Magnetic polarity reversals

The Earth’s magnetic field is changing all the time, in its intensity, direction and, now and again, its polarity.  It’s the last that proved the key to sea-floor spreading and plate tectonics, though ocean-floor magnetic “stripes”, and which has become a key stratigraphic tool for correlation and approximate dating.  Along with palaeomagnetic pole determinations, that are vital to continental reconstructions, the whole field still remains largely empirical.  Although widely agreed to be connected to changes in motions in the core, exactly what happens during reversals of geomagnetic polarity remains enigmatic, despite 40 years having passed since they were first recognised.  There is no doubt that they are quick events, but to judge their pace and what happens to field strength and direction during a “flip” requires high quality data that is well-calibrated to time.  Most early work focussed on magnetisation in igneous rocks, where the signal is strong.  Minerals such as igneous magnetite acquire a permanent magnetisation once they cool below their Curie temperature, but since accurate radiometric dating gives an age, not a range of ages, it might seem that all that is possible with lavas and intrusions is to obtain a series of points.  Fine for a time series, but useless for the details of reversals.  However, by modelling the cooling history of an igneous body, it is possible to calibrate different levels within it to time.  With careful choice, it has proved possible to find flows in flood basalt sequences that include the brief progress of a reversal.  The results seem very odd, the pole itself seeming to migrate rather than jump from north to south, and gross changes in intensity over a short time.  Improved instrumentation allows a shift from strongly magnetic basalts, to sediments that preserve much weaker signals.  These are due to the alignment with the field of magnetic grains as they slowly settle.  Marine sediment cores can now be magnetically characterised – the principle behind magneto-stratigraphy.  For geomagnetists the most recent reversals have proved especially instructive, when the sedimentary record is analysed (Clement, B.M. 2004.  Dependence of the duration of geomagnetic polarity reversals on site latitude.  Nature, v. 428, p. 637-640).  On average, the last four “flips” took about 7000 years to complete by migration of the magnetic poles.  Yet there is an oddity in the detail.  Sites at low latitude show significantly shorter periods (down to 2000 years) than those at high latitude (as much as 10000 years).  Clement’s explanation for the difference is the persistence of the lower intensity non-dipole field, which might suggest different core processes or a single process with several components that evolve at different rates.

Sulphur cycling and sea-level change

Sulphur is one the major prerequisites for life after carbon, hydrogen, oxygen and nitrogen, and the bulk of it is supplied by sulphate ions.  After chlorine, the SO42- ion is the most abundant anion in the oceans.  Not very much is added annually by river drainage, and although anaerobic bacteria remove some by reducing it to hydrogen sulphide so that it is removed from solution as a result of precipitation of insoluble iron sulphide, the sulphur cycle has been considered to be the most sluggish of all the major geochemical rhythms at the Earth’s surface.  Because iron sulphide is highly reactive in oxidising conditions, should marine sulphide-rich sediments become exposed at the surface their oxidation to sulphuric acid and iron hydroxide would rapidly add sulphate ions to seawater.  Studies of sulphur isotopes seem to suggest that this is not very important however.  Through sulphate-sulphide reducing bacteria, sulphur is implicated in the carbon cycle because of its sheer abundance, not so much from the encouragement and burial of the bacteria, but because they induce the highly reducing conditions that help a larger proportion of dead organic matter to remain unoxidised and become buried.  In a roundabout way, sulphur has a role in climate controls.  In fact, two roles.  Sulphate ions affect the alkalinity of seawater, and on that depends the oceans’ ability to dissolve CO­2 from the atmosphere.  The big question is, “Does the sulphate content of seawater ever change fast enough to have some impact on climate in the short term?”.  Most studies of the S-cycle have focused on sulphur isotopes, so a new twist is bound to be interesting.  Alexandra Turchyn and Daniel Schrag of Harvard University looked instead at the isotopes of oxygen within barium sulphate contained within seafloor sediments since the Late Miocene (about 10 Ma ago) (Turchyn, A.V. & Schrag, D.P. 2004.  Oxygen isotope constraints on the sulfur cycle over the past 10 million years.  Science, v. 303, p. 2004-2007).  Up until 6 Ma, the barite d18O (measured against mean ocean water values) stayed constant at about 9.5‰, and then rose to around 12.5‰ by 3.5 Ma.  Through the Late Pliocene and Pleistocene, the period of repeated glacial-interglacial cycles, it fell dramatically to its present level of 7.9‰.  In that later period, the average d16O of deep water foraminifera rose significantly.  The decline in “heavy” oxygen in marine sulphates can be linked to increased exposure of pyrite-bearing marine sediments during glacial sea-level falls when “light” atmospheric oxygen enters the sulphate ions that are produced.  Modelling suggests sulphate ions in seawater increased by as much as 20% during the Great Ice Age.  Whether that had an influence on the oceans’ take-up of carbon dioxide from the atmosphere in the last 3 Ma is yet to be evaluated.  However, Turchyn and Schrag’s detection of a short term shift in the sulphur cycle, and attributing it to falling sea level, may allow a new approach to global sea-level change, which has mainly been deduced from features in stratigraphy.

See also:  Derry, L.A. & Murray, R.W. 2004.  Continental margins and the sulfur cycle.  Science, v. 303, p. 1981-1982


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