The micrometeorite flux to Earth during the earliest Paleogene reconstructed in the Bottaccione section (Umbrian Apennines), Italy

Abstract

Based on sediment‐dispersed extraterrestrial spinel grains in the Bottaccione limestone section in Italy, we reconstructed the micrometeorite flux to Earth during the early Paleocene. From a total of 843 kg of limestone, 86 extraterrestrial spinel grains (12 grains > 63 μm, and 74 in the 32–63 μm fraction) have been recovered. Our results indicate that the micrometeorite flux was not elevated during the early Paleocene. Ordinary chondrites dominated over achondritic meteorites similar to the recent flux, but H chondrites dominated over L and LL chondrites (69%, 22%, and 9%, respectively). This H‐chondrite dominance is similar to that recorded within an enigmatic 3He anomaly (70, 27, and 3%) in the Turonian, but different from just before this 3He anomaly and in the early Cretaceous, where ratios are similar to the recent flux (~45%, 45%, and 10%). The K‐Ar isotopic ages of recently fallen H chondrites indicate a small impact event on the H‐chondrite parent body ~50 to 100 Ma ago. We tentatively suggest that this event is recorded by the Turonian 3He anomaly, resulting in an H‐chondrite dominance up to the Paleocene. Our sample spanning the 20 cm above the Cretaceous–Paleogene (K–Pg) boundary did not yield any spinel grains related to the K–Pg boundary impactor.

Asteroid break-ups and meteorite delivery to Earth the past 500 million years

“…In assemblages representing the time about 1 million y before the LCPB break-up the L chondrites already constitute about one-third of the ordinary chondrites with a flux rate on the same order of magnitude as the Phanerozoic background flux. In the Cambrian time windows, we see some of the lowest L-chondritic percentages of the Phanerozoic, 12 to 17%, but also in the early Paleocene, ∼66 to 61 Ma ago, are such low values recorded (26).…”

“…However, the anomalies only reflect minor increases in the flux of 3 He-carrying dust, typically by a factor two to five over periods of 0.1 to 2 Ma. We have searched three of the 3 He-rich intervals for extraterrestrial spinels (>32 μm), but could not detect any micrometeorite flux increases larger than a factor of two (5,26,35,36).…”