24 - Space Radiation
And it powered and triggered some end-of-last-glacial events
A previous post identified “a large number of highly unusual events” that occurred around the time of the Younger Dryas (YD; 12,850 – 11,700 BP ), so it is worthwhile to explore some as yet unexplained events before moving on to the Holocene:
A return to active volcanism: the Laacher See (Germany) volcano (VEI = 6) around 12,900 years ago[3] as well as Mount St. Helens (USA) during the "Swift Creek Stage" (13,000–10,000 BP)[2]
(Solar) cosmic ray intensity was 50 times higher than present around 16,000 BP, declining to 15 times higher by 12,000 BP.[4]
An abrupt doubling of atmospheric radiocarbon around the start of the YD[5]
An abnormally low geomagnetic field strength, a directional magnetic excursion and a failed geomagnetic reversal around the start of the YD[6]
The peak of the Late Pleistocene megafaunal extinction that killed off almost 80% of the large to medium-sized mammals in North and South America.[7]
Most of these events, such as the megafaunal extinction, have never been historically recorded, so were therefore unusual and very likely catastrophic in nature. The next posts will use an “as-uniformitarian-as-possible” approach.
Space Radiation
Some of the main problems when summarizing space research are the ever-evolving meanings of terms such as “solar flare”, the cornucopia of terms that essentially (or according to some don’t) describe exactly the same event such as “Solar Particle Event” and “Solar Radiation Storm”, and the large number of obsolete or confusing terms that really shouldn’t be used anymore, but that are so common they are unavoidable. One such term in the latter category is “Space Radiation”, which can be broadly defined as all forms of energy in space that are emitted as rays, electromagnetic waves, and particles.
The principle types of space radiation particles are:
galactic cosmic rays (GCRs), which aren’t really rays at all but fully-charged, high-velocity particles
the solar wind, which isn’t a wind in the classical sense but a fairly constant stream of partially to fully-charged low-velocity particles
solar particle events (SPE’s)
magnetically-trapped (GCR, SPE, solar wind, cosmic dust, …) particles in Earth’s Van Allen Belts.
Solar wind and Solar Particle Events (SPEs)
A previous post already introduced the terms solar wind and Solar Particle Events (SPEs). SPEs are energetic streams of charged particles - mostly protons - that are irregularly emitted by the Sun. SPEs occur randomly though more frequently during solar maxima, can last from a few hours to a few days, and occur ~1,000 (small) to ~10 (larger) times per year during solar maxima. Very large SPEs occur once every ~450 years.
The solar wind consists of a stream of charged particles - primarily protons - that is ejected from the Sun’s corona at supersonic speeds, and is accompanied by stream-embedded magnetic energy. A main difference with SPEs is that it forms the regular, fairly steady plasma stream that carries the interplanetary heliomagnetic field.
Galactic Cosmic Rays (GCRs)
Galactic cosmic rays (GCRs) are fully-ionized atomic nuclei - protons and the bare nuclei of heavier elements - emitted by energetic sources outside of the solar system such as stars and supernovae. The most abundant particles by far are hydrogen (protons; ~85%) and helium nuclei (~13%)[8], although all elements are present in proportion to their relative cosmic abundance.
The Earth-incident stream of GCR particles is modulated by both the heliomagentic (solar wind) and geomagnetic fields, e.g. a strong heliomagnetic field strength caused by a high-velocity outward flow of solar wind particles electromagnetically diverts weaker-strength GCR particles around the solar system. This causes GCR particle numbers to vary inversely with the 11-year solar cycle, i.e. inversely with sunspot numbers and solar wind strength. Similarly, an intense SPE can also suppress the intensity of the GCR flux (Forbrush decrease).
Earth's atmosphere is also partially shielded from GCRs by its geomagnetic field, whose strength has varied significantly over the last 45 ka, e.g.[9]:
At the midpoint of the [42 ka] Laschamps excursion, the global cosmic ray flux reaching the Earth’s atmosphere was up to three times as much as today’s value
Space radiation particle energy
GCR particles typically have velocities that approach the speed of light. Their energies can therefore be very high, even up to the TeV - EeV range, though their energy distribution peaks around 0.3 GeV. SPE particles have similar energies, with protons typically in the 100’s of MeV range, and heavier ions in the 100’s of GeV range. It can therefore often be very difficult to determine whether an Earth- or Moon-incident energetic proton is a GCR or SPE particle without considering additional contextual information. No such issues exist with solar wind particles, whose lower velocities (300-800 km/s) result in much lower particle energies on the order of 0.5-10 keV. Note that SPEs are only partially ionized, and therefore are less likely to be deflected by the magnetosphere than the fully ionized GCRs.
