Geochemical Evidence for a Comet Shower in the Late Eocene

Analyses of pelagic limestones indicate that the flux of extraterrestrial helium-3 to Earth was increased for a 2.5-million year (My) period in the late Eocene. The enhancement began ~1 My before and ended ~1.5 My after the major impact events that produced the large Popigai and Chesapeake Bay craters ~36 million years ago. The correlation between increased concentrations of helium-3, a tracer of fine-grained interplanetary dust, and large impacts indicates that the abundance of Earth-crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced. These observations provide evidence for a comet shower triggered by an impulsive perturbation of the Oort cloud...

Our preferred explanation for these data is that a short-lived burst of long-period comets occurred in the late Eocene. If this explanation is correct, then the Popigai and Chesapeake Bay impactors are likely to have been comets. This suggestion is consistent with the approximately chondritic elemental abundances inferred for the impactor component in the Popigai impact ejecta (1), although identification of the impactor by this technique is difficult (26), particularly in the case of Popigai, where the target lithologies are complex and poorly characterized (27). Additional evidence for a comet impact at this time comes from the recent discovery of abundant Ni-rich spinel, probably of cometary origin, associated with the 5.61-m Ir anomaly at Massignano (28).

This topic was posted 03/26/2009.

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