COSMOGENIC ISOTOPES
D. Lal, Scripps Institute of Oceanography, University of
California San Diego, La Jolla, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
In different settings, spanning from the extraterrestrial to the terrestrial, naturally occurring nuclides
offer unique possibilities for being deployed as tracers for studying a great variety of physical, chemical, and biological processes, occurring over a wide
range of timescales. This article discusses the continuous production of several stable and radioactive
isotopes as a result of nuclear reactions of cosmic ray
particles in the Earth’s atmosphere and the hydrosphere, and their potential usefulness as tracers for
studying oceanic processes. The great merit of cosmic ray produced (cosmogenic) isotopes as tracers
lies in the fact that their source functions in the different geospheres can be determined, and that several
nuclides with a wide range of half-lives and chemical
properties are available.
Cosmic radiation, which consists of energetic H,
He and heavier nuclei, with kinetic energies much
greater than tens of mega electronvolts (MeV), (with
particles of energies much beyond 10
10 MeV), produce a great variety of nuclides by their interactions
with target nuclei in the atmosphere, hydrosphere
and the lithosphere. The predominant cosmic ray
interaction is fragmentation of the target nuclei by
primary and secondary particles of the cosmic radiation. Some nuclides are produced following the
capture of thermal (very slowly moving) neutrons by
target nuclei, which are abundant in the secondary
cosmic radiation as a result of slowing down of fast
neutrons emitted in energetic cosmic ray-produced
nuclear reactions.
Radiocarbon,
14 C, was the first cosmic ray-produced isotope to be discovered in 1947 in sewage
methane. Soon thereafter it was applied for archaeological/anthropological dating. This discovery
was a milestone in the use of cosmic ray-produced
(cosmogenic) isotopic changes as a tool for learning
about planetary sciences. It laid the foundations of
the field of cosmic ray geophysics/geochemistry.
Subsequently, in the early 1960s, about 25 cosmogenic radionuclides produced in the earth’s atmosphere, with half-lives ranging from B1 h to millions
of years were detected. The driving force for the
studies of cosmic ray-produced nuclides was the
realization that if they could be detected in different
dynamic reservoirs of the geospheres, they could be
used as tracers to obtain important information
about the timescales involved in the transport of
materials through the atmosphere to the hydrosphere, oceans, and the cryosphere, and that in some
cases they could be used as clocks to introduce
timescales into the diverse proxy records of earth’s
climate. Oceans are central to the dynamic interplay
between the dynamic reservoirs, and therefore considerable emphasis has been placed on understanding
the nature of the principal mixing/transfer processes,
of the marine biogeochemical cycles, and of the
large-scale ocean circulation. All oceanic investigations, in one way or the other, are linked to the
central question of what processes control the earth’s
climate. Geochemical tracers serve as tools to
understand these processes and their rates.
As mentioned above, the field of cosmic ray produced (cosmogenic) tracers caught roots in 1947,
with the discovery of
14 C. It grew rapidly thereafter
in the late 1950s/early 1960s, and to date it is still
one of the frontier areas in modern geochemistry.
There are two reasons for this sustained hold and
value of the cosmogenic tracers: continued development of new and powerful techniques for measuring
their distribution in natural settings at very low
concentrations, and the emergence of new biogeochemical questions which crop up as our understanding of the terrestrial climate system improves.
However, there are often no other (suitable) tracers
available to study the short- and long-term behavior
of oceans on large space scales. This article considers
the essentials of the cosmogenic tracers, their potentials, and how new advances continue to keep this
field growing.