Atmospheric reactions caused by energetic particles
Higher-energy (typically > 1GeV) SPE or GCR particles can cause nuclear reactions when they collide with an atom, usually in Earth’s upper atmosphere, in a process known as “spallation”. Note that on planets or moons with a thin atmosphere and a weak magnetic field, such as Mercury, Mars and the Moon, spallation causes observable surface weathering.
Atmospheric radiocarbon (14C) is one of the particles created by spallation reactions: when a high-energy (GCR or SPE) particle collides with an atmospheric atom it creates secondary cosmic rays in the form of energetic neutrons, which in turn can collide with 14N (seven protons, seven neutrons) atoms to form a 14C atom (six protons, eight neutrons) and a hydrogen atom (a proton).
Atmospheric radiocarbon dating
Atmospheric radiocarbon plays an important role in the reconstruction of events surrounding the end of the Last Glacial Period, as its radioactive decay allows scientists to date layers containing organic carbon:
Age = [ ln (P) / (-0.693) ] x H
where ln is the natural logarithm, P is the fraction of remaining 14C, and H is the half-life of 14C, ~5,700 years. For example, if only 1/4 of the original 14C is remaining then:
Age = [ln(0.25)/ (-0.693) ] x H = 2 x 5,700 years = 11,400 (radiocarbon) years
In practice, all radiocarbon ages must be converted - “calibrated” - to a calendar age as the atmospheric 14C/12C ratio has historically not remained constant. For example, as mentioned above, at the midpoint of the 42 ka Laschamps geomagnetic excursion the GCR flux reaching the Earth’s atmosphere was three times higher, which is noticeable as a steepening of the calibration curve below. This substack posts use the (industry-standard) IntCal20 calibration curve as implemented in R’s rintcal package.
Miyake Events
Miyake events are characterized by abrupt rises in cosmogenic isotopes, e.g. an abrupt rise in measured 14C in tree rings, or abrupt rises in 10Be and 36Cl in ice cores. The two best-investigated - the 774 and 993 AD (CE) events - have been attributed to very large SPEs. Büntgen et al. claim[10]:
Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation.
although no recorded SPE, including the 1859 Carrington event, significantly increased atmospheric 14C suggesting[11]:
that the larger of the two events (AD 774/5) was at least five times stronger than any instrumentally recorded solar event.
Although there is therefore some wiggle room as to their origin, most scientists agree that Miyake events are caused by highly energetic charged particles. More vital for this substack series: these charged particle events almost certainly interacted with (red auroras) and transferred a large part of their energy to the geomagnetic field.
“SPE” occurrences around the time of ice sheet melting
Miyake events (MEs) are characterized by highly energetic particles that cause an abrupt increase in atmospheric 14C and therefore cause any organic carbon dating their occurrence to calibrate to a younger age. MEs very likely transfer a large part of their enormous energy to Earth’s Outer Core via their deformation of the geomagnetic field. Similar to the 1859 Carrington Event they therefore abruptly raise the Outer Core temperature and possibly lead to magnetic excursions and/or increased volcanic activity. Two geomagnetic excursions (~800 AD and ~1000 AD) were very likely caused by the 774 and 993 AD MEs (see next post).
The model presented above can be used to identify possible Miyake events from the IntCal20 curve. Note that a frequency of 1 very large event per 450 years suggests there should be around 30 such very large MEs between 24 and 10 ka.
Note that the fairly unconvincing blip around 14.3 ka represents the largest Miyake event recognized from tree ring data, which was estimated to be ~100 (80-160) times larger than the Carrington event. A medium-sized 11.2 ka 14C deviation/ME was also recognized from 10Be and 36Cl data measured in ice cores from Greenland and Antarctica, and was interpreted to be an SPE whose strength was estimated to be ~100-500 times larger than the Carrington event[12]. Also note that many recognized “events'“ correspond - as expected - to key geothermal melting events:
23.8, 23 ka: start of the geothermal melting of the circum-Antarctic ice cover
18.5 ka: start of the Oldest Dryas, peak of the Lake Missoula megaflooding and Mount St. Helens (USA) volcanism during the “Couger Stage”
17.6, 17.2 ka: time of the Patagonian Ice Sheet geothermal melting
14.8 ka: around the start of the Bølling-Allerød (BA), when geothermal heat caused the retreat of the North Atlantic sea ice cover to just offshore Greenland
14.3, 14.1, 13.8, 13.5 ka: coincide with BA large meltwater pulses entering the North Atlantic, so were very likely responsible for the increased melting or preventing North Atlantic sea ice cover regrowth or both.
