Rapid changes in the nature of the H chondrites falling to Earth

We have previously identified a subgroup of Antarctic H chondrites that are significantly different from H chondrites among the modern falls in terms of induced thermoluminescence (TL), metallographic cooling rate, and cosmogenic inert gas contents. Here we examine their terrestrial and thermal history as apparent in their natural TL and radioactive cosmogenic isotope abundances. These meteorites have a tendency towards high Al-26 activities and fairly short C-14 and Cl-36 terrestrial ages (generally <100 ka). They also sometimes exhibit unusually high natural TL levels, which we have previously interpreted as indicating orbital evolution from perihelia >1.2 AU to similar to 1 AU within the last <10(5) years. We suggest that the nature of the meteorites falling to Earth is not independent of time but depends on stochastic events, such as the breakup of parent bodies and recent variations in orbit.