We show that Earth's sedimentary strata can provide a record of the collisional evolution of the asteroid belt. From 1652 kg of pelagic Maiolica limestone of Berriasian-Hauterivian age from Italy, we recovered 108 extraterrestrial spinel grains (32-250 μm) representing relict minerals from coarse micrometeorites. Elemental and three oxygen isotope analyses were used to characterize the grains, providing a first-order estimate of the major types of asteroids delivering material at the time. Comparisons were made with meteorite-flux time "windows" in the Ordovician before and after the L-chondrite parent-body breakup. In the Early Cretaceous, ~80% of the extraterrestrial spinels originated from ordinary chondrites. The ratios between the three groups of ordinary chondrites, H, L, LL, appear similar to the present, ~1:1:0.2, but differ significantly from Ordovician ratios. We found no signs of a hypothesized Baptistina LLchondrite breakup event. About 10% of the grains in the Maiolica originate from achondritic meteorite types that are very rare (<1%) on Earth today, but that were even more common in the Ordovician. Because most meteorite groups have lower spinel content than the ordinary chondrites, our data indicate that the latter did not dominate the flux during the Early Cretaceous to the same extent as today. Based on studies of three windows in deep time, we argue that there may have been a gradual long-term (a few hundred million years) turnover in the meteorite flux from dominance of achondrites in the early Phanerozoic to ordinary chondrites in the late Phanerozoic, interrupted by short-term (a few million years) meteorite cascades from single asteroid breakup events.
The breakup of the L-chondrite parent body in the asteroid belt 466 million years (Ma) ago still delivers almost a third of all meteorites falling on Earth. Our new extraterrestrial chromite and3He data for Ordovician sediments show that the breakup took place just at the onset of a major, eustatic sea level fall previously attributed to an Ordovician ice age. Shortly after the breakup, the flux to Earth of the most fine-grained, extraterrestrial material increased by three to four orders of magnitude. In the present stratosphere, extraterrestrial dust represents 1% of all the dust and has no climatic significance. Extraordinary amounts of dust in the entire inner solar system during >2 Ma following the L-chondrite breakup cooled Earth and triggered Ordovician icehouse conditions, sea level fall, and major faunal turnovers related to the Great Ordovician Biodiversification Event.
We present the first reconstruction of the micrometeorite flux to Earth in the Silurian Period. We searched 321 kg of condensed, marine limestone from the Late Silurian Cellon section, southern Austria, for refractory chrome‐spinel grains from micrometeorites that fell on the ancient sea floor. A total of 155 extraterrestrial spinel grains (10 grains >63 μm, and 145 in the 32–63 μm fraction) were recovered. For comparison, we searched 102 kg of similar limestone from the mid‐Ordovician Komstad Formation in southern Sweden. This limestone formed within ~1 Ma after the breakup of the L chondrite parent body (LCPB) in the asteroid belt. In the sample we found 444 extraterrestrial spinel grains in the >63 μm fraction, and estimate a content of at least 7000 such grains in the 32–63 μm fraction. Our results show that in the late Silurian, ~40 Ma after the LCPB, the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude, almost down to background levels. Among the ordinary chondrites, the dominance of L‐chondritic micrometeorites has waned off significantly, from >99% in the post‐LCPB mid‐Ordovician to ~60% in the Late Silurian, with ~30% H‐, and ~10% LL‐chondritic grains. In the Late Silurian, primitive achondrite abundances are similar to today's value, contrasting to the much higher abundances observed in pre‐LCPB mid‐Ordovician sediments.
We have reconstructed the distribution of extraterrestrial chrome spinels in a marine limestone section across the Frasnian-Famennian stratotype section at Coumiac in southern France, providing the first insights on the types of micrometeorites and meteorites that fell on Earth at this time. The data can test whether the small cluster of roughly coeval, large impact structures is related to an asteroid breakup and shower with possible bearings also on the late Devonian biodiversity crisis. A total of ~180 extraterrestrial spinel grains (>32 um) were recovered from 957 kg of rock. Noble-gas measurements of individual grains show high solar-wind content, implying an origin from decomposed micrometeorites. Element analyses indicate a marked dominance of ordinary chondritic over achondritic grains, similar to the recent flux. The relation between H, L and LL meteorites is ~29-58-13%, similar to the late Silurian flux, ~31-63-6%, but different from the distribution, ~45-45-10%, in the recent and the Cretaceous flux. Our data show no indication of a generally enhanced late Devonian micrometeorite flux that would accompany an asteroid shower. However, in a single limestone bed that formed immediately before the Upper Kellwasser horizon, that represents the main end-Frasnian species-turnover event, we found an enrichment of ~10 ordinary chondritic grains (>63 um) per 100 kg of rock, compared to the ~1-3 grains per 100 kg that characterize background. The anomalously abundant grains are of mixed H, L and LL types and may be related to an enhanced flux of extraterrestrial dust during postulated minima in both the 405 ka and 2.4 Ma Earth-orbit eccentricity cycles at the onset of the Upper Kellwasser event. In the present solar system the dust accretion at Earth is the highest at eccentricity minima because of the spatial distribution of dust bands of the zodiacal cloud. Besides this small grain anomaly the data here and in previous studies support a stable meteorite flux through the late Silurian and Devonian, in contrast to the mid-Ordovician, when achondritic meteorites that are rare on Earth today were common, followed by the influx of a flood of debris related to the breakup of the L-chondrite parent body. Our accumulated data for six time windows through the Phanerozoic indicate that the ordinary chondrites make up a major fraction in the meteorite flux since at least the mid-Ordovician. We note that the sources in the asteroid belt of the H and L meteorites, the two most common types of meteorites today and through much of the Phanerozoic, remain elusive. Revision NotesHighlights for review: First data on the meteorite flux to Earth during the Frasnian-Famennian bioevent. No support of asteroid shower, but comet shower cannot be ruled out. Meteorite flux supports eccentricity minimum in Earth's orbit at bioevent. First picture of the variations in meteorite flux through the Phanerozoic. Ordinary chondrites made up a major fraction of the flux since at least 466 Ma ago.
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