Econontersect: You may not want to forget about global warming but much bigger events could quickly change the planet. We have heard about the danger of asertoid impacts that could create catastrophic damage to the planet (and has done so in the past). But a recent article in Earth and Planetary Science Letters discusses an event that occurred about 41,000 years ago which saw the earth's magnetic reverse. Compass needles, if any existed at that time, would have pointed south for a period of approximately 250 years. This is known because there actually have been "compass needles" throughout geologic time in the form of magnetic particles suspended in sediments and volcanic lava. When these lavas cooled and sediments are incorporated in conglomerates while the reverse polarity is still active, a permanent record of the reverse alignment of the earth's magnetic field can result.
The new paper is a significant revision to what has been generally known about geomagnetic reversals. In the current Wikipedia listing it says:
A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was the opposite. These periods are called chrons. The time spans of chrons are randomly distributed with most being between 0.1 and 1 million years with an average of 450,000 years. Most reversals are estimated to take between 1,000 and 10,000 years. The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago. Brief disruptions that do not result in reversal are called geomagnetic excursions.
The new studies have brought the the most recent reversal into the near geologic time associated with the rise of man on the planet. The earth's magnetic field at the time of the reversal was much less than today (about 1/20). That means the radiation exposure from the sun was much greater and made the surface of the earth much more hazardous. Early humanoids may have found caves a refuge from more than cold and predatory animals, although it is unlikely that they had health physicists at that time to inform them of the risk.
This time period from 39,000 to 41,000 years ago saw several abrupt climate changes in the middle of the last ice age and unusual volcanic activity. About 39,400 the largest super volcano eruption of the last 100,000 years occured in present day Italy, spreading rock and lava eastward over Asia Minor and northeast in to central Russia.
Here is an excerpt from R&D Magazine:
These three extreme scenarios, a short and fast reversal of the Earth's magnetic field, short-term climate variability of the last ice age and the volcanic eruption in Italy, have been investigated for the first time in a single geological archive and placed in precise chronological order.
Econintersect has found no evidence that these three events had any interdepedence. That will apparently remain on the list of things to be studied further.
There is one observation that should be pointed out, with respect to the very low magnetic field strength for the earth 41,000 years ago. The earth's magnetic field is still showing significant fluctuations. Another paper to be published in the November 15 issue of the same journal investigates the significant swings in the strength of the earth's magnetic field between 200 and 1400 AD.
Click on graphic for larger image.
Here is the abstract of the polarity reversal paper:
Investigated sediment cores from the southeastern Black Sea provide a high-resolution record from mid latitudes of the Laschamp geomagnetic polarity excursion. Age constraints are provided by 16 AMS14C ages, identification of the Campanian Ignimbrite tephra (39.28±0.11 ka), and by detailed tuning of sedimentologic parameters of the Black Sea sediments to the oxygen isotope record from the Greenland NGRIP ice core. According to the derived age model, virtual geomagnetic pole (VGP) positions during the Laschamp excursion persisted in Antarctica for an estimated 440 yr, making the Laschamp excursion a short-lived event with fully reversed polarity directions. The reversed phase, centred at 41.0 ka, is associated with a significant field intensity recovery to 20% of the preceding strong field maximum at˜50 ka. Recorded field reversals of the Laschamp excursion, lasting only an estimated ˜250 yr, are characterized by low relative paleointensities (5% relative to 50 ka). The central, fully reversed phase of the Laschamp excursion is bracketed by VGP excursions to the Sargasso Sea (˜41.9 ka) and to the Labrador Sea (˜39.6 ka). Paleomagnetic results from the Black Sea are in excellent agreement with VGP data from the French type locality which facilitates the chronological ordering of the non-superposed lavas that crop out at Laschamp–Olby. In addition, VGPs between 34 and 35 ka reach low northerly to equatorial latitudes during a clockwise loop, inferred to be the Mono lake excursion.
- Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments (N. R. Nowaczyk et al, Abstract, October 2012)
- Earth’s brief polarity reversal linked to other extreme events (R&D Magazine, 16 October 2012)
- Improving our knowledge of rapid geomagnetic field intensity changes observed in Europe between 200 and 1400 AD (Miriam Gomex-Paccard, et al, Earth and Planetary Science Letters, Vol. 355-356, 15 November 2012)
- Geomagnetic Reversal (Wikipedia)