Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1195–1203 www.elsevier.com/locate/jastp External forcing of the geomagnetic ÿeld? Implications for the cosmic ray .ux—climate variability Jens Wendler Geoscience Department, Bremen University, Division of Historical Geology and Paleontology, P.O. Box 330 440, 28334 Bremen, Germany Abstract Data on the relation of past climate variations and changes in the geomagnetic ÿeld can contribute to the research on a cosmic ray–climate link. This paper presents a synthesis of geological and astrophysical results about the link of magnetic ÿeld variations and Earth’s climate and external forcing. On millennial timescales, geomagnetic intensity lows are apparently in phase with times of low solar magnetic activity and climate cooling. Reconstructions of quaternary continental climate systems indicate colder and dryer conditions during and after geomagnetic reversals. In the Milankovitch frequency band geomagnetic ÿeld reversals can occur in conjunction with minima in the carbonate content related to the 100 ka eccentricity cycle of the Earth’s orbit. A quasi 100 ka cyclicity (in phase with orbital eccentricity) is discussed in the literature for both geomagnetic intensity and solar magnetic activity. On mega-cycle periods external forcing seems to play a role in the appearance of superchrons of the geomagnetic ÿeld. When applying current spiral arm models superchrons developed during times when the solar system was located between spiral arms. This suggests that the galactic environment may force the geomagnetic ÿeld to switch from its reversing mode into superchron stages probably via modulations of the Sun. Interestingly, there is a link to the structuring of magnetic ÿelds in our galaxy. While the galactic magnetic ÿelds between spiral arms are homogenous, the passage of the solar system through turbulent magnetic ÿelds along the spiral arms parallels high geomagnetic reversal rates. Occurring in conjunction with a low intrinsic cosmic ray .ux the superchrons probably lowered the cosmic ray .ux at Earth even further. This may also have played a role in the evolution of life on Earth and the timing of some mass extinction events. c 2004 Elsevier Ltd. All rights reserved. Keywords: Geomagnetic reversals; Superchrons; Orbital forcing; Solar activity; Cosmic ray .ux; Spiral arms; Carbonate cycles; Palaeoclimatic change 1. Introduction at stimulating such synthetic research between sciences by discussing the role of magnetic ÿelds in shaping Earth’s cli- Space weather in.uences the climate on Earth by mate. Three magnetic ÿelds are known so far to have the variations in the cosmic ray .ux (CRF). In order to im- potential to cause variations in the CRF: the galactic mag- prove our knowledge of this kind of external forcing, netic ÿelds which control the distribution of cosmic rays in multi-disciplinary syntheses of astrophysical, geophysi- the Milky Way (Binney and Tremaine, 1988; Shaviv, 2002, cal and geological data are crucial. The investigation of 2003), the interplanetary magnetic ÿeld produced by the phenomena in the geological record which are related Sun (Hoyt and Schatten, 1997; van Geel et al., 1999; Beer to CRF variations can play a key role for the under- et al., 2000; Wagner et al., 2001), and the Earth’s magnetic standing of the processes of the cosmic ray–climate link ÿeld (van Geel et al., 1999; Wagner et al., 2000a, b). which are still controversially discussed. This study aims One of the most important issues in the research on a cosmic ray–climate connection is the search for the physical mechanisms. The way galactic and solar cos- E-mail address: [email protected] (J. Wendler). mic rays aEect the cloud formation by tropospheric ion 1364-6826/$ - see front matter c 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2004.05.003 1196 J. Wendler / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1195–1203 production comprises an area of intensive research (e.g. evidence and is discussed in the second part of the present Ney, 1959; Friis-Christensen and Lassen, 1991; Svensmark paper. and Friis-Christensen, 1997; Tinsley, 1996, 1997, 2000; Svensmark, 1998; Marsh and Svensmark, 2000; Todd and Kniveton, 2001; Solanki, 2002). To which extent do the 2. Data magnetic ÿelds of the galaxy, Sun, and Earth contribute to the modulation of the CRF? Is the cloud formation sig- This study represents a synthesis based on a review of niÿcantly in.uenced by changes in the CRF? Is climate data from the literature. In the ÿrst part of the paper the few cooling an eEect of increased CRF? In order to evaluate the studies available so far on a geomagnetic reversal-climate impact of cosmic rays on climate we need to investigate the relation in the Quaternary are summarised and interpreted. system Earth during various stages of the diEerent magnetic Tertiary sedimentary cycles from the lower Danian section ÿelds which operate in the system. Both the