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.
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