Terrestrial Cosmogenic Isotopes and
their Production Rates
Most of the cosmic ray energy (498%) is dissipated
in the earth’s atmosphere in the nuclear reactions
they produce. The atmospheric column represents
about 13 mean free paths for nuclear interactions of
fast protons and neutrons. After traversal through
the atmosphere, the secondary particles of the cosmic
radiation continue to produce nuclear reactions with
the surficial terrestrial reservoirs: the hydrosphere,
225
D. Lal, Scripps Institute of Oceanography, University of
California San Diego, La Jolla, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
In different settings, spanning from the extraterrestrial to the terrestrial, naturally occurring nuclides
offer unique possibilities for being deployed as tracers for studying a great variety of physical, chemical, and biological processes, occurring over a wide
range of timescales. This article discusses the continuous production of several stable and radioactive
isotopes as a result of nuclear reactions of cosmic ray
particles in the Earth’s atmosphere and the hydrosphere, and their potential usefulness as tracers for
studying oceanic processes. The great merit of cosmic ray produced (cosmogenic) isotopes as tracers
lies in the fact that their source functions in the different geospheres can be determined, and that several
nuclides with a wide range of half-lives and chemical
properties are available.
Cosmic radiation, which consists of energetic H,
He and heavier nuclei, with kinetic energies much
greater than tens of mega electronvolts (MeV), (with
particles of energies much beyond 10
10 MeV), produce a great variety of nuclides by their interactions
with target nuclei in the atmosphere, hydrosphere
and the lithosphere. The predominant cosmic ray
interaction is fragmentation of the target nuclei by
primary and secondary particles of the cosmic radiation. Some nuclides are produced following the
capture of thermal (very slowly moving) neutrons by
target nuclei, which are abundant in the secondary
cosmic radiation as a result of slowing down of fast
neutrons emitted in energetic cosmic ray-produced
nuclear reactions.
Radiocarbon,
14 C, was the first cosmic ray-produced isotope to be discovered in 1947 in sewage
methane. Soon thereafter it was applied for archaeological/anthropological dating. This discovery
was a milestone in the use of cosmic ray-produced
(cosmogenic) isotopic changes as a tool for learning
about planetary sciences. It laid the foundations of
the field of cosmic ray geophysics/geochemistry.
Subsequently, in the early 1960s, about 25 cosmogenic radionuclides produced in the earth’s atmosphere, with half-lives ranging from B1 h to millions
of years were detected. The driving force for the
studies of cosmic ray-produced nuclides was the
realization that if they could be detected in different
dynamic reservoirs of the geospheres, they could be
used as tracers to obtain important information
about the timescales involved in the transport of
materials through the atmosphere to the hydrosphere, oceans, and the cryosphere, and that in some
cases they could be used as clocks to introduce
timescales into the diverse proxy records of earth’s
climate. Oceans are central to the dynamic interplay
between the dynamic reservoirs, and therefore considerable emphasis has been placed on understanding
the nature of the principal mixing/transfer processes,
of the marine biogeochemical cycles, and of the
large-scale ocean circulation. All oceanic investigations, in one way or the other, are linked to the
central question of what processes control the earth’s
climate. Geochemical tracers serve as tools to
understand these processes and their rates.
As mentioned above, the field of cosmic ray produced (cosmogenic) tracers caught roots in 1947,
with the discovery of
14 C. It grew rapidly thereafter
in the late 1950s/early 1960s, and to date it is still
one of the frontier areas in modern geochemistry.
There are two reasons for this sustained hold and
value of the cosmogenic tracers: continued development of new and powerful techniques for measuring
their distribution in natural settings at very low
concentrations, and the emergence of new biogeochemical questions which crop up as our understanding of the terrestrial climate system improves.
However, there are often no other (suitable) tracers
available to study the short- and long-term behavior
of oceans on large space scales. This article considers
the essentials of the cosmogenic tracers, their potentials, and how new advances continue to keep this
field growing.
Terrestrial Cosmogenic Isotopes and
their Production Rates
Most of the cosmic ray energy (498%) is dissipated
in the earth’s atmosphere in the nuclear reactions
they produce. The atmospheric column represents
about 13 mean free paths for nuclear interactions of
fast protons and neutrons. After traversal through
the atmosphere, the secondary particles of the cosmic
radiation continue to produce nuclear reactions with
the surficial terrestrial reservoirs: the hydrosphere,
225