Radionuclide 10 Be

Beryllium has only one stable isotope,⁹Be, which can be found only in naturally occurring minerals.⁹Be has an abundance of 4 – 6 ppm in the Earth’s crust and the uppermost solid mantel. The radioactive isotope¹⁰Be is another commonly used indicator of solar activity on millennial time scales due to its long half-life of about 1.39×10⁶ years (Korschinek et al. 2010). It was discovered independently by two groups (Arnold 1956; Goel et al.

1957). Due to its long decay time and very low concentration, ¹⁰Be can be precisely measured only by the Accelerator Mass Spectrometry (AMS) technique invented in the 1980s. This technique achieves an accuracy (ratio of¹⁰Be/⁹Be) better than 10⁻¹³. ¹⁰Be is originally measured in concentration (in units of [atoms/g]) and is often converted into production rate or flux (in units of [atoms/sec/cm²]) if possible.

99.9% of¹⁰Be is produced in the terrestrial atmosphere through cosmic ray spallation while the remaining 0.1% is from the in-situ production when the GCRs-induced secon-dary particles interact with mineral/rock surfaces. In the atmospheric cascade,¹⁰Be can be produced through multiple reactions as the following:

n+¹⁴7N→¹⁰₄Be+3p+2n, (2.14) p+¹⁴7N→ ¹⁰₄Be+4p+n, (2.15) n+¹⁶₈O→¹⁰₄Be+4p+3n, (2.16) p+¹⁶₈O→¹⁰₄Be+5p+2n. (2.17) Similar to¹⁴C, the production rate of¹⁰Be can be described as a function of modulation potential and the geomagnetic dipole moment (see Fig. 3 inVonmoos et al. 2006). The total energy of the cascade (increases with altitude) and the the number of secondary particles (decrease with altitude) balance each other at an altitude of 10 – 15 km, where most of the global¹⁰Be (about 55 – 70%) is produced (e.g.,Lal and Peters 1967;Masarik and Beer 1999,2009;Usoskin and Kovaltsov 2008;Kovaltsov and Usoskin 2010).

In contrast to ¹⁴C, which participates in the global circulation, ¹⁰Be takes a different geochemical path after its production. After the radionuclide is produced, ¹⁰Be beco-mes attached to the atmospheric aerosols, such as sulfates, sea salts and ammonium salts.

2.4 Cosmogenic isotopes Since aerosols often act as CCN,¹⁰Be can thereby easily fall out to the Earth’s surface by either wet or dry deposition. The wet deposition is an efficient and precipitation-related process, which includes all processes that involve water in many forms (e.g., rain, snow, hail). The dry deposition involves turbulence and the¹⁰Be is removed from the atmosp-here when it touches and adatmosp-heres to any surface, such as soil and vegetation. Compared to the dry deposition, which is only significant in very dry regions, the wet deposition is the main mechanism removing¹⁰Be from the atmosphere.

The signal of ¹⁰Be can be found in many types of water-related reservoirs, such as oceans, glaciers, lake sediments and polar ice (e.g.,Beer et al. 1988;Wieland et al. 1991).

Among all these, ice cores are the most commonly-employed resource for studying the long-term solar activity on the basis of¹⁰Be concentration. Although the ice cores have the traceable layers, it is difficult to date them correctly especially with increasing depth.

This is due to the thinning effect of the ice, which describes the ice annual thickness decreasing logarithmically with increasing depth owing to the increasing pressure of the ice.

The aerosol-bound¹⁰Be in the stratosphere has a residence time of about 1 – 2 years while the¹⁰Be in the troposphere has a short residence time of about a few weeks (Rais-beck et al. 1981; Masarik and Beer 1999). Therefore, the¹⁰Be in the stratosphere could be slightly atmospheric-mixed compared to the ¹⁰Be in the troposphere (Raisbeck et al.

1981; Masarik and Beer 1999). Generally, the sampled polar ice cores have a greater proportion¹⁰Be (60 – 64%) from the stratosphere, with only 20 – 22% from the polar region troposphere, and less than 16 – 18% from the troposphere at mid-latitude (Beer et al. 2012). The opposite hemisphere and the equatorial region have negligible effects on the polar ice. Additional evidence for the¹⁰Be signal being highly climate-influenced is the halved¹⁰Be concentrations during the Holocene compared to that in the last gla-cial period. This is attributed to the fact that the water cycle was reduced during the last glacial period, causing a lower precipitation rate in the polar regions (≈50% of today’s value), and resulting in an approximately doubled¹⁰Be concentration. Consequently, the measured¹⁰Be concentrations in the polar ice contain both production signal (solar acti-vity) and the local climatic/precipitation signals (e.g.,Steig et al. 1996;Bard et al. 1997).

The common solar signal can be qualitatively observed by comparing the¹⁰Be series from Greenland and Antarctica with the globally-mixed¹⁴C series.

