The effect of changes in the Earth's moment of inertia during glaciation on geomagnetic polarity excursions and reversals: Implications for Quaternary chronology

摘要

Geomagnetic polarity reversals and excursions in the Quaternary correlate well with interglacial-to-glacial transitions and glacial maxima. It is suggested that this relationship results from interactions between the Earth's mantle and core that accompany decreases in the Earth's moment of inertia during ice accumulation, which weaken the geomagnetic field in order to try to counter the decrease in differential rotation between the mantle and inner core that is being forced. In the Late Pleistocene, geomagnetic excursions directly correlate with brief phases of rapid ice growth that accompany falls in global sea-level, notably during the Younger Dryas stage, Dansgaard-Oeschger interstadials 5 and 10 that precede the rapid melting events during Heinrich events H3 and H4, and during the transitions between oxygen isotope stages 5c-5b, and 5e-5d. It is proposed that similar relationships between instabilities in climate and the geomagnetic field also typefied the Middle Pleistocene. As a result of the transfer of some of the mass of the oceans into polar ice sheets, the climate instabilities that initiate these rapid ice accumulations redistribute angular momentum and rotational kinetic energy between the Earth's mantle and inner core. These changes weaken the Earth's magnetic field, facilitating geomagnetic excursions and also causing enhanced production of cosmogenic nuclides, including C-14. The subsequent phases of rapid ice melting, Heinrich events, reverse this effect: strengthening the field. This explanation, of forcing of geomagnetic excursions by climate instabilities, provides a natural explanation for why, during the Middle-Late Pleistocene, excursions have been numerous but none has developed into a polarity reversal: the characteristic duration of the climate instabilities is too short. River terrace aggradation, in Europe at least, is also likely to be concentrated during Heinrich events. The most important of these can now be dated throughout the Middle and Late Pleistocene, as they are expected to lag the geomagnetic excursions by no more than similar to2 ka. Timings of these fluvial aggradations could also be constrained by observation of in situ production spikes of cosmogenic nuclides such as Be-10, which would allow direct correlation with the geomagnetic excursions.