4
Weferetal.
Mangerud et al. 1984). Many other types of events
are used for correlation, including magnetic reversals, and first and last appearances of microfossils.
The single most important event recorded in deepsea sediments is the mass extinction at the end of
the Cretaceous. It is commonly ascribed to the
Chixculub meteorite impact on the Yucatan Peninsula (Mexico), whose debris occurs in association
with the extinction.
Diagenetic changes within the sediment can
greatly affect the quality ofthe paleoceanographic
record. Selective removal ofthe more readily dissolving foraminifersl tests dramatically alters the
species composition (see summaries in Vincent and
Berger 1981; Hemleben et al. 1989). Stable isotope
ratios of planktic foraminifers are sensitive as well,
with deviations of up to 0.3%0 in 8180 of planktic
foraminifers attributable to partial dissolution
(Berger and Gardner 1975). Early diagenetic processes such as dissolution are an important part of
the record. Dissolution patterns, for example, allow the reconstruction oflysocline and calcite compensation depth (CCD) and thereby provide clues
about the past carbonate saturation state of the
ocean, which can be tied to the deep water circulation. Other diagenetic influences (which make
it very difficult to interpret stable isotopes in
foraminifers from Mesozoic deposits) include secondary crystal precipitation and recrystallization of
tests. The effects of diagenetic change are usually
quite evident. Other signal-destructive processes
are more subtle. Surface samples can
be homogenized by burrowing organisms
(bioturbation) degrading resolution below a time
frame of several thousand years. In areas of low
sedimentation rates, the record of extreme conditions can be destroyed by mixing of the materials
from widely different conditions.
When studying sediment-generating processes
in the water column, we are faced with the problem that plankton tow samples document only a very
short time interval (compared to the surface sediment), possibly not reflecting the optimal growth
period of a particular group of organisms. In recent
years a sufficient number of sediment-trap experiments have been conducted to obtain a good idea
of seasonal and interannual variability in sediment
production.
In addition to well-documented proxies, an accurate reconstruction of past environmental conditions requires a satisfactory time framework. An
important foundation is the evolution of fossils, especially microfossils. An approximate age can usually be determined relatively quickly and easily using
first and last appearance data of various genera,
species, or phenotypic variations. Stratigraphic
range charts (see Berggren et al. 1995) are continuously updated, so that there is always a current
and internationally recognized age scale available.
Stratigraphic correlations are greatly aided by
magnetostratigraphy, which is also constantly refined. Substantial shifts in age assignments are still
occurring, even in relatively young sediments. The
prime example is the Brunhes/Matuyama boundary, which has recently been assigned an age of
between 775 and 790 ky based on orbital tuning
(Shackleton et al. 1990; Berger et al. 1995), replacing the previously accepted age of 734 ky
(SPECMAP time scale, Imbrie et al. 1984).
Since the pioneering work of Shackleton and
Opdyke (1973) and Imbrie et al. (1984) the most
important basis for dating the Quaternary has been
oxygen isotope stratigraphy (Prell et al. 1986;
Martinson et al. 1987). Ages are assigned by comparing the isotope fluctuations to seasonal insolation patterns in northern latitudes (as first proposed
by Emiliani 1955), thereby "tuning" the record to the
orbital signal (e.g. Berger and Loutre 1991) to take
variations in sedimentation rates into account. In
recent years the significance of astronomic forcing has been brought into discussion as a controlling agent of climate variability within a long-term
Tertiary cooling trend (Hilgen 1991; Lourens and
Hilgen 1994; Maslinetal. 1995).
A continuous sediment sequence is required for
accurate reconstructions of past environmental
change. The continuity and completeness of the
sediment sequences retrieved has been greatly
improved through the use of an advanced piston
coring system (APC) employed on the ODP drilling vessel JOIDES Resolution. It has become
standard practice on paleoceanographic cruises to
drill three or more parallel holes at each site to
ensure recovery of a continuous record. Based on
a characteristic hiatus pattern coinciding with core
breaks it has long been suspected that a portion of
Weferetal.
