Cosmogenic radionuclides in environmental archives – A paleo-perspective on space climate and a synchronizing tool for climate records

Summary, in English

The Sun is the primary source for Earth’s climate system.
Its fluctuations in irradiance are also known to have an impact on climate. In
addition, changes in solar activity modulate the atmospheric production rates
of cosmogenic radionuclides (e.g. ¹⁰Be, ¹⁴C, ³⁶Cl)
that all eventually deposit to different environmental archives. The signal of
the changing solar activity through time can thus be retrieved and measured
from these archives, such as ice cores, tree rings, or lake sediments. The Sun can
also display a more chaotic behavior by erupting flashes of light, plasma and
magnetic fields whereby energetic particles can be accelerated, and sometimes
hit Earth. These all can damage our spacecraft technologies, harm astronauts,
and also affect transformer, electric, and electronic infrastructures on the
ground. In the case of extreme events, they may also pose a challenge to
air-travel safety. At the same time, when solar energetic particles enter the
atmosphere, they can enhance the production rate of cosmogenic radionuclides.

The objectives of this thesis are twofold. First, the
potential of using ¹⁰Be, ¹⁴C, and ³⁶Cl as
tracers of extreme solar storms is explored in depth. Second, the common
production signal of ¹⁰Be and ¹⁴C caused by the longer term
changes in solar activity is used to synchronize climate records from different
environmental archives from different regions in order to assess the relative timing
of a prominent climate oscillation, over 11,000 years before present.

Two large signatures of cosmic-ray increase date to AD 774/5
and AD 993/4 are conclusively attributed to extreme solar energetic particle
events that have hit Earth and left a clear imprint on the production rates of ¹⁴C
as measured in tree rings all around the world, and of ¹⁰Be and ³⁶Cl
in ice cores from Greenland and Antarctica. The inferred energy spectrum and flux
of particles of these events indicate that they were an order of magnitude
stronger than any solar high-energy event observed during the space era. To
infer the energy spectrum of ancient events, it is shown that the relative
differences in the energy dependency of the production rates of ¹⁰Be
and ³⁶Cl by solar particles can be used. An additional, and
similarly extreme, solar storm is also suggested to have hit Earth 2,610 years BP.
The events from AD 774/5 and AD 993/4 are further explored to test, and
thereafter reject, the hypothesis that nitrate enhancements in ice cores can be
reliably linked to the occurrence of solar storms or to assess their magnitude.
Two high resolution and continuous records of ice-core ³⁶Cl
concentration spanning the past several centuries are also presented. They show
several increases in ³⁶Cl possibly linked to solar energetic
particle events including one that is coeval with the geomagnetic storm of
September 1909 CE. These records also show that there is no enhancement in ³⁶Cl
production rate following the Carrington event of 1859 CE. A theoretical
experiment also proposes that it is possible that major solar storms with a
large flux of lower energy particles could lead to a significant increase in
ice-core ³⁶Cl concentrations but not in ¹⁰Be. Finally, it
is shown that wiggles in cosmogenic radionuclides caused by longer-term changes
in solar activity can be used to synchronize lake sediment records from Europe
to Greenland ice cores. The investigation of both archives on the same
time-scale suggests that the climate oscillations observed in Greenland, and
subsequently in Western Europe could be attributed, in part, to solar forcing.

• Author: Florian Mekhaldi
• Department/s: Quaternary Sciences; MERGE: ModElling the Regional and Global Earth system
• Publishing year: 2018-12
• Language: English
• Volume: 86
• Document type: Dissertation
• Publisher: Lund University
• Topic: Geology; Astronomy, Astrophysics and Cosmology
• Keywords: Comogenic radionuclides; 10Be; 36Cl; Solar energetic particles; Solar storms; Paleoclimate; Time-scales; Solar activity
• Status: Published
• Supervisor: Raimund Muscheler; Florian Adolphi; Svante Björck; Markus Czymzik
• ISBN/ISSN/Other: ISBN: 978-91-87847-45-5; ISBN: 978-91-87847-44-8
• Defence date: 18 January 2019
• Defence time: 13:15
• Defence place: Sal Pangea, Geocentrum II, Sölvegatan 12, Lund
• Opponent: Mats Faurschou Knudsen (Ass. Professor)