Cosmogenic nuclides
in the atmosphere
The atmosphere and
the chemical and physical processes, which occur in it, have profound effect
on the terrestrial environment and on human population. In spite of the
importance of atmospheric processes there is still a great deal which is
unknown about how the atmosphere function. The cosmogenic radionuclides
10Be
and 36Cl are unique
among tracer species measured in ice core in two important ways:
-
the production rates of
these nuclides are related directly to the galactic cosmic ray (GCR) intensity
which is strongly influenced both by solar activity and by the geomagnetic
field strength, and
-
these nuclides are
produced in the stratosphere and upper troposphere. In contrast, other
atmospheric trace chemical species, are predominantly produced and influenced
by boundary layer processes at their source regions such as continental
or ocean surfaces. The concentration of such trace species, in ice cores
and in the atmosphere is therefore strongly influenced by variations of
wind speed and direction at the source region and by changes in production
path. Cosmogenic nuclides, on the other hand, are introduced into the atmosphere
in a uniform and predictable way.
We have been involved
for a number of years in an effort to simulate cosmogenic nuclide production
rates in the extraterrestrial and later terrestrial samples, including
those from polar ice cores, to investigate long-term variations in their
concentration and to relate these variations to the fundamental processes
at work. The concentration of the cosmogenic nuclides 10Be
and 36Cl are influenced
both by factors external to the atmosphere, such as changes in solar activity
or in the galactic cosmic ray flux, and by internal factors, such as changes
in the atmospheric circulation, in stratosphere-troposphere exchange processes,
in snow accumulation rat, and in the geomagnetic field. Our previous work
provided already basic information for the interpretation of experimental
results. Questions, however, remain:
-
What is the relation between
solar activity changes and climate?
-
How constant has solar
variability been?
-
To what extent do the
polar ice sheets comprise a record of global change and to what extent
do they reflect change primarily in the polar region?
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How are short-term variations
reflected on the time scale of the annual sampling?
-
What is the case of the
still unexplained peak at ~40 ky?
-
In what ways do the Antarctic
and Arctic differ in the extent to which the are coupled to the global
atmosphere system?
We propose here to directly
attack these questions and to examine the fundamental geophysics and geochemistry,
which influence the cosmogenic radionuclide record in polar ice. The project
is intended to provide a detailed model description of cosmogenic nuclides
production in the atmosphere and earth's surface. We are proposing to calculate
worldwide distribution of 3H,
7Be,
10Be,
14C,
and 36Cl. In order
to study processes which affect the atmospheric production of cosmogenic
nuclides and subsequent global distribution of these nuclides. The comparison
with the results of measurements carried out in ice cores at the Lawrence
Livermore National Laboratory and ETH accelerator mass spectrometry facilities
will provide information invaluable in understanding the production and
mixing of cosmogenic radionuclides in the atmosphere, will aid in the interpretation
of the very detailed GRIP and GISP-II records and will provide useful information
about the special atmospheric circulation processes and stratosphere-troposphere
exchange mechanisms.
To simulate in detail
the development of the cascade and to calculate the corresponding production
rates of cosmogenic isotopes in the atmosphere the GEANT and MCNP
code systems were applied. These codes use only basic physical quantities
and parameters, without including any free parameters, to numerically simulate
all processes relevant in particle production and transport. This enables
to trace the fate of each individual particle and in doing so to study
in detail the effects of various parameters on the production rate such
as geomagnetic and solar modulation for a wide range of possible conditions.
Particular contribution expected
-
It will be particularly instructive to
compare the ice core records with the results of our simulations.
-
The comparison will help to separate global
from local effects at the different drill sites.
-
Study long-term variation in several important
geophysical variables, including solar activity, geomagnetic field, atmospheric
circulation, snow accumulation rates and others. The resulting time series
of nuclide concentrations will be applied to three main problem areas:
deducing the history of solar activity, deducing the history of variations
in the geomagnetic field, and a detailed comparison of 10Be,
36Cl
production and the d 14C
profile.
-
We will use cosmogenic nuclide concentration
to study climate history through the effects of atmospheric circulation
and of atmospheric chemistry on nuclide deposition.
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In addition, comparison of calculated
profiles with annual 10Be
and 36Cl profiles
in a shallow cores will test the hypothesis that solar cosmic rays produce
36Cl
in polar regions and that this contribution can be significant in some
cases.