Alan R. Hildebrand
Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4 email@example.com
A decade has passed since widespread acceptance has been reached that the Chicxulub is the source crater of the Cretaceous/Tertiary boundary ejecta; the final links were the demonstration that the lithologies of the crater matched the isotopic characteristics (including age) of the layers supporting the previously derived field relationships and elemental composition congruence. Investigation of the Chicxulub crater and K/T ejecta has continued addressing three themes: (1) delineating Chicxulubs structure as a proxy for the structure of large complex impact craters, (2) using the impact generated deposits in Chicxulub (and the crater structure) and its associated ejecta deposits as a means to explore and constrain the impact process, and (3) exploring which Chicxulub impact effects sufficiently perturbed the terrestrial environment to destroy much of terrestrial life. The first and second themes are relevant to understanding energy and mass flux inputs to the resulting environmental perturbations, but this work focuses on assessing the degree of lethality associated with proposed extinction mechanisms. An underlying assumption is that most K/T extinctions resulted within <1 year of the Chicxulub impact although its environmental perturbations may have been responsible for a tail of extinctions over a following period of up to a million years.
Table 1: Suggested agents of environmental damage induced by the Chicxulub impact; this table is updated and modified from that produced by Wolbach et al. (1990); see Toon et al. (1997) for a similar tabulation. The modifiers "proximal" and "regional" indicate potential extinction agents operating within ~1000 and ~5000 km radial distance of Chicxulub, respectively. (Modified from Hildebrand et al., 1998)
|Environmental change agent||Duration||Reference(s)|
|1. Dust veil (darkness and cold)||Months||i, ii, iii|
|2. Proximal wind||Hours||iv|
|3. Proximal giant waves||Hours||iv, v, vi, vii, viii|
|4. Proximal to regional fireball irradiance||Minutes||ix, xxviii|
|5. Regional thermal pulse from re-entering ejecta||Hours||x|
|6. Acid rain (nitrogen- and sulfur-based)||Years||xi, xii, xiii, xiv, xv, xvi|
|7. Stratospheric aerosols (cold)||Decades||xiv, xv, xvii, xviii|
|8. Ozone layer depletion (ultraviolet exposure)||Decades||xiv, xvii|
|9. H2O greenhouse||Decades||iv|
|10. CO2 greenhouse||Millennia||xii, xiv, xix, xx|
|11. Poisons and mutagens||Years to millennia?||xi, xii, xxi, xxii, xxiii, xxiv, xxv|
|12. Oscillatory/disrupted climate||Million years||xxvi, xxvii|
Extinction Mechanisms. Table 1 lists extinction mechanisms judged most lethal and/or stemming from agents evidenced in the geological record (including nature of the Chicxulub projectile and the impacted lithologies of the Yucatan platform), or stemming from our current understanding of the impact process. Wolbach et al. (1990) tabulated possible extinction mechanisms shortly before the recognition of the K/T impact site; knowledge of the latter added only cooling from atmospheric aerosols as an extinction mechanism. Additional theoretical considerations added a thermal pulse from re-entering ejecta, and K/T stratigraphic studies allowed recognition of an extended period of oscillatory/disrupted climate. Toon et al. (1997) published a thorough review of most of these extinction mechanisms (with consideration based on a realistic impact energy value for Chicxulub of ~1 X 1031 ergs). As discussed by Hildebrand et al. (1998) and illustrated in Fig. 1, the impacts environmental perturbations produce a sequence of temperature changes coupled with a halt in photosynthesis, generation of toxins, and destruction of the ozone layer. This succession of effects may mean that multiple agents were necessary for species extinction, but it remains desirable to understand which one or one(s) were the most lethal to determine the threshold to induce extinctions (and to assess the current impact hazard), and to understand why obvious impact induced mass extinctions are not more common in the geologic record. For example, is the type of impactor, impact velocity, or type of impacted rocks important? Or just an impacts magnitude? As discussed by Hildebrand et al. (1998) and illustrated in Fig. 1, the impacts environmental perturbations produce a sequence of temperature changes
Figure 1. Schematic illustration of temperature perturbations induced by the Chicxulub impact (Modified from Hildebrand et al., 1998). The log of the temperature variation (in degrees Kelvin) is plotted against the log of time (in seconds). Durations of other well evidenced environmental perturbations are also indicated.