Since the concentration of¹⁰Be is subjected to the local climate due to partial atmos-pheric mixing (Kocharov et al. 1989;McHargue and Damon 1991), the transport process has to be taken into account when converting the concentration in natural archives to the production rate in the atmosphere. The computations and models of¹⁰Be production have been developed since the 1960s, with various approaches and numerical methods (Bhandari et al. 1966; Lal and Peters 1967;Lal and Suess 1968;O’brien 1979; Masarik and Beer 1999). Modern atmospheric 3-D general circulation models (e.g., the NASA GISS model;Field et al. 2006) for simulating the air-mass transport and wet/dry depo-sition processes (e.g.,Field et al. 2006; Heikkilä et al. 2009) allow one to estimate the local climatic effects. The models, that are based on full Monte Carlo simulations of at-mospheric cascades, show the globally-averaged production rate of¹⁰Be is about 0.02 – 0.03 atoms/sec/cm²(Webber and Higbie 2003;Webber et al. 2007;Usoskin et al. 2006b;

Usoskin and Kovaltsov 2008; Kovaltsov and Usoskin 2010). It is about two orders of magnitude smaller than that of¹⁴C (Sect.2.4.1) because most of the neutrons produced in

70°

Figure 2.8: The drilling sites of the six¹⁰Be series used in this thesis.

the cascade process end up producing the¹⁴C (through neutron capture), and only those neutrons and protons with energy>10 MeV can further produce¹⁰Be.

The validity of the models has been demonstrated byUsoskin et al. (2009) and Pe-dro et al. (2011). However, the choice of transportation models can result in different solar modulation potentials, φ, for the same measured ¹⁰Be value. Two extreme scena-rios, which assume either: (1)¹⁰Be is only produced at high latitudes or, (2)¹⁰Be is only globally-mixed, have been tested to be unrealistic (i.e., they either over or underestimate the solar activity). The actual scenario should be somewhere in between these two ex-trema.

There have been numerous ice cores drilled at different locations in both Greenland and Antarctica, such asCamp Century, Greenland (Beer et al. 1988), GRIP, Greenland (Yiou et al. 1997), Dye 3, Greenland(Beer et al. 1990), GISP2, Greenland (Finkel and Nishiizumi 1997), Milcent, Greenland (Beer et al. 1983), NGRIP, Greenland (Dahl-Jensen et al. 2002; Hvidberg et al. 2002; Berggren et al. 2009), Dome C (Concordia), Antarctica (Raisbeck et al. 1981), South Pole, Antarctica (Raisbeck et al. 1990; Bard et al. 1997),Dome Fuji, Antarctica(Horiuchi et al. 2007),EPICA Dronning Maud Land (EDML), Antarctica (Ruth et al. 2007; Steinhilber et al. 2012). Different records from these sites provide different temporal resolutions and cover different periods of time. In this thesis, we use three¹⁰Be series from Greenland (GRIP, NGRIP and Dye3) and three from Antarctic (EDML, Dome Fuji and South Pole). Their locations are shown in the Fig.

2.8.

3 Solar activity over nine millennia:

A consistent multi-proxy reconstruction

*This chapter is published as a journal article “Solar activity over nine millennia: A con-sistent multi-proxy reconstruction” (DOI: 10.1051/0004-6361/201731892) in Astronomy

& Astrophysics.

Credit: C.-J Wu^1,2, I. G. Usoskin^3,4, N. A. Krivova¹, G. A. Kovaltsov⁵, M. Baroni⁶, E.

Bard⁶ and S. K. Solanki^1,7, A&A, 2018, reproduced with permission©ESO.

Abstract

Aims. Solar activity in the past millennia can only be reconstructed from cosmogenic radionuclide proxy records in terrestrial archives. However, because of the diversity of the proxy archives, it is difficult to build a homogeneous reconstruction. All previous studies were based on individual, sometimes statistically averaged, proxy datasets. Here we aim to provide a new consistent multi-proxy reconstruction of solar activity over the last 9000 years, using all available long-span datasets of¹⁰Be and¹⁴C in terrestrial archives.

Methods. A new method, based on a Bayesian approach, was applied for the first time to solar activity reconstruction. A Monte-Carlo search (using the χ² statistic) for the most probable value of the modulation potential was performed to match data from dif-ferent datasets for a given time. This provides a straightforward estimate of the related uncertainties. Six¹⁰Be series of different lengths (from 500 – 10000 years) from both Greenland and Antarctica, as well as the global ¹⁴C production series were used. The

10Be series were re-sampled to match wiggles, related to the Grand minima, in the ¹⁴C reference dataset. Stability of the long data series was tested.

Results.The Greenland GRIP (Greenland Ice-core Project) and Antarctic EDML (EPICA Dronning Maud Land)¹⁰Be series diverge from each other during the second half of the Holocene, while the¹⁴C series lies in between them. A likely reason for the discrepancy is the insufficiently precise beryllium transport and deposition model for Greenland, which leads to under-correction of the GRIP series for the geomagnetic shielding effect. A slow 6

1Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, Göttingen, Germany

2Georg-August-Universität Göttingen, Institute for Astrophysics, Göttingen, Germany

3Space Climate Research Unit, University of Oulu, Finland

4Sodankylä Geophysical Observatory, University of Oulu, Finland

5Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia

6CEREGE, Aix-Marseille University, CNRS, Collège de France, Technopôle de l’Arbois, Aix-en-Provence, France

7School of Space Research, Kyung Hee University, Yongin, Gyeonggi-Do,446-701, Republic of Korea

– 7-millennia variability with lows ca. 5500BCand 1500ADin the long-term evolution of solar activity is found. Two components of solar activity can be statistically distinguished:

the main component, corresponding to the ‘normal’ moderate level, and a component corresponding to Grand minima. A possible existence of a component representing Grand maxima is hinted, but it cannot be separated from the main component in a statistically significant manner.

Conclusions.A new consistent reconstruction of solar activity over the last nine millennia is presented with the most probable values of decadal sunspot numbers and their realistic uncertainties. Independent components of solar activity corresponding to the main mo-derate activity and the grand-minimum state are identified, possibly related to different modes of the operation of the dynamo.