If indeed an asteroid shower in the Late Eocene was triggered by minute changes in the orbits of the inner planets, these changes could have left also a signal in Earth\'s climate. Brown et al. (2009) observe a disturbance over the 3He-rich interval in the Massignano section of the orbitally controlled cyclicity in calcite, but this could represent overprinting climatic effects of repeated asteroid impacts. It is generally accepted that Earth\'s climate variations are primarily astronomically driven, i.e. diurnal, seasonal and Milankovitch cycles, but the fundamental transition after 250 Ma of greenhouse to icehouse climate in the Late Eocene is generally explained in terms of Earth-bound causes, such as the closing or opening of different seaways or changes in atmospheric CO2 (Zachos et al., 2001). Our data cannot alone challenge such explanations in favor of astronomical causation, but an additional argument comes from the K–Ar ages of recently fallen ordinary chondrites (Swindle et al., 2014). These meteorites show an enigmatic bimodal pattern with gas-retention ages either >3.4 Ga>3.4 Ga or <1.5 Ga<1.5 Ga (and mostly <1 Ga) (Fig. 4). The many young ages indicate that major collisions involving ordinary chondrites were much more common in the past 1 Ga than in the 1 Ga before that. The period on Earth from ca. 2 to 1 Ga before the present, sometimes referred to as the Boring Billion (Hazen, 2012), does not have any documented ice age, and there is only one confirmed impact crater from this long period (for a detailed discussion, see Schmitz, 2013). It is compelling that both the ordinary chondrites and Earth\'s climate show a similar trend, with stability in the period 2 to 1 Ga before the present, followed by a period of instability up to the present. In the past ∼0.8 Ga∼0.8 Ga there have been seven or eight major Nolatrexed episodes, starting with the Snowball Earth glaciations. The onset of Snowball Earth coincides approximately with the formation of the Copernicus crater, the largest young (<2 Ga) crater on the Moon. This crater formed ∼0.8 Ga∼0.8 Ga ago (Bogard et al., 1994) and some evidence indicate that chlorophyll b formed during an asteroid shower (Zellner et al., 2009). Only the ordinary chondrites among recent meteorites show common young (<1 Ga<1 Ga) gas retention ages, indicating that only the inner asteroid belt may have been affected by gravity perturbations. Shoemaker (1998) argued based on lunar and terrestrial crater abundances that “the long-term average cratering rate may have increased late in geological time, perhaps as much as a factor of two” (see also, McEwen et al., 1997). If ordinary chondrites have been the dominating type of projectile impacting Earth, as indicated by the present study and previous work on the mid-Ordovician, then the bimodal distribution of gas retention ages of the recently fallen ordinary chondrites lends support for an increase in the impact rate the past ca. 0.8 Ga or so. Some support for this scenario comes also from studies of craters on the Moon, where Kirchoff et al. (2013) have noticed “lulls” in the impact rate >600 Ma long and approximately coinciding with the Boring Billion years. Dating uncertainties for the lunar craters, however, add complexities to the issue. The ordinary chondrites clearly dominate the meteorite flux today, and a major fraction of them originate from the Late Eocene or mid-Ordovician events. If one takes these two events out, the recent meteorite flux (and most likely also the asteroid flux) would have been significantly lower. The breakup of the Baptistina asteroid family at 160 Ma ago has been suggested to have increased the flux of km-sized bodies by at least a factor of two over the last 100 Ma (Bottke et al., 2007), however, there is no known obvious link between any of the recent common meteorite types and this event (see further Schmitz, 2013).
↧