IMPACT-CRATER AGES AND MICROMETEORITE PALEOFLUXES COMPARED: EVIDENCE FOR THE IMPORTANCE OF ORDINARY CHONDRITES IN THE FLUX OF METEORITES AND ASTEROIDS TO EARTH THE PAST 500 MILLION YEARS

Although the ~200 impact craters known on Earth represent only a small fraction of the craters originally formed, the available data suggest an excess by one order-of-magnitude of craters, by number, in the interval ~470-440 Ma in the Ordovician. Most of these "excess" craters may be related to the breakup of the L-chondrite parent body (LCPB) in the asteroid belt 465.8 ±0.3 Ma. This is the only obvious peak in the crater-age record that can currently be attributed to an asteroid shower and breakup event. Spatial crater densities in regions with high potential for crater preservation (e.g., Canada, Scandinavia) support a one order-of-magnitude increase in the flux of large (>0.1 km) impactors following the LCPB breakup. A similar pattern as seen in the cratering record is emerging in studies of the flux of micrometeoritic chrome-spinel through the Phanerozoic, with so far only one major spike in the flux, and associated with the LCPB breakup. Similarly, the record of K-Ar and (U-Th)/He gas retention ages of recently fallen meteorites only locates one major breakup, the LCPB event, during the Phanerozoic. On the other hand, astronomical backtracking studies of the orbits of asteroid family members indicate about 70 major family-forming breakups within the past ~540 m.y. which apparently have not left any clear imprint in Earth's geological record. An observed nearly constant flux of ordinary chondritic chrome-spinel grains throughout the Phanerozoic, except after the LCPB event, indicates that the present situation - with a clear dominance of ordinary chondritic matter in the large (>500 μm) micrometeorite and macroscopic meteorite fractions - has prevailed at least for the last 500 m.y. This is also supported by generally high ratios in our samples of chrome-spinel grains from ordinary chondrites compared to other types of spinel-bearing meteorites. The chrome-spinel data together with the abundance of fossil meteorites (1-21 cm in diameter) on the Ordovician sea floor also sets an upper limit at one order of magnitude in the increase in flux of large (>0.1 km-diameter) L-chondritic projectiles to Earth following the LCPB. Such an increase would not stand out in the global cratering record if ordinary chondritic impactors had only represented a small fraction of all Phanerozoic impactors. We argue that the origin of impactors delivered to Earth during the past 500 m.y. has mirrored the flux of large micrometeorites and meteorites, with ordinary chondrites being an important or, most likely, the dominant (in numbers) component throughout.