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Micrometeorites: A possible bias on the sedimentary magnetic record
No full textMicrometeorites are strongly magnetic and continuously accumulate at the Earth's surface. On the basis of previously acquired magnetic data, we investigated at which conditions micrometeorites can bias the sedimentary palaeomagnetic and rock magnetic record. We calculated the probabilities for a sediment sample (discrete samples or U-channel samples) to have its detrital remanent magnetization deviated by the presence of a micrometeorite. Our model shows that direction anomalies >5° caused by micrometeorites may be rather frequent (more than 1% of measured samples), even for sediments with typical values of sedimentation rate (up to 10 cm/kyr) and remanent magnetization (up to 5 × 10-3 A/m). Excursions >30° caused by micrometeorites have probabilities >1% in sediments with remanent magnetization 1% for sediments with remanent magnetization <2 × 10-4 A/m and sedimentation rate <1 cm/ka. On the other hand, only sediments with magnetic susceptibilities <10-5 SI and sedimentation rates <1 cm/ka can be significantly affected by the presence of micrometeorites
Constraining the terrestrial age of micrometeorites using their record of the Earth's magnetic field polarity
No full textWe propose a new nondestructive method that uses the paleomagnetic record of micrometeorites in Earth's polar regions to constrain the age of their fall. During atmospheric entry, melted micrometeorites acquire a thermal remanent magnetization and record the polar subvertical geomagnetic field. When the fall vector can be determined, due to the location of bubbles, iron-nickel droplets, or grain-size gradients, it is possible to ascribe the fall to a normal or reverse polarity interval of the geomagnetic field. We tested this concept on a set of eight melted micrometeorites from the Transantarctic Mountains (Antarctica). Two micrometeorites have magnetization directions consistent with a normal polarity of the Earth's magnetic field, whereas four others have recorded a reverse polarity, and therefore fell to Earth at least 0.78 m.y. ago. One micrometeorite has a magnetization that is seemingly unrelated to the inferred entry direction. The fall direction could not be determined with certainty for one micrometeorite. These results provide new evidence suggesting that the Transantarctic Mountains micrometeorite traps are 1-2 m.y. old, and confirm that they contain the oldest non-fossil micrometeorites available
A new micrometeorite collection from Antarctica and its preliminray charcterization by microobservation, microanalysis and magnetic methods
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