Schmitz, and 6 co-authors report that 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 today. Our new extraterrestrial chromite and 3He 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 Bio-diversification Event.
The results of all the parameters studied across the Hällekis section show that the first signal of the LCPB breakup occurs 1 m below the base of the Arkeologen bed, in the L. variabilis Conodont Zone. At this level, the authors observed the gradual onset of a marked rise in the number of extraterrestrial chromite grains. There is also a sharp change at −1 m in the ratios between different meteorite groups toward complete dominance of L-chondritic grains. Extremely high numbers of extraterrestrial chromite grains continue upward through the section for at least 8 m, corresponding to 2 to 4 Ma.
Extraterrestrial 3He shows a marked and sudden rise at the −1-m level, which indicates that the first fine-grained dust arrived on Earth at the same (±50 ka) time as the first abundant LCPB-related micrometeorites. Osmium isotopes also show a major shift at this level, indicative of increased delivery of extraterrestrial matter to the seafloor. Both the He and Os isotopes indicate elevated abundances of extraterrestrial matter for some meters upward through the section, in concordance with the chromite data. The authors conclude that detailed studies of the sudden increase in extraterrestrial material at −1 m reflects the arrival of the first LCPB dust on Earth and cannot be explained by a stratigraphic hiatus.
In the almost 3,000 Antarctic micrometeorites dissolved in HF, authors only found a single ordinary chondritic chromite grain of >32 um, an angular grain of ~60 um in diameter, and two chrome-spinel grains ~20 um large from other meteorite types (see data file S4 for details).The grains were found in the melted micrometeorites, which
is consistent with chromite and other chrome-spinel types being among the minerals in meteorites with the highest melting temperatures. Chromite apparently can survive atmospheric passage when most other minerals melt or recrystallize. From other studies of recent micrometeorite assemblages, the authors deduced that about one-fifth of the micrometeorites that the authors dissolved were ordinary chondrites. This means that about 1 in 600 ordinary chondritic micrometeorites contains a chromite grain of >32 um. Thus, the up to ~50 chromite grains >32 um kg−1 in post–LCPB breakup limestone represent the residue on the order of 30,000 micrometeorites (>0.1 mm) per kilogram of sediment.
Read on ..... interesting paper. Well done.
An earlier paper on the subject dated 2007: https://i2mconsultants.com/downloads/corp/2007M+PS___42__113K.pdf
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