Cross-references
Biogenic Barium
Contourites
Currents
Deep-sea Sediments
Diatoms
Dinoflagellates
Foraminifers (Benthic)
Foraminifers (Planktonic)
Ice-rafted Debris (IRD)
Paleoceanographic Proxies
Paleoceanography
Pteropods
Radiogenic Tracers
Radiolarians
GEOCHRONOLOGY: URANIUM-SERIES DATING OF
OCEAN FORMATIONS
Vladislav Kuznetsov
Institute of Earth Sciences, Saint Petersburg State
University, St. Petersburg, Russia
Definition
Uranium-series methods allow estimating quantitatively
an age of various types of ocean and terrestrial formations
and are based on phenomenon of radioactive decay/accumulation of isotopes from two naturally occurring
U-series, with parent isotopes being
238 U and
235
U.
Introduction
Deep-sea sediments are deposited under a thick water
layer and protected from external influences such as climate change, wind erosion, and other destruction processes. Therefore, deep-sea sediments represent unique
well-preserved record of climatic and geological events
that occurred during the Quaternary period. Hence, they
can be used for reconstruction of these events in time
applying the U-series dating methods. Theory says that
in natural objects in which the activities of all of the
nuclides are equal, all isotopes from the same decay chain
are in radioactive secular equilibrium (Figure 1). In most
environmental materials, however, a break in the decay
chain and a state of radioactive disequilibrium are
observed, and it is the main prerequisite for the use of
U-series dating methods.
The chemical behavior of U-series nuclides, their contents, and mechanisms of accumulation in the ocean environment were widely researched in the second half of the
twentieth century. As a result, the basic principles and
methods of radioisotope geochronology of ocean sediments are established and well summarized (Kuznetsov,
1976; Smart, 1991; Ivanovich and Harmon, 1992;
Cochran, 1992; Wagner, 1998; Cochran and Masque,
2003; Edwards et al., 2003; Henderson and Anderson,
2003; Kuznetsov, 2008; Kuznetsov and Maksimov, 2012).
For dating different ocean formations, two types of the
U-series disequilibrium methods are available which are
based on radioactive (1) decay or (2) accumulation of
daughter isotope (Ku, 1976; Ivanovich and Harmon,
1992).
At the present time, the most widespread and wellfounded methods are
230 Th excess ,
231 Pa excess ,
230 Th/
234
U,
and
231
Pa/
235
U dating methods of different ocean formations. This entry further discusses the use of such shortliving U-series nuclides as
234
Th (half-life ¼ 24.1 day),
224
Ra (3.64 day), etc., allowing assess rates of processes
in the modern ocean, as well as
210
Pb (22 years), allowing
estimates of accumulation rates of the marine shelf and
lacustrine sediments.
230 Th excess and
231 Pa excess dating methods
In oxidizing aqueous conditions typical for most seawater,
U forms the soluble uranyl carbonate species, and the most
part of this element content is held in the ocean waters.
230
Th and
231
Pa are produced by decay of
238
U (and
234
U) and
235
U, respectively. Both elements form the
positively charged or neutral hydroxide species
which are strongly absorbed onto suspended particles
and rapidly removed from the water to the ocean floor
(Sackett, 1960; Scott et al., 1972; Kuznetsov, 1976;
Anderson et al., 1983a; Anderson et al., 1983b; Taguchi
et al., 1989; Cochran, 1992). Thus, significant concentrations/activities of
230
Th, as well as
231
Pa, are accumulated
on the surface of the ocean floor, much more than in seawater (Moore and Sackett, 1964; Kuznetsov, 1976).
230
Th (and
231
Pa) in the sediments consists of both
the
230 Th (
231
Pa) derived from
234
U (
235
U) decay in
the seawater and small portion of
230
Th (
231
Pa) contained
in mineral detrital component. Only the unsupported
230
Th or
231
Pa, the so-called
230 Th excess (or
230
Th xs ) and
231
Pa excess (or
231
Pa xs ), derived directly from seawater
234
U (
235
U) decay can be used to calculate the sediment
age. Therefore, both isotope activities measured in the
sediment samples must be corrected for this small portion
of detrital
230
Th (and
231 Pa) to estimate
230
Th xs (
231
Pa xs ).
The correction technique is described (see Henderson
and Anderson, 2003 for details).
There are a number of ocean objects suitable for
230
Th xs
(or
231
Pa xs ) dating: (1) deep-sea sediments of different
origin (foraminiferal or metalliferous sediments,
for instance) and (2) ferromanganese formations
(Fe-Mn nodules and crusts). The theoretical distribution
of
230
Th xs or
231
Pa xs radioisotopes in sediments and
Fe-Mn formations reflects exponential decay due to their
half-life time.