interplane- Bjala, Bulgaria (Preisinger et al., 2002) are discussed. The tary magnetic ÿeld and the geomagnetic ÿeld modulate compilation of data on the geomagnetic history, galactic mo- the CRF in the vicinity of our planet. The most reliable tion of the solar system and geological characteristics of the information can be obtained from the geomagnetic ÿeld of Ordovician through Recent (Fig. 1) comprises phenomena which the polarity reversal history preserved in the rocks which have previously only partly been compared regarding is well-known and dated for the last 320 Ma (Eide and their temporal distribution and causal links. Palaeomagnetic Torsvik, 1996; Jacobs, 2001). Geomagnetic reversals repre- data (panel A) are based on the compilation and calculations sent extrema when the geomagnetic ÿeld strength reached a of Eide and Torsvik (1996). The occurrences of large ig- minimum, i.e. the geomagnetic contribution in modulating neous provinces were summarised by Wignall (2001). Data cosmic ray-forced atmospheric processes was low. Exam- on the meteorite cosmic ray exposure (panel B) and the ples for a link between geomagnetic variations and climate galactic spiral arm crossings (panel C) are based on the change are discussed in the ÿrst part of the present paper. works of Voshage and Feldmann (1979) and Shaviv (2002, Besides the potential relation of geomagnetic reversals 2003). A second model for the spiral arm passages of the and climate change, external forcing also seems to play a solar system (Leitch and Vasisht, 1998) was used to evalu- signiÿcant role in controlling climate modes on longer time ate the time uncertainties induced by modelling diEerences. scales. Calculations of the dynamics of our 4-arm spiral The sequences (panel D) of transgression/regression on the galaxy (VallÃee, 2002) indicate that the solar system has an North American craton (Sloss, 1963) and the Russian plat- orbital position which takes it around the galactic center form (Sloss, 1972) are chosen to indicate signiÿcant global in about 250 Ma with a rotational speed which is about changes in sedimentary deposition which have been previ- 10 km=s=kpc faster than the 4 spiral arms (Shaviv, 2002, ously shown by Larson (1991) to correlate with superchrons. 2003 and references therein) which are commonly believed Further sedimentary data comprise the accumulation of oil, to be density waves (Binney and Tremaine, 1988). Recent coal and gas (panel I; after Larson, 1991) and the deposi- research on the link of galactic dynamics and Earth history tion of black shales (panel J; Frakes et al., 1992). The data suggests that long-term climate variability may have been in panels G and H on the climate modes are based on the forced considerably by intrinsic variations in galactic cos- comprehensive overview of Frakes et al. (1992) modiÿed mic radiation. A model of Shaviv (2002, 2003) and Shaviv according to more recent updates (http://www.scotese.com; and Veizer (2003) oEers an attractive way of triggering Frakes, 1999). The compilation of panels E and F must be ice age epochs (Frakes et al., 1992) by variations in the regarded as preliminary due to scarcity of data on the length CRF. This .ux is higher within the galaxy’s spiral arms of day perturbations (Machetel and Thomassot, 2002), and which are cyclically passed by the solar system during its true polar wander (Van der Voo, 1994; PrÃevot et al., 2000; galactic revolution. Passages of spiral arms were modelled Evans, 2003). very diEerently and are currently thought to have hap- Of course, when relating these phenomena to each other pened once every 143 Ma years according to Shaviv (2002, one must be aware of the low resolution of time in the range 2003) or with a ∼ 180 Ma year periodicity according to of millions of years with tolerance ranges and lag times of Leitch and Vasisht (1998). The spiral arms are charac- the same order of magnitude. terised by a higher density of stars, abundant interstellar clouds and strong magnetic ÿelds. These magnetic ÿelds conÿne cosmic radiation to the spiral arm regions. The CRF 3. Discussion is increased within spiral arms mainly because of abun- dant star formation and supernovae in these regions of the 3.1. Cyclic geomagnetic, climatic, and sedimentary galaxy. Further evidence is needed to check the hypothesis variability? of periodic spiral arm passages, and to evaluate the eEects of such galactic forcing on the system Earth. The long-term To verify the role of the geomagnetic ÿeld in cli- alternation of the geomagnetic ÿeld between its reversing mate forcing it is essential to ÿnd unequivocal geological and non-reversing (superchron) state might represent such hints. If low geodynamo intensities during a reversal were J. Wendler / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1195–1203 1197 Fig. 1. Compilation of data on the geomagnetic history, galactic motion of the solar system and geological characteristics of the Ordovician through Recent.