At http://phys.org/print273396491.html … we are back at the C14 boundary for dating objects, around 35 to 40 thousand years ago. This boundary, it seems, may have been caused by a bombardment of cosmic rays at a time when Earth's magnetosphere failed. Evidence for the event has been found in ocean sediment cores by French researchers. The cores show variations in concentrations of beryllium 10, a radioactive isotope produced by cosmic rays on the oxygen and nitrogen atoms in the atmosphere. The paper is published in the Journal of Geophysical Research and represents the first step in a new method of studying the history of Earth's magnetic field. The magnetosphere forms an efficient shield that deflects charged particles (plasma) of cosmic origin but it seem the magnetic field is not constant and has undergone many reversals. A pity that Peter Warlow is not around anymore as he would have loved this, as it seems the North magnetic pole can shift to the South geographic pole. The last complete reversal is set at 780,000 years ago, one of the corner stones of the uniformitarian hypothesis. However, excursions of the Magnetic pole have occurred since then – but these have all recovered their 'normal' polarity (or what is assumed to be the norm). The most recent excursion took place 41,000 years ago – which brings us back to the beginning. Something happened at that boundary – but what? We know from other fields of science it involved the kill-off of large numbers of mammal species – including the iconic Ice Age mammoths.
See also http://tallbloke.wordpress.com/2012/12/01/science-meeting-at-st-andrews-… and http://hinode.nao.ac.jp/meeting/SOLARC120813/files/Solanki_Solar_C_Meeti…. From the comments at Tall Blokes Talk Shop are some nice points on the Maunder Minimum. The temperatures then were warmer than what they were in the early 1960s. Does less sun spot activity really = cooler times?