Mangerud et al. 1984). Many other types of events
are used for correlation, including magnetic reversals, and first and last appearances of microfossils.
The single most important event recorded in deepsea sediments is the mass extinction at the end of
the Cretaceous. It is commonly ascribed to the
Chixculub meteorite impact on the Yucatan Peninsula (Mexico), whose debris occurs in association
with the extinction.
Diagenetic changes within the sediment can
greatly affect the quality ofthe paleoceanographic
record. Selective removal ofthe more readily dissolving foraminifersl tests dramatically alters the
species composition (see summaries in Vincent and
Berger 1981; Hemleben et al. 1989). Stable isotope
ratios of planktic foraminifers are sensitive as well,
with deviations of up to 0.3%0 in 8180 of planktic
foraminifers attributable to partial dissolution
(Berger and Gardner 1975). Early diagenetic processes such as dissolution are an important part of
the record. Dissolution patterns, for example, allow the reconstruction oflysocline and calcite compensation depth (CCD) and thereby provide clues
about the past carbonate saturation state of the
ocean, which can be tied to the deep water circulation. Other diagenetic influences (which make
it very difficult to interpret stable isotopes in
foraminifers from Mesozoic deposits) include secondary crystal precipitation and recrystallization of
tests. The effects of diagenetic change are usually
quite evident. Other signal-destructive processes
are more subtle. Surface samples can
be homogenized by burrowing organisms
(bioturbation) degrading resolution below a time
frame of several thousand years. In areas of low
sedimentation rates, the record of extreme conditions can be destroyed by mixing of the materials
from widely different conditions.
When studying sediment-generating processes
in the water column, we are faced with the problem that plankton tow samples document only a very
short time interval (compared to the surface sediment), possibly not reflecting the optimal growth
period of a particular group of organisms. In recent
years a sufficient number of sediment-trap experiments have been conducted to obtain a good idea
of seasonal and interannual variability in sediment
production.
In addition to well-documented proxies, an accurate reconstruction of past environmental conditions requires a satisfactory time framework. An
important foundation is the evolution of fossils, especially microfossils. An approximate age can usually be determined relatively quickly and easily using
first and last appearance data of various genera,
species, or phenotypic variations. Stratigraphic
range charts (see Berggren et al. 1995) are continuously updated, so that there is always a current
and internationally recognized age scale available.
Stratigraphic correlations are greatly aided by
magnetostratigraphy, which is also constantly refined. Substantial shifts in age assignments are still
occurring, even in relatively young sediments. The
prime example is the Brunhes/Matuyama boundary, which has recently been assigned an age of
between 775 and 790 ky based on orbital tuning
(Shackleton et al. 1990; Berger et al. 1995), replacing the previously accepted age of 734 ky
(SPECMAP time scale, Imbrie et al. 1984).
Since the pioneering work of Shackleton and
Opdyke (1973) and Imbrie et al. (1984) the most
important basis for dating the Quaternary has been
oxygen isotope stratigraphy (Prell et al. 1986;
Martinson et al. 1987). Ages are assigned by comparing the isotope fluctuations to seasonal insolation patterns in northern latitudes (as first proposed
by Emiliani 1955), thereby "tuning" the record to the
orbital signal (e.g. Berger and Loutre 1991) to take
variations in sedimentation rates into account. In
recent years the significance of astronomic forcing has been brought into discussion as a controlling agent of climate variability within a long-term
Tertiary cooling trend (Hilgen 1991; Lourens and
Hilgen 1994; Maslinetal. 1995).
A continuous sediment sequence is required for
accurate reconstructions of past environmental
change. The continuity and completeness of the
sediment sequences retrieved has been greatly
improved through the use of an advanced piston
coring system (APC) employed on the ODP drilling vessel JOIDES Resolution. It has become
standard practice on paleoceanographic cruises to
drill three or more parallel holes at each site to
ensure recovery of a continuous record. Based on
a characteristic hiatus pattern coinciding with core
breaks it has long been suspected that a portion of