A 35 Myr record of helium in pelagic limestones from Italy: Implications for interplanetary dust accretion from the early Maastrichtian to the middle Eocene

We have determined the helium concentration and isotopic composition of a suite of early Maastrichtian through middle Eocene pelagic limestones in the Italian Apennines. The results provide a 35 Myr record of the implied flux of extraterrestrial ^3He, which is a proxy for the accretion rate of interplanetary dust particles (IDPs). Our measurements show that the ^3He flux was fairly constant in the Maastrichtian, except for possible minor increases (factor of two or less) from ∼70.5 Ma to 68 Ma and at ∼66 Ma, which probably reflect transient increases in the accretion rate of asteroidal and/or cometary IDPs. We find no evidence for an increase in IDP accretion at or immediately before the K/T boundary, implying that the K/T impact was not associated with enhanced solar system dustiness. This observation precludes the possibility that the K/T impactor was a member of a major comet shower, and is more consistent with impact of a lone comet or asteroid. Our data suggest a 2 to 4 fold increase in IDP accretion between 57 and 54 Ma, followed by a factor of three decrease over an ∼4 Myr period in the early to middle Eocene. The duration and magnitude of this variability is inconsistent with previous observations attributed to a shower of long period comets, and is more likely the result of collisions in the asteroid belt/and or Kuiper belt. In the entire 35 Myr record we find no evidence for major enhancements of the IDP accretion rate of the type expected from comet showers. Our results, in combination with earlier ^3He measurements, do not support models that predict recurrent comet showers with periods of