EVIDENCE FOR AN ASTEROID SHOWER TO EARTH FOLLOWING THE BREAKUP OF THE L-CHONDRITE PARENT BODY 466 MA

Although the ~200 impact craters known on Earth represent only a small fraction of the craters originally formed, the data suggest an excess by one order-of-magnitude of craters, by number, in the interval ~470-440 Ma in the Ordovician [1, 2]. It is a matter of debate whether the Ordovician "excess" craters reflect a preservation bias, or are related to the breakup of the L-chondrite parent body (LCPB) in the asteroid belt 465.8 ±0.3 Ma. Here five independent lines of empirical evidence are summarised in support of the latter view. The strongest evidence comes from the >130 fossil L chondrites (1-21 cm large) recovered during quarrying of marine limestone that formed within 2 Myr after the LCPB. Both the spatial density of the meteorites on the seafloor and the ratio of L chondrites versus other types of meteorites found provide evidence for a two orders-of-magnitude increase in the flux of L chondrites. Only one non-L-chondritic meteorite has been found. The record of K-Ar gas retention ages of many different types of recently fallen meteorites only documents one major breakup in the asteroid belt during the Phanerozoic, the LCPB event. Among the meteorites falling on Earth today about a third originate from this event. Apparently, the peak in L-chondritic K-Ar ages at ~470 Ma is matched by a corresponding enhanced flux of L chondrites to Earth shortly after the breakup. A similar pattern as in the cratering and K-Ar isotopic records, with only one prominent flux peak, is emerging in studies of the flux of micrometeoritic chrome spinel through the Phanerozoic. Searches for such chrome-spinel grains in 8484 kg of sediment representing fifteen different time windows through the past 540 Myr indicate a nearly constant flux of ordinary chondritic grains [1]. The only exception is after the LCPB event, when the flux increased by at least two orders of magnitude. The recovered chrome-spinel grains dominantly represent large micrometeorites (>300 μm) and as such are representative also of the flux of larger meteorites to Earth. Additional evidence for an asteroid shower after the LCPB comes from crater spatial densities in North America and Baltoscandia and the flux of extraterrestrial He-3 in deep time.

References: [1] B. Schmitz et al. (2022) Geol. Soc. Am. Spec. Pap. 557, c. 18. [2] M. Schmieder & D.A. Kring (2020) Astrobiology 20, p. 91-141.