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172 CHAPTER 6. FORSCHUNGSSTELLE “RADIOMETRIE”
6.1.12 Reconstruction of the geomagnetic field strength over the past
300.000 years derived from 10 Be data of deep sea sediments from
the North and South Atlantic Ocean
Frank Bernsdorff
Abstract In this project we use the anti correlation between the 10 Be production in the earths
stratosphere via galactic cosmic rays and the strength of the geomagnetic field to determine its variation
over a time period of 300.000 years. Hence two deep sea sediment cores (ODP) from the Atlantic
Ocean, which act as 10 Be archives are investigated.
Figure 6.13: Schematic pathway of the production and deposition of cosmogenic 10 Be
Background The cosmogenic nuclide 10 Be is
mainly produced in the lower stratosphere by
inter-action of galactic cosmic rays with oxygen
and nitrogen atoms, and its production is
known to be strongly anti-correlated with the
solar- and/or geomagnetic field strength. After a
short atmospheric residence time of about 1 year
10 Be is removed from this part of the atmosphere
and deposited onto land, ice sheets and (mainly)
the ocean surface. Therefore it should be possible
to extract a record of geomagnetic paleointensity
(GPI) from depositional profiles of these radionuclides
in marine, terrestrial and ice core archives.
In this study we are investigating two deep sea
sediment cores from the North and Northwest Atlantic
Ocean (ODP-Site 983 and ODP-Site 1063)
for highly resolved 10 Be profiles. The application
of a special correction procedure (involving uranium
and thorium measurements) is indispensable
to quantify the transport of 10 Be in the ocean, so
that the global 10 Be-production can be extracted
from marine records. Based on these profiles, a
marine 10 Be stratigraphy will be developed that
can be matched with 10 Be records from Greenland
(GRIP, GISP II) and Antarctic (EPICA) ice
cores.
Methods and results In addition to the above
specified goals a new analytical technique had to
be developed and applied for the uranium and
thorium measurements to get more precise data
sets in shorter times (compared to Alpha spec-
trometry). This was implemented by the application
of an ICP-SF-MC-MS (Inductively Coupled
Plasma Sector Field Multi Collector Mass Spectrometer)
for the uranium and thorium measurements.
To assure consistent data sets different
standard materials (e.g. certified standard solutions
of thorium and uranium isotopes as well as
deep sea sediment samples of known isotopic composition)
were tested intensively. After these initial
experiments we could assure the quality of
the following measurements concerning the deep
sea sediment cores: ODP 983 and 1063. Those
data sets will be processed and evaluated within
the next month.
Nevertheless, first results of the 230 Th xs(0) corrected
data sets of 10 Be are showing already that
we are able to reconstruct at least the LaChampand
Jamaica Event.
Outlook/Future work Outlook/Future work
According to the goals of this project the future
work will mainly comprise 10 Be measurements of
the deep sea sediments from ODP site 983 and
1063, its normalisation to 230 Th xs(0) and comparison
to the data set of ODP site 1089 (South
Atlantik).
Funding DFG Schwerpunktprogramm: Integrated
Ocean Drilling Program/Ocean Drilling
Program (IODP/ODP); (SPP 527)
Main publication Muscheler et. al [2005]