Final Report Summary - EDIFICE (Changes in the geomagnetic dipole (Earth Dipole Field Intensity from Cosmogenic Elements))
Understanding which processes govern the geomagnetic field requires a detailed knowledge of field evolution in the past, specifically its intensity. The EDIFICE (Earth Dipole Field Intensity from Cosmogenic Elements) project has focused first on constructing a composite curve of geomagnetic field intensity variations for the past five million years. The originality and the challenge of the program was to combine paleointensity (RPI) measurements using the magnetization intensity of worldwide marine sedimentary sequences with information derived from the 10Be cosmogenic isotope normalized by 9Be. However, the geomagnetic signal is frequently masked or altered by other components that have to be taken into account. For that purpose, we analyzed the beryllium production peaks associated with the last geomagnetic reversal in sediments from five widely distinct oceanic sites. Due to atmospheric mixing of beryllium, all peaks were expected to be identical. In fact, we observed large differences that were mathematically declined in terms of additive and multiplicative components. The additive component has a similar behavior at all sites, while the multiplicative factor is related to the global climatic changes. Outside the reversal period, the climatic contribution can represent up to 60% of the signal and thus requires to select sites with minimal environmental contributions. Oceanic circulation has been put forward as an important factor that has to be taken into account. These results point out that the beryllium technique is not as optimal as believed and emphasize further the importance of combining the two approaches.
The distribution of tektite layers found within sediments have allowed to model the responses of a geochemical tracer as well as sediment magnetization to a field impulse. Beryllium convolution by sedimentary processes is heavier than for magnetization, but is restricted within the upper 15-20 cm which correspond to the bioturbated zone. The model shows that the magnetization is acquired with a delay and thus offset with respect to the Beryllium record. This feature was tested by the data and the Beryllium features used to date with precision major geomagnetic events such as the last reversal. This bioturbation-based magnetization model represents a major step forward which shows that smearing imposed by late re-alignment of the magnetic grains is lower than expected, but large enough to generate significant smoothing of rapid field changes. We have shown effectively from actual records of the last reversal that sediments are not appropriate for the study of rapid events, except in presence of very high deposition rates.
An unprecedented scale of 3679 beryllium and 153.000 magnetic measurements were performed from various sediments. Dating was obtained from oxygen isotope studies. Nine cores integrate both Be and RPI measurements. They show together the succession of intensity lows associated with reversals and excursions. These datasets were favorable to calculate a first composite curve for the 0.7-2.14 Ma time interval that was calibrated in terms of geomagnetic dipole moment that will have to be refined. The older period going back to 4.5 Ma requires a few additional beryllium measurements that were delayed by the Covid-19 situation. This huge amount of data has yet to be compiled and integrated after cleaning persistent climatic overprint on the basis of the findings mentioned above.
Edifice was also concerned by the direction and intensity variations during the reversal process from volcanic sequences. After investigating several locations, we finally obtained two new records of the last reversal. They confirm the dominance of a complex geometry of the transitional field. The São Miguel record in the Azores shows a rebound which strengthen the scenario in which three successive phases would have punctuated the period of the reversing field during the M-B transition.
The distribution of tektite layers found within sediments have allowed to model the responses of a geochemical tracer as well as sediment magnetization to a field impulse. Beryllium convolution by sedimentary processes is heavier than for magnetization, but is restricted within the upper 15-20 cm which correspond to the bioturbated zone. The model shows that the magnetization is acquired with a delay and thus offset with respect to the Beryllium record. This feature was tested by the data and the Beryllium features used to date with precision major geomagnetic events such as the last reversal. This bioturbation-based magnetization model represents a major step forward which shows that smearing imposed by late re-alignment of the magnetic grains is lower than expected, but large enough to generate significant smoothing of rapid field changes. We have shown effectively from actual records of the last reversal that sediments are not appropriate for the study of rapid events, except in presence of very high deposition rates.
An unprecedented scale of 3679 beryllium and 153.000 magnetic measurements were performed from various sediments. Dating was obtained from oxygen isotope studies. Nine cores integrate both Be and RPI measurements. They show together the succession of intensity lows associated with reversals and excursions. These datasets were favorable to calculate a first composite curve for the 0.7-2.14 Ma time interval that was calibrated in terms of geomagnetic dipole moment that will have to be refined. The older period going back to 4.5 Ma requires a few additional beryllium measurements that were delayed by the Covid-19 situation. This huge amount of data has yet to be compiled and integrated after cleaning persistent climatic overprint on the basis of the findings mentioned above.
Edifice was also concerned by the direction and intensity variations during the reversal process from volcanic sequences. After investigating several locations, we finally obtained two new records of the last reversal. They confirm the dominance of a complex geometry of the transitional field. The São Miguel record in the Azores shows a rebound which strengthen the scenario in which three successive phases would have punctuated the period of the reversing field during the M-B transition.