LONG-TERM TRACER CHANGES
F. von Blanckenburg, Universita ¨ t Bern, Bern,
Switzerland
Copyright & 2001 Elsevier Ltd.
Introduction
Ocean tracers that record long-term changes preserve certain water column information within the
sediment. This information comprises (1) the tracers’
fluxes in the past, such as erosional input from the
continents, hydrothermal activity at mid-ocean ridges, input of extraterrestrial material, or carbonate
recycling; (2) the distribution of water masses in the
past and the state of the past global thermohaline
circulation. Inorganic isotope tracers whose isotope
ratios are modified by radioactive decay in their
source materials are ideally suited for these studies.
Their original water column values can be measured
in materials such as biogenic carbonates, ferromanganese crusts and nodules, and the authigenic
phase of deep-sea sediments.
Studies of tracer fluxes in the past are favored
by those tracers whose residence time in the ocean
(t, defined below) is long relative to the turnover
time of the thermohaline circulation (1500 y),
such as Sr and Os. Tracers of which t is of the order
of, or shorter than the oceans turnover time (Nd,
Hf, Pb, Be) offer the ability to label water masses
isotopically. In this case, long-term isotope changes
of these intermediate-t tracers are potentially
caused by variations of the thermohaline circulation.
However, secular variations of these isotope
tracers can also be caused by regional variations in
these tracers’ fluxes, mostly resulting from changes
in weathering. It is not always straightforward to
distinguish between these two causes of tracer
variations.
Certainly the globally uniform seawater isotope
evolution of Sr, Os, and potentially also Be, offer
excellent tools for isotope stratigraphy on long (My)
timescales.
Definitions and Concepts
Long-term tracers are those elements whose isotopic
compositions provide information on the physical
and chemical state of the oceans on timescales
of several thousands of years to millions of years
(My). For example, paleo-oceanographers aim to
reconstruct past water mass distributions and the
mode of the thermohaline circulation. For this purpose it would be desirable to reconstruct past
oceanographic water mass characteristics such as
salinity, temperature, silica, or phosphorus content
from the sedimentary record. Similarly, the reconstruction of the past land–sea transfer of certain
tracers is desirable in order to reconstruct changes in
the weathering history of the continents. However,
these present-day tracers are usually not conserved in
the sedimentary record. Even if they were precipitated chemically and stored in sediments, their
changes in concentration as measured in a sedimentary column back through time cannot be directly related to past water mass properties. This is
because the tracers’ concentrations in sediments depend on factors such as sedimentation rate, diagenesis, partitioning into a certain phase, uptake by
organisms, and additions of the same element by
detrital hemipelagic or aeolian material. Therefore
ocean chemists make use of proxy tracers which are
not routinely analyzed in surveys of present-day
water masses, because their measurement presents a
considerable effort compared with tracers such as
salinity and temperature. However, their characteristics can be directly related to those well-known
oceanographic seawater tracers.
The conditions that need to be met for an element
to be of use as a proxy tracer are that (1) it conserves
a characteristic chemical or isotopic property when
transferred from the water column into the sediment;
(2) the elements or their isotopic composition can be
extracted from the sediment; (3) the age of the
sediment is known so that changes of the tracer over
time can be reconstructed.
One such proxy makes use of element ratios. For
example, the ratio of Cd to Ca in foraminiferal tests
is a proxy for the PO 4 content of the past water mass
in which the foraminifera formed and therefore
provides information on the past thermohaline circulation. This tracer is explained in detail in the
article on trace elements in foraminiferal tests.
Similarly, the ratio of the intermediate uranium
decay products
231
Pa and
230 Th, measured in bulk
sediment, may under certain conditions provide information on the advection of water masses in the
overlying water column in the past (see Cosmogenic
Isotopes and Uranium-Thorium Series Isotopes in
Ocean Profiles). Isotope ratios are ideally suited as
long-term proxy tracers. Some of these isotope ratios
are characteristic of certain seawater properties and
123
F. von Blanckenburg, Universita ¨ t Bern, Bern,
Switzerland
Copyright & 2001 Elsevier Ltd.
