The micrometeorite flux to Earth through the Phanerozoic Eon : Reconstructed using sediment-dispersed extraterrestrial spinels
Abstract: The purpose of this thesis is to add an astronomical component to the history and evolution of Earth by reconstructing the micrometeorite flux to Earth at different time intervals during the Phanerozoic Eon, using sediment-dispersed extraterrestrial spinel group minerals as a proxy. Chromite and chrome spinel are common refractory members of the spinel group, occurring in both extraterrestrial and terrestrial rocks. Their chemistry is indicative both of the petrogenesis of the rock and type of host meteorite. A meteorite on Earth’s surface decays relatively quickly, and with the exception of chrome spinel the minerals composing the meteorite are replaced. By the dissolution of large samples of condensed pelagic sediments, and the extraction and analysis of chrome spinel, the flux of different meteorite types in different geological times can be determined.Covered in nine papers, nine sedimentary sections located in Sweden, Italy, Austria, United States, France, and Russia, deposited during seven different time windows were studied: the mid-Ordovician, late Silurian, late Devonian, early and late Cretaceous, and early Paleogene. The main objectives in this thesis are to search for spinel grains and describe the flux of micrometeorites in: late Silurian sediments following the L-chondrite parent body (LCPB) breakup in the mid-Ordovician (paper II); Turonian (late Cretaceous) sediments in the Bottaccione section, Italy, within and below the “K3” 3He-anomaly, possibly linked to dust from large lunar impacts (paper III); and Albian-Aptian (early Cretaceous) sediments in search for the lunar Tycho crater ejecta, formed ~109 Ma ago (paper VIII). Additional aims of this thesis, are to search for spinel grains and describe the flux of micrometeorites in: early Cretaceous, with the potential of finding grains from the LL chondritic Baptistina family-forming event (paper I); late Devonian, during a large biotic crisis (paper V); a multiproxy-approach to resolve the timing of the LCPB breakup and the causality with terrestrial climatic and biotic turnovers (paper VI); early Paleogene after the Cretaceous-Paleogene mass extinction 66 Ma ago (paper VII), including a zircon provenance study that helps to resolve the origin of terrestrial chrome spinels (paper IV); and lastly the terrestrial crater record is compared to the spinel data from the Phanerozoic (paper IX).Results: in the late Silurian, L chondrites still dominate the flux of ordinary chondrites but is almost back to background levels; in the Turonian 3He anomaly, no lunar grains were found but there is a dominance of H chondritic grains and a ca. 5-fold increase in achondritic grains that could be signs of perturbations and smaller collisions in the asteroid belt. Several levels contain translucent Fe-rich MgAl spinel not previously detected; in the Aptian and Albian, only one tentatively lunar grain was found indicating that the Tycho-forming event could be older than 109 Ma. In summary, from the extracted equilibrated ordinary chondritic spinels, only one major breakup event is recorded during the Phanerozoic, the LCPB breakup. The flux of micrometeorites from this event continues well into the Devonian Period. The record is intercepted with a few minor, factor of two increase of H-chondritic grains during the time between ca. 60 and 170 Ma ago, and 500 Ma ago. The flux of achondritic micrometeorites were likely more common in earlier times. The sediment-dispersed spinel approach has proven to be a reliable proxy for the micrometeorite flux during geological time. The development to also characterize the flux in smaller size fractions and that of carbonaceous micrometeorites would improve the characterization of the overall flux in ancient times.
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