Dennis V. Kent*†, Benjamin S. Cramer*^, Luca Lanci*, Daming Wang‡, James D. Wright*, Rob Van der Voo‡
*Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854 USA
†Lamont-Doherty Earth Observatory, Palisades, NY 10964 USA
^now at: Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan
‡Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 USA
The Paleocene/Eocene (P/E) boundary (~55 Ma) is marked by an abrupt carbon isotope excursion (CIE) recorded in marine and terrestrial systems (e.g., (1, 2)) that is coincident with an equally rapid oxygen isotope excursion interpreted as the P/E thermal maximum (3). Closely associated with the P/E boundary was the largest deep-sea benthic extinction of the past 90 m.y. (4) as well as a major radiation of mammals (5). A widely accepted explanation for the CIE is the sudden dissociation of 12C-enriched marine gas hydrates (6-8). Such a major dissociation event would seem to require either a thermal (6) or mechanical (9) precursor. However, high-resolution oxygen and carbon isotope records have yet to demonstrate a significant warming immediately preceding the CIE (e.g., (10-12)) and a geologic event of sufficient magnitude to provide a plausible mechanical trigger has not been identified.
We suggest (13) that the rapid onset of the CIE may have resulted from the accretion of a significant amount of 12C-enriched carbon (14-16) from the impact of a ~10 km comet, an event that would also trigger rapid greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. The massive introduction of carbon directly into the atmosphere would produce newly corrosive and warmer (and hence less oxygenated) bottom waters that have previously been suggested as proximate causal mechanisms for the massive extinction of benthic organisms coincident with the CIE (1, 4). Indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite-rich shelf sediments that closely coincide with the onset and nadir of the CIE at three drill sites on the Atlantic Coastal Plain (Fig. 1). After considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron-rich nanophase material at the Cretaceous/Tertiary boundary (17, 18), we suggest that the CIE occurrence was derived from an impact plume condensate. The sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket. Published reports (19, 20) of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact.
Figure 1. Lithologic, isotopic and magnetic hysteresis profiles across the CIE interval in three drill sites on the Atlantic Coastal Plain of New Jersey, eastern North America. Double-arrows mark the onset of the CIE interval. Magnetostratigraphic, biostratigraphic, lithologic and benthic foraminiferal isotopic data for Bass River is from (23), lithologic data for Clayton from (24). The sand-fraction record does not differentiate between largely quartz and glauconite sand below the CIE and exclusively biogenic (e.g., foraminiferal) sand within the CIE interval. Bulk carbonate carbon isotope data were generated for this study. Samples for carbon isotope analysis were ground and reacted in phosphoric acid (H3PO4) at 90°C in a Multiprep system attached to a Micromass Optima mass spectrometer. The close correspondence between bulk carbonate and benthic foraminiferal d13C records at Bass River indicates that the bulk carbonate d13C records accurately reflect local seawater dissolved inorganic carbon at the time of deposition. The magnitude of the d13C decrease recorded in these sections (-4 to –5 ‰) is substantially larger than that recorded in deep-sea records, but is similar to the atmospheric/surface-ocean records of surface-dwelling planktonic foraminifera (1, 21) and terrestrial carbonates (2, 25). Saturation magnetization (Ms) and the ratio of saturation remanence to saturation magnetization (Msr/Ms) were determined from hysteresis measurements on ~20 mg bulk sediment samples using an alternating gradient force magnetometer (Micromag 2900) to a maximum field of 1000 mT; Ms is determined after standard correction for the high field paramagnetic slope from 7000-1000 mT.
A testable prediction of the comet impact hypothesis is the production of virtually instantaneous environmental effects in the atmosphere and surface ocean with later repercussions in the deeper ocean. We believe that recently reported high-resolution isotopic and faunal records from a deep-sea sediment section (21, 22) are in agreement with such a scenario.
References
1. J. P. Kennett, L. D. Stott, Nature 353, 225-229 (1991).
2. P. L. Koch, J. Zachos, P. D. Gingerich, Nature 358, 319-322 (1992).
3. J. C. Zachos, K. C. Lohmann, J. C. G. Walker, S. W. Wise, The Journal of Geology 101, 191-213 (1993).
4. E. Thomas, N. J. Shackleton, in Correlation of the Early Paleogene in Northwest Europe R. W. O. B. Knox, R. M. Corfield, R. E. Dunay, Eds. (Geological Society, London, 1996), vol. Special Publication No. 101, pp. 401-441.
5. M. C. Maas, M. R. L. Anthony, P. D. Gingerich, G. F. Gunnell, D. K. Krause, Palaeogeography, Palaeoclimatology, Palaeoecology 115, 181-207 (1995).
6. G. R. Dickens, J. R. O'Neil, D. K. Rea, R. M. Owen, Paleoceanography 10, 965-971 (1995).
7. G. R. Dickens, M. M. Castillo, J. C. G. Walker, Geology 25, 259-262 (1997).
8. K. Kaiho et al., Paleoceanography 11, 447-465 (1996).
9. M. E. Katz, B. S. Cramer, G. S. Mountain, S. Katz, K. G. Miller, Paleoceanography 16, 549-562 (2001).
10. T. J. Bralower et al., Geology 25, 963-966 (1997).
11. D. J. Thomas, T. J. Bralower, J. C. Zachos, Paleoceanography 14, 561-570 (1999).
12. S. Bains, R. M. Corfield, R. D. Norris, Science 285, 724-727 (1999).
13. D. V. Kent et al., Earth and Planetary Science Letters in final revision (2003).
14. E. Jessberger, K., J. Kissel, in Comets in the Post-Halley Era R. L. Newburn, e. al., Eds. (Kluwer Academic Publishers, Amsterdam, 1991), vol. 2, pp. 1075-1092.
15. A. H. Delsemme, Philosophical Transactions of the Royal Society of London A325, 509-523 (1988).
16. E. K. Jessberger, Space Science Reviews 90, 91-97 (1999).
17. T. J. Wdowiak et al., Meteoritics & Planetary Science 36, 123-133 (2001).
18. H. C. Verma et al., Lunar and Planetary Science 32, 1270.pdf (2001).
19. B. Schmitz et al., Palaeogeography, Palaeoclimatology, Palaeoecology 133, 49-68 (1997).
20. T. Dolenec, J. Pavsic, S. Lojen, Terra Nova 12, 199-204 (2001).
21. D. J. Thomas, J. C. Zachos, T. J. Bralower, E. Thomas, S. Bohaty, Geology 30, 1067-1070 (2002).
22. D. C. Kelly, Paleoceanography 17, 23/1-23/13 (2002).
23. B. S. Cramer et al., Bulletin Societe Geologie France 170, 883-897 (1999).
24. T. G. Gibson, L. M. Bybell, D. B. Mason, Sedimentary Geology 134, 65-92 (2000).
25. G. J. Bowen et al., in Paleocene-Eocene Stratigraphy and Biotic Change in the Bighorn and Clarks Fork Basins, Wyoming P. D. Gingerich, Ed. (Museum of Paleontology, The University of Michigan, Ann Arbor, 2001), vol. University of Michigan Papers on Paleontology, No. 33, pp. 73-88.