|coupled with a halt in photosynthesis, generation of toxins, and destruction of the ozone layer. This succession of effects may mean that multiple agents were necessary for species extinction, but it remains desirable to understand which one or one(s) were the most lethal to determine the threshold to induce extinctions (and to assess the current impact hazard), and to understand why obvious impact induced mass extinctions are not more common in the geologic record. For example, is the type of impactor, impact velocity, or type of impacted rocks important? Or just an impacts magnitude?|
Where is Progress to be Made? Neither impact modelling nor Chicxulub crater studies have added significantly to our understanding of potential lethal effects of the impact during the last half decade, although some uncertainties have been removed. Laboratory experiments have provided constraints on shock devolatisation of limestone and anhydrite, but scaling to the necessary magnitude introduces uncertainties similar to those associated with the impact energy released at Chicxulub. Discovery of altered fragments of the projectile (Kyte, 2000), and the Cr isotopic signature of the boundary layers (Shukolyukov and Lugmair, 1998) have usefully constrained possible projectile types, but leave a puzzle to be solved concerning Chicxulubs impact energy.
Recent significant advances have come from discovery and study of new and old K/T boundary localities. Vajda et al. (2001) established that the forest was destroyed at a New Zealand nonmarine locality apparently demonstrating that deforestation was a global effect. This work also established that a barren zone occurs after the extinction level before the fern spore abundance anomaly occurs (V. Vajda, written commun.) possibly indicating complete destruction of the vegetation rather than just the canopy (Sweet, 2001). Belcher et al. (submitted) have established that no charcoal occurs in the soot-bearing boundary layers at nonmarine sites in North America. This apparently indicates that the thermal pulse from re-entering ejecta mechanism is not capable of igniting the forests even relatively near Chicxulub, relegating this mechanism a very minor role at best, as fireball irradiance will dominate thermal effects close to the crater.
The remaining candidates for prompt global lethal agents appear to be: 1) global dust/soot cloud, 2) acid rain, 3) stratospheric aerosols, and 4) ozone layer destruction. The last is thought to be incapable of causing sufficiently widespread extinctions amongst the extant biota leaving only three candidate mechanisms as the global K/T killers. Detailed studies of extant and still-to-be discovered K/T boundary strat sections currently seem the most likely route to further constrain extinction mechanisms.
Belcher, C.M., Collinson, M.E., Sweet, A.R., Hildebrand, A.R. and Scott, A.C. (submitted) The role of thermal radiation in the K/T event: Evidence from the charcoal record of North America
Hildebrand, A.R., Pilkington, M., Ortiz-Aleman, C., Chavez, R.E., Urrutia-Fucugauchi, J., Connors, M., Graniel-Castro, E., Camara-Zi, A., Halpenny, J.A. and Niehaus, D. 1998. Mapping Chicxulub crater structure with gravity and seismic reflection data: in, Grady, M.M., Hutchison, R., McCall, G.J.H., and Rothery, D.A. (Eds), Meteorites: Flux with Time and Impact Effects. Geological Society, London, Special Publications, 140, 153-173.
Kyte, F.T. 1998. A meteorite from the Cretaceous/Tertiary boundary: Nature, 396, 237-239.
Shukolyukov, A. and Lugmair, G.W. 1998. Isotopic evidence for the Cretaceous-Tertiary impactor and its type. Science, 282, p.927-929.
Sweet, A.R., 2001, Plants, a yardstick for measuring the environmental consequences of the Cretaceous-Tertiary boundary event. Geoscience Canada, 28, 127-138.
Vajda, V., Raine, J.I. and Hollis, C.J. 2001. Indication of global deforestation at the Cretaceous-Tertiary boundary by New Zealand fern spike. Science, 294, 1700-1702.
Toon, O.B., Zahnle, K., Morrison, D., Turco, R.P. and Covey, C. 1997. Environmental perturbations caused by the impacts of asteroids and comets: Reviews of Geophysics, 35, 41-78.
Wolbach, W.S., Gilmour, I. and Anders, E. 1990. Major wildfires at the Cretaceous/Tertiary boundary: in Sharpton, V.L. and Ward, P.D., eds., Global Catastrophes in Earth History; An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality. Geological Society of America Special Paper 247, 391-400.