Formerly, the alpha-emitting nuclides, such as
230
Th
and
231 Pa, were measured by alpha spectrometry following analytical procedures needed to extract the isotopes
from a sample. Effective age ranges, depending on the isotope’s half-life (T 1/2 ¼ 75.2 kyr for
230 Th and T 1/2 ¼ 34.3
kyr for
231 Pa) and activity level in a sample, are
ca. 3–350 kyr for the
230 Th xs method and ca. 5–150 kyr
GEOCHRONOLOGY: URANIUM-SERIES DATING OF OCEAN FORMATIONS
271
Biogenic Barium
Contourites
Currents
Deep-sea Sediments
Diatoms
Dinoflagellates
Foraminifers (Benthic)
Foraminifers (Planktonic)
Ice-rafted Debris (IRD)
Paleoceanographic Proxies
Paleoceanography
Pteropods
Radiogenic Tracers
Radiolarians
GEOCHRONOLOGY: URANIUM-SERIES DATING OF
OCEAN FORMATIONS
Vladislav Kuznetsov
Institute of Earth Sciences, Saint Petersburg State
University, St. Petersburg, Russia
Definition
Uranium-series methods allow estimating quantitatively
an age of various types of ocean and terrestrial formations
and are based on phenomenon of radioactive decay/accumulation of isotopes from two naturally occurring
U-series, with parent isotopes being
238 U and
235
U.
Introduction
Deep-sea sediments are deposited under a thick water
layer and protected from external influences such as climate change, wind erosion, and other destruction processes. Therefore, deep-sea sediments represent unique
well-preserved record of climatic and geological events
that occurred during the Quaternary period. Hence, they
can be used for reconstruction of these events in time
applying the U-series dating methods. Theory says that
in natural objects in which the activities of all of the
nuclides are equal, all isotopes from the same decay chain
are in radioactive secular equilibrium (Figure 1). In most
environmental materials, however, a break in the decay
chain and a state of radioactive disequilibrium are
observed, and it is the main prerequisite for the use of
U-series dating methods.
The chemical behavior of U-series nuclides, their contents, and mechanisms of accumulation in the ocean environment were widely researched in the second half of the
twentieth century. As a result, the basic principles and
methods of radioisotope geochronology of ocean sediments are established and well summarized (Kuznetsov,
1976; Smart, 1991; Ivanovich and Harmon, 1992;
Cochran, 1992; Wagner, 1998; Cochran and Masque,
2003; Edwards et al., 2003; Henderson and Anderson,
2003; Kuznetsov, 2008; Kuznetsov and Maksimov, 2012).
For dating different ocean formations, two types of the
U-series disequilibrium methods are available which are
based on radioactive (1) decay or (2) accumulation of
daughter isotope (Ku, 1976; Ivanovich and Harmon,
1992).
At the present time, the most widespread and wellfounded methods are
230 Th excess ,
231 Pa excess ,
230 Th/
234
U,
and
231
Pa/
235
U dating methods of different ocean formations. This entry further discusses the use of such shortliving U-series nuclides as
234
Th (half-life ¼ 24.1 day),
224
Ra (3.64 day), etc., allowing assess rates of processes
in the modern ocean, as well as
210
Pb (22 years), allowing
estimates of accumulation rates of the marine shelf and
lacustrine sediments.
230 Th excess and
231 Pa excess dating methods
In oxidizing aqueous conditions typical for most seawater,
U forms the soluble uranyl carbonate species, and the most
part of this element content is held in the ocean waters.
230
Th and
231
Pa are produced by decay of
238
U (and
234
U) and
235
U, respectively. Both elements form the
positively charged or neutral hydroxide species
which are strongly absorbed onto suspended particles
and rapidly removed from the water to the ocean floor
(Sackett, 1960; Scott et al., 1972; Kuznetsov, 1976;
Anderson et al., 1983a; Anderson et al., 1983b; Taguchi
et al., 1989; Cochran, 1992). Thus, significant concentrations/activities of
230
Th, as well as
231
Pa, are accumulated
on the surface of the ocean floor, much more than in seawater (Moore and Sackett, 1964; Kuznetsov, 1976).
230
Th (and
231
Pa) in the sediments consists of both
the
230 Th (
231
Pa) derived from
234
U (
235
U) decay in
the seawater and small portion of
230
Th (
231
Pa) contained
in mineral detrital component. Only the unsupported
230
Th or
231
Pa, the so-called
230 Th excess (or
230
Th xs ) and
231
Pa excess (or
231
Pa xs ), derived directly from seawater
234
U (
235
U) decay can be used to calculate the sediment
age. Therefore, both isotope activities measured in the
sediment samples must be corrected for this small portion
of detrital
230
Th (and
231 Pa) to estimate
230
Th xs (
231
Pa xs ).
The correction technique is described (see Henderson
and Anderson, 2003 for details).
There are a number of ocean objects suitable for
230
Th xs
(or
231
Pa xs ) dating: (1) deep-sea sediments of different
origin (foraminiferal or metalliferous sediments,
for instance) and (2) ferromanganese formations
(Fe-Mn nodules and crusts). The theoretical distribution
of
230
Th xs or
231
Pa xs radioisotopes in sediments and
Fe-Mn formations reflects exponential decay due to their
half-life time.
Formerly, the alpha-emitting nuclides, such as
230
Th
and
231 Pa, were measured by alpha spectrometry following analytical procedures needed to extract the isotopes
from a sample. Effective age ranges, depending on the isotope’s half-life (T 1/2 ¼ 75.2 kyr for
230 Th and T 1/2 ¼ 34.3
kyr for
231 Pa) and activity level in a sample, are
ca. 3–350 kyr for the
230 Th xs method and ca. 5–150 kyr
GEOCHRONOLOGY: URANIUM-SERIES DATING OF OCEAN FORMATIONS
271