Mass Extinctions and The Sun’s Encounters with Spiral Arms

The terrestrial fossil record shows that the exponential rise in biodiversity since the Precambrian period has been punctuated by large extinctions, at intervals of 40 to 140 Myr. These mass extinctions represent extremes over a background of smaller events and the natural process of species extinction. We point out that the non-terrestrial phenomena proposed to explain these events, such as boloidal impacts (a candidate for the end-Cretaceous extinction), and nearby supernovae, are collectively far more effective during the solar system’s traversal of spiral arms. Using the best available data on the location and kinematics of the Galactic spiral structure (including distance scale and kinematic uncertainties), we present evidence that arm crossings provide a viable explanation for the timing of the large extinctions. The literature is replete with suggestions of non-terrestrial phenomena as the candidate causes for large scale extinctions. The most frequently invoked are supernovae and boloidal impacts (e.g. comets from the Oort Cloud), the latter a strong candidate for the K/T extinction ever since the discovery of the Iridium anomaly in the K/T boundary clay (Alvarez et al. 1990). Certainly the most violent events in the solar neighborhood during geologic history would have been supernovae (barring the possibility of a nearby γ-ray burst, which is far less likely; Thorsett 1995); that the structure of the very local interstellar medium is considered to be the result of a supernova, possibly related to the Geminga pulsar (at 150 pc) (Bignami & Caraveo 1996), is an impressive reminder of their potential impact. Supernovae and young supernova remnants, especially those occurring at distances < ∼ 10 pc (Ruderman 1974), can result in biospheric imbalance through a variety of processes, including ozone depletion by enhanced ionizing radiation and cosmic rays (Ellis & Schramm 1995, Koyama et al. 1995), the absorption of visible light by the formation of NO2 (Crutzen & Brühl 1996), and in rare cases the direct deposition of supernova debris. Tidal and collisional encounters with intermediate-sized gas or dust clouds might focus cometary activity (∼ 10 comets) to the inner solar system, by scattering of Oort Cloud member bodies, a mechanism proposed to explain the K/T boundary event. In addition, the passage of the Sun through a cloud of density n > ∼ 10 4 cm could raise the solar luminosity significantly, through Bondi accretion, as well as raise the opacity of Earth’s atmosphere, directly affecting the insolation on Earth (McCrea 1975). While the recent association of the Chicxulub crater with the K/T boundary lends credence to the boloidal impact model, the large concentration of Ir deposited at the boundary may indicate that accretion also played an important role (Yabushita & Allen 1997). The proposed mechanisms constitute a set of plausible external agents for any one extinction, yet do not of themselves suggest any explanation for the timing of the mass extinctions, or for the large variation in severity of the observed extinctions. Hatfield and Camp (1970) were among the first to suggest that extinctions might be correlated with Galactic-plane crossing due to the solar orbit’s vertical oscillations. Rampino and Stothers (1984), as well as Schwartz and James (1984), have invoked these z-oscillations in connection with the suggested ∼ 26 − 30 Myr periodicity of minor extinctions, as virtually all of the postulated extinction mechanisms concentrate toward the Galactic plane. However, the fact that we