Daming Wang

A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion

Abstract We hypothesize that the rapid onset of the carbon isotope excursion (CIE) at the Paleocene/Eocene boundary (∼55 Ma) may have resulted from the accretion of a significant amount of 12C-enriched carbon from the impact of a ∼10 km comet, an event that would also trigger greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. 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. 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, 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 of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact.

The hysteresis properties of multidomain magnetite and titanomagnetite/titanomaghemite in mid-ocean ridge basalts

Hysteresis measurements have been carried out on a suite of ocean-floor basalts with ages ranging from Quaternary to Cretaceous. Approximately linear, yet separate, relationships between coercivity ( B c ) and the ratio of saturation remanence/saturation magnetization ( M rs / M s ) are observed for massive doleritic basalts with low-Ti magnetite and for pillow basalts with multi-domain titanomagnetites (with x =0.6). Even when the MORB has undergone low-temperature oxidation resulting in titanomaghemite, the parameters are still distinguishable, although offset from the trend for unoxidized multidomain titanomagnetite. The parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types. This plot provides a previously underutilized and non-destructive method to detect the presence of low-titanium magnetite in igneous rocks, notably MORB.

Why is the remanent magnetic intensity of Cretaceous MORB so much higher than that of mid to late Cenozoic MORB

The fact that the natural remanent magnetization (NRM) intensity of mid-oceanicridge basalt (MORB) samples shows systematic variations as a function of age has long been recognized: maximum as well as average intensities are generally high for very young samples, falling off rather rapidly to less than half the recent values in samples between 10 and 30 Ma, whereupon they slowly rise in the early Tertiary and Cretaceous to values that approach those of the very young samples. NRM intensities measured in this study follow the same trends as those observed in previous publications. In this study, we take a statistical approach and examine whether this pattern can be explained by variations in one or more of all previously proposed mechanisms: chemical composition of the magnetic minerals, abundance of these magnetization carriers, vectorial superposition of parallel or antiparallel components of magnetization, magnetic grain or domain size patterns, low-temperature oxidation to titanomaghemite, or geomagnetic field behavior. We find that the samples do not show any compositional, petrological, rockmagnetic, or paleomagnetic patterns that can explain the trends. Geomagnetic field intensity is the only effect that cannot be directly tested on the same samples, but it shows a similar pattern as our measured NRM intensities. We therefore conclude that the geomagnetic field strength was, onaverage, significantly greater during the Cretaceous than during the Oligocene and Miocene.

Reply to a comment on ‘‘A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion’’ by G.R. Dickens and J.M. Francis

Contrary to Dickens and Francis’s claim [1] that we ‘challenge the idea of a massive CH4 release during the PETM (Paleocene/Eocene thermal maximum)’, our consideration of an extraterrestrial carbon contribution to the carbon isotope excursion (CIE) is speci¢cally limited to the initial and most rapid decrease in N13C, which accounts for less than half of the full magnitude of the CIE [2]. Thermal dissociation in response to the warming at the PETM is explicitly allowed in our hypothesis, as reiterated in our conclusions that the impact ‘may have triggered a more gradual thermal dissociation of sea£oor methane hydrates’ [2]. We directly challenge only that portion of the hydrate dissociation hypothesis that relies on gradual warming intrinsic to Earth’s climate system as the triggering mechanism [3]. Such a mechanism is not consistent with the documented essentially synchronous and instantaneous warming and decrease in N13C values at the onset of the event [3,4] and is also at odds with the occurrence of the CIE during an interval of low amplitude orbital forcing of climate [5]. Instead, we postulate a comet impact as an explanation for the rapid onset of the event. Dickens and Francis state that the ‘primary di⁄culty with invoking a comet T is that there is no supporting evidence’ and then list four points from our paper that, taken out of context, are construed as damaging to our hypothesis : (1) ‘There is no crater’. If this were to be taken as a fatal problem with hypothesizing an impact then the idea of a K/T (Cretaceous/Tertiary) impact would never have gained any traction ^ it took 10 years to identify the smoking gun at Chicxulub crater [6^8]. In addition, it is hardly ‘contrived’ to acknowledge that the P/E (Paleocene/ Eocene) impact may have occurred on oceanic crust, which constitutes more than half of Earth’s surface area and where impact craters of any age have been very di⁄cult to ¢nd. (2) ‘The remarkable fossil turnovers T strongly contrast to those across the Cretaceous/Tertiary Boundary T.’ In fact, although we noted that the two events are ‘clearly diierent’ [2] and that an a priori assumption that big impacts should be