rafted debris (IRD», and particular
components such as microfossil contents and elemental and isotopic compositions.
Mass flux data of various components (e.g. CO,g'
opal, carbonate) give a general impression of the
productivity (related to nutrient availability) ofthe
surface waters. However, they also provide information aboutthe characteristics of the deep ocean,
for example, the undersaturation of bottom waters
with calcite. The proportion ofIRD (grain size> 2
mm) in deep-sea sediment allows reconstruction of
iceberg drift paths during melting phases of the polar
ice caps. However, the most important and accurate information is provided by various microfossils,
either in the form of species assemblages or through
their elemental and isotopic compositions. Surfaceand bottom-dwelling organisms are sensitive recorders of environmental changes, with different species reacting to different stimmuli. These relationships are the subject of investigations of patterns
in the recent ocean (using plankton tows and sediment-trap experiments) and by culturing experiments on shell-bearing organisms such as
foraminifers, diatoms and nannoplankton, in the
laboratory.
The investigation of the processes in biogenous
sediments resulted in a new appreciation of
seasonality of production and the importance of
unusual events. Much of the biogenous sediment
accummulating on the sea floor arrives during a few
months ofthe year (Wefer 1989; Berger and Wefer
1990). In extreme cases, given a strong seasonality
of production ("pulsed production"), only a short
time period of the year will be represented in the
sediments. For example, approximately 90% of the
annual production of diatoms in the Bransfield Strait
(Antarctic) occurs within a single month (Gersonde
and Wefer 1987; Wefer et al. 1988). In addition,
there are large year-to-year differences, such that
the output from a few years may be greatly overrepresented in a time period of several decades.
Isolated events such as abrupt sediment flows
(turbidites) or volcanic eruptions can severely disturb the sediment pattern. On the other hand, such
events may provide a useful and accurate time
marker for correlating cores over a large region.
A well-known example is the ash layer at about
10,000 years in the North Atlantic (Vedde Ash;
3
water temperatures for the Last Glacial Maximum
(LGM). Large glacial-to-postglacial temperature
differences were found in high latitudes, while differences in the tropics are small. The systematic
exploration of oxygen isotopes in foraminifera, taken
to be a proxy for the waxing and waning of global
ice mass, led to the recognition of orbital forcing of
ice-age climate, that is, Quaternary sedimentation
is dominated by Milankovitch cycles. Thanks to
orbital tuning, the sequence of warm and cold periods over the past few million years can be reconstructed with high temporal resolution from the deepsea sediments, and then related to Quaternary deposits on land. Pre-Quaternary sediments are accessible through deep-ocean drilling. Major research themes include the warm climate of the
Cretaceous, the CretaceouslTertiary boundary, and
climate change throughout the Tertiary, especially
in connection with the opening and closing of ocean
passages and the concomitant modification of circulation systems in the individual ocean basins.
The sediment record is not limited to information relating to the history of the oceans; it also
contains messages from adjacent land masses providing insight into the history of climate and of vegetation cover on the continents. In fact, marine
sediments usually yield better information on the
climate of the land masses than outcrops on land
because the ocean deposits are more nearly continuous, and age assignment is generally more reliable. Examples for this approach include evidence
for a worldwide desertification during the Last Glacial Maximum about 20,000 years ago (Sarnthein
1978), the Late Quaternary vegetation history of
northern Africa (Hoogchiemstra 1988; Dupont
1993) showing the influence of precession-controlled monsoons (Rossignol-Strick 1983) and pulses
of aridity in tropical Africa (Janssen and van Iperen
1991 ).
The accuracy of reconstructions of past environmental conditions, including circulation and productivity in the ocean, is dependent on the quality
of the database. To make optimal use of sedimentderived information, it is necessary to calibrate sediment properties to present-day oceanographic and
biological (productivity) conditions mapping modem
mass fluxes of carbonate, opal, organic carbon,
terrigenous material, (wind- and river-transported,
as well as ice-rafted debris (IRD», and particular
components such as microfossil contents and elemental and isotopic compositions.
Mass flux data of various components (e.g. CO,g'
opal, carbonate) give a general impression of the
productivity (related to nutrient availability) ofthe
surface waters. However, they also provide information aboutthe characteristics of the deep ocean,
for example, the undersaturation of bottom waters
with calcite. The proportion ofIRD (grain size> 2
mm) in deep-sea sediment allows reconstruction of
iceberg drift paths during melting phases of the polar
ice caps. However, the most important and accurate information is provided by various microfossils,
either in the form of species assemblages or through
their elemental and isotopic compositions. Surfaceand bottom-dwelling organisms are sensitive recorders of environmental changes, with different species reacting to different stimmuli. These relationships are the subject of investigations of patterns
in the recent ocean (using plankton tows and sediment-trap experiments) and by culturing experiments on shell-bearing organisms such as
foraminifers, diatoms and nannoplankton, in the
laboratory.
The investigation of the processes in biogenous
sediments resulted in a new appreciation of
seasonality of production and the importance of
unusual events. Much of the biogenous sediment
accummulating on the sea floor arrives during a few
months ofthe year (Wefer 1989; Berger and Wefer
1990). In extreme cases, given a strong seasonality
of production ("pulsed production"), only a short
time period of the year will be represented in the
sediments. For example, approximately 90% of the
annual production of diatoms in the Bransfield Strait
(Antarctic) occurs within a single month (Gersonde
and Wefer 1987; Wefer et al. 1988). In addition,
there are large year-to-year differences, such that
the output from a few years may be greatly overrepresented in a time period of several decades.
Isolated events such as abrupt sediment flows
(turbidites) or volcanic eruptions can severely disturb the sediment pattern. On the other hand, such
events may provide a useful and accurate time
marker for correlating cores over a large region.
A well-known example is the ash layer at about
10,000 years in the North Atlantic (Vedde Ash;