Introduction
Ocean tracers that record long-term changes preserve certain water column information within the
sediment. This information comprises (1) the tracers’
fluxes in the past, such as erosional input from the
continents, hydrothermal activity at mid-ocean ridges, input of extraterrestrial material, or carbonate
recycling; (2) the distribution of water masses in the
past and the state of the past global thermohaline
circulation. Inorganic isotope tracers whose isotope
ratios are modified by radioactive decay in their
source materials are ideally suited for these studies.
Their original water column values can be measured
in materials such as biogenic carbonates, ferromanganese crusts and nodules, and the authigenic
phase of deep-sea sediments.
Studies of tracer fluxes in the past are favored
by those tracers whose residence time in the ocean
(t, defined below) is long relative to the turnover
time of the thermohaline circulation (1500 y),
such as Sr and Os. Tracers of which t is of the order
of, or shorter than the oceans turnover time (Nd,
Hf, Pb, Be) offer the ability to label water masses
isotopically. In this case, long-term isotope changes
of these intermediate-t tracers are potentially
caused by variations of the thermohaline circulation.
However, secular variations of these isotope
tracers can also be caused by regional variations in
these tracers’ fluxes, mostly resulting from changes
in weathering. It is not always straightforward to
distinguish between these two causes of tracer
variations.
Certainly the globally uniform seawater isotope
evolution of Sr, Os, and potentially also Be, offer
excellent tools for isotope stratigraphy on long (My)
timescales.
Definitions and Concepts
Long-term tracers are those elements whose isotopic
compositions provide information on the physical
and chemical state of the oceans on timescales
of several thousands of years to millions of years
(My). For example, paleo-oceanographers aim to
reconstruct past water mass distributions and the
mode of the thermohaline circulation. For this purpose it would be desirable to reconstruct past
oceanographic water mass characteristics such as
salinity, temperature, silica, or phosphorus content
from the sedimentary record. Similarly, the reconstruction of the past land–sea transfer of certain
tracers is desirable in order to reconstruct changes in
the weathering history of the continents. However,
these present-day tracers are usually not conserved in
the sedimentary record. Even if they were precipitated chemically and stored in sediments, their
changes in concentration as measured in a sedimentary column back through time cannot be directly related to past water mass properties. This is
because the tracers’ concentrations in sediments depend on factors such as sedimentation rate, diagenesis, partitioning into a certain phase, uptake by
organisms, and additions of the same element by
detrital hemipelagic or aeolian material. Therefore
ocean chemists make use of proxy tracers which are
not routinely analyzed in surveys of present-day
water masses, because their measurement presents a
considerable effort compared with tracers such as
salinity and temperature. However, their characteristics can be directly related to those well-known
oceanographic seawater tracers.
The conditions that need to be met for an element
to be of use as a proxy tracer are that (1) it conserves
a characteristic chemical or isotopic property when
transferred from the water column into the sediment;
(2) the elements or their isotopic composition can be
extracted from the sediment; (3) the age of the
sediment is known so that changes of the tracer over
time can be reconstructed.
One such proxy makes use of element ratios. For
example, the ratio of Cd to Ca in foraminiferal tests
is a proxy for the PO 4 content of the past water mass
in which the foraminifera formed and therefore
provides information on the past thermohaline circulation. This tracer is explained in detail in the
article on trace elements in foraminiferal tests.
Similarly, the ratio of the intermediate uranium
decay products
231
Pa and
230 Th, measured in bulk
sediment, may under certain conditions provide information on the advection of water masses in the
overlying water column in the past (see Cosmogenic
Isotopes and Uranium-Thorium Series Isotopes in
Ocean Profiles). Isotope ratios are ideally suited as
long-term proxy tracers. Some of these isotope ratios
are characteristic of certain seawater properties and
123