Tag Archives: Geomagnetic field reversals

Ancient oceanic lithosphere beneath the eastern Mediterranean

The extensive active subduction zones around the Pacific ocean are responsible for a dearth of oceanic lithosphere older than about 200 Ma that still remains where it formed. Trying to get an idea of pre-Mesozoic ocean-floor processes depends almost entirely on fragmented ophiolites thrust or obducted onto continent at destructive plate margins. Yet the characteristically striped magnetic signature above in situ oceanic lithosphere offers a good chance of spotting any old oceanic areas, provided the stripes are not imperceptible because of thick sediment cover.  One of the most intriguing areas of ocean floor is that beneath the eastern Mediterranean Sea in the 3 km deep Herodotus Basin, which has long been thought to preserve a relic of old ocean floor.  Roi Granot of Ben-Gurion University of the Negev, Israel has analysed magnetic data gathered along 7 000 km of survey lines and indeed there are vague traces of stripy geomagnetic variation that has a long wavelength, to be precise there are two bands of . Mathematical analysis of the magnetic profiles suggest that they have a source  about 13 to 17 km beneath the seabed: probably crystalline crust beneath thick Mesozoic sediments (Granot, R. 2016. Palaeozoic oceanic crust preserved beneath the eastern Mediterranean. Nature Geoscience, doi:10.1038/ngeo2784).

English: Age of oceanic lithosphere Deutsch: A...

Ages of oceanic lithosphere (credit: Wikipedia)

The shape of the anomalies cannot be matched with those of younger magnetic stripes, but can be modelled to fit with a sequence of normal-reverse-normal magnetic polarity preserved in continental sequences of early Carboniferous age, about 340 Ma ago. At that age, the lithosphere would by now be old, cold and dense enough to subside to the observed depth, but the fact that it escaped subduction during amalgamation of Pangaea in the Upper Palaeozoic or when Africa collided with Eurasia in the early Cenozoic is a puzzle. Granot reckons that it most likely formed in Pangaea’s great eastern ocean embayment, known as Palaeotethys. An interesting view, but one that does not seem likely to lead any further, simply because of the great depth to which the oceanic material is buried. The deepest yet to be achieved is only 12 km in the onshore Kola Superdeep Borehole in Russia. So the changes of getting samples are slim, even if the overlying sedimentary pile proves to have hydrocarbon potential.

English: Pangea animation

Pangea break-up animation ( credit: Wikipedia)

The core’s influence on geology: how does it do it?

Although no one can be sure about the details of processes in the Earth’s core what is accepted by all is that changes in core dynamics cause the geomagnetic field to change in strength and polarity, probably through some kind of physical interaction between core and deep mantle at the core-mantle boundary (CMB). Throughout the last 73 Ma and especially during the Cenozoic Era geomagnetism has been more fickle than at any time since a more or less continuous record began to be preserved in the Jurassic to Recent magnetic ‘stripes’ of the world ocean floor. Moreover, they came in bursts: 5 in a million years at around 72 Ma; 10 in 4 Ma centred on 54 Ma; 17 over 3 Ma around 42 Ma; 13 in 3 Ma at ~24 Ma; 51 over a period of 12 Ma centring on 15 Ma. During the Late Jurassic and Early Cretaceous the core was similarly ‘busy’, the two time spans of frequent reversals being preceded by quiet ‘superchrons’ dominated by the same normal polarity as we have today i.e. magnetic north being roughly around the north geographic pole.

The Cenozoic history of magnetic reversals - black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

The Cenozoic history of magnetic reversals – black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

Until recently geomagnetic ‘flips’ between the two superchrons were regarded as random , perhaps suggesting chaotic behaviour at the CMB. But such a view depends on the statistical method used. A novel approach to calculating reversal frequency through time, however, shows peak-trough pairs recurring 5 times through the Cenozoic Era, approximately 13 Ma apart: maybe the chaos is illusory (Chane, J. et al. 2015. The 13 million year Cenozoic pulse of the Earth. Earth and Planetary Science Letters, v. 431, p. 256-263). So, here is a kind of yardstick to see if there may be any connection between core processes and those at the surface, which Chen of the Fujian Normal University, Fushou China and Canadian and Chinese colleagues compared with the very detailed Cenozoic oxygen-isotope (δ18O) record preserved by foraminifera in ocean-floor sediments, which is a well established proxy for changes in climate. Removing the broad trend of cooling through the Cenozoic resulted in a plot of more intricate climatic shifts that matches the geomagnetism record in both shape and timing of peak-trough pairs. It also turns out, or so the authors claim, that both measures correlate with changes in the rate of Cenozoic subduction of oceanic lithosphere (a measure of plate tectonic activity), albeit negative – peaks in magnetism and climate connecting with slowing in the pace of tectonics.

The analyses involved some complicated maths, but taken at face value the correlations beg the questions why and how? Long-term climate change contains an astronomical signal, encapsulated in the Milankovich hypothesis which has been tested again and again with little room for refutation. So is this all to do with gravitational influences in the Solar System. More exotic still is the possibility of 13 Ma cyclicity linking the Milankovich mechanism with the vaster scale of the Sun’s orbit oscillating through the disc of the Milky Way galaxy and theoretical hints of a mysterious role for dark matter in or near the galaxy. Or, is it a relationship in which climate and the magnetic field are modulated by plate tectonics through varying volcanic emissions of greenhouse gases and the deep effect of subduction on processes at the CMB respectively? To me that seems more plausible, but it is still as exceedingly complex as the maths used to reveal the correlations.