Historical “solar flare” strengths
Based on a study of microcraters in lunar rocks, and employing a few fairly large assumptions, Zook et al.[4] concluded “solar flare” intensity decreased from 50 times higher than present at 16 ka to 15 times higher by 12 ka, indicating solar electromagnetic radiation and/or SPE strength may have previously been larger than has been historically recorded. It is obvious the authors are referring to SPE intensity, as the term “solar flare” was later redefined to mean an intense local emission of solar electromagnetic radiation, which is incapable of causing microcraters in Moon rocks. One of Zook et al.’s inherent assumptions was GCR intensity was constant over the last 20 ka, when in fact it was almost certainly higher during the 24-10 ka period than today. Earth’s orbital inclination angle was maximal around 32 ka, and is has since been decreasing. The heliomagnetic field would therefore not have deflected as many GCR particles around the solar system between 32 ka to present, so GCR particles could plausibly have contributed to a higher observed “solar flare” intensity.
Historical “SPE” power
A previous post estimated the Earth-incident peak power of the Carrington event at 250 TW. Estimates of the 774 AD Miyake event power [11] suggest it was 5 times stronger than the Carrington event, so a peak power on the order of 1000 TW. A quick look at the fluence of the very large SPEs indicates the 1/10000 year event is roughly two orders of magnitude stronger than the 1/450 year event, so a peak power on the order of 100,000 TW, or several orders of magnitude larger than the estimated Outer Core to mantle heat flux power of 4 TW. Any very large SPE energy pulse would therefore have caused severe temporary Outer Core heating, geomagnetic field disruptions, an enormous increase in geothermal heat flux and volcanism, and vigorous glacial melting.
Summary
Several extremely large SPEs/Miyake Events almost certainly hit Earth during the 14,000+ years the ice sheets were melting. These extremely energetic events very likely caused extreme Outer Core heating, which in turn would have caused the geothermal heat pulses that played a large role in ending the last glacial period.
References
[1] Andersen, B., Borns, H., 1994, The Ice Age World: An Introduction to Quaternary History and Research with Emphasis on North America and Northern Europe During the Last 2.5 Million Years. Oxford University Press, ISBN: 978-8200218104
[2] Mullineaux, D., 1996, Pre-1980 Tephra-Fall Deposits Erupted From Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1563
[3] Schmincke, H.; Park, C.; Harms, E., 1999, Evolution and environmental impacts of the eruption of Laacher See Volcano (Germany) 12,900 a BP. Quaternary International, 61, 61–72. doi:10.1016/S1040-6182(99)00017-8.
[4] Zook H.A., Hartung J.B., Storzer D., 1977, Solar flare activity: Evidence for large-scale changes in the past. Icarus, 32,106-126.
[5] LaViolette, P. A., 2011, Evidence for a solar flare cause of the Pleistocene mass extinction, Radiocarbon 53, p 303-323.
[6] Pavón-Carrasco, F.J., Osete, M.L., Torta, J.M., De Santis, A., 2014, A geomagnetic field model for the Holocene based on archaeomagnetic and lava flow data, Earth and Planetary Science Letters, 388, 98-109, //doi.org/10.1016/j.epsl.2013.11.046.
[7] Channell, J. , Vigliotti, L., 2019, The Role of Geomagnetic Field Intensity in Late Quaternary Evolution of Humans and Large Mammals. Reviews of Geophysics, 57, 709–738, doi:10.1029/2018RG000629.
[8] Nelson, G.,2016, Space Radiation and Human Exposures, A Primer. Radiation Research, 185, 349 - 358.
[9] Jiawei G. et al., 2022, Geomagnetic field shielding over the last one hundred thousand years. J. Space Weather Space Clim., 12, DOI: 10.1051/swsc/2022027
[10] Büntgen, U. et al., 2018, Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE. Nature Communications.
[11] Mekhaldi, F. et al., 2015, Multiradionuclide evidence for the solar origin of the cosmic-ray events of AD 774/5 and 993/4. Nature Communications, 6, 8611
doi: 10.1038/ncomms9611.
[12] Paleari, C. et al., 2022, Cosmogenic radionuclides reveal an extreme solar particle storm near a solar minimum 9125 years BP. Nat Commun 13, 214. https://doi.org/10.1038/s41467-021-27891-4