The extraterrestrial impact evidence at the Palaeocene-Eocene boundary and sequence of environmental change on the continental shelf

Abstract

We have identified clear evidence of an extraterrestrial impact within the onset of the carbon isotope excursion (CIE) that defines the Palaeocene-Eocene (P-E) boundary hyperthermal event (approx. 56 Ma) from several sites on the eastern Atlantic Coastal Plain and offshore. We review and update the state of the evidence for an impact at the P-E boundary, including a K-Ar cooling age of the ejecta that is indistinguishable from the depositional age at the P-E, which establishes the ejecta horizon as an isochronous stratigraphic indicator at the P-E. Immediately above the ejecta peak at the base of the coastal plain Marlboro Clay unit, we identify a sharp increase in charcoal abundance coincident with the previously observed dramatic increase in magnetic nanoparticles of soil pyrogenic origin. We therefore revisit the observed sequence of events through the P-E boundary on the western Atlantic Coastal Plain, showing that an extraterrestrial impact led to wildfires, landscape denudation and deposition of the thick Marlboro Clay, whose base coincides with the spherule horizon and CIE onset. The Sr/Ca ratio of the spherules indicates that the carbon responsible for the onset may be vaporized CaCO₃ target rock mixed with isotopically light carbon from the impactor or elsewhere. Crucially, we do not argue that the impact was responsible for the full manifestation of the CIE observed globally (onset to recovery approx. 170 kyr), rather that a rapid onset was triggered by the impact and followed by additional carbon from other processes such as the eruption of the North Atlantic Igneous Province. Such a scenario agrees well with recent modelling work, though it should be revisited more explicitly.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

Keywords: Palaeocene–Eocene Thermal Maximum; carbon cycle; extraterrestrial impact; hyperthermal; impact ejecta.

Figure 1.

Stratigraphic distribution of P-E spherules from Millville (a), Wilson Lake B (b), and ODP Hole 1051B (c) (modified from Schaller et al. [18]). The Millville and Wilson Lake B core depths are indicated in drilling units (du) of decimal feet; core depth in Hole 1051B is in metres below seafloor (mbsf). The bulk carbonate δ¹³C from Millville and Wilson Lake B is from Wright & Schaller [19], and Hole 1051B from Katz et al. [13]. (d) Map showing Atlantic margin locations, including exposure in Medford, NJ, and Site 1051, Blake Nose. Ejecta in the Mid-Atlantic palaeo-continental shelf sites are found in the basal Marlboro Clay Formation.

Figure 2.

Electron backscatter (15 kV) images of some representative P-E spherules (microtektites and microkrystites) from Hole 1051B, Wilson Lake B, and Millville cores (modified from Schaller et al. [18]). Insets are light micrographs. (i–p) show P-E boundary spherule cross sections mounted in epoxy (modified from Schaller et al. [18]). Note the classic dendritic quench textures. Shocked quartz grain in (m) is shown in detail in figure 6. See Schaller et al. [18] for details. (Online version in colour.)

Figure 3.

Location map showing distribution of sites along the Mid-Atlantic Salisbury Embayment on the palaeo-continental shelf (modified from Kent et al. [31] and Kopp et al. [32]). Sites discussed in the text: M, Millville; WL, Wilson Lake-B. Thick red line shows the fall line; thin red contours are the approximate thickness of the Marlboro Clay. Circles represent other core locations with characteristic single-domain magnetic nanoparticles within the Clay unit [32]. ‘Potomac’ and ‘Susquehanna’ represent the approximate location of the drainage outlets of these major river systems in the latest Palaeocene. (Online version in colour.)

Figure 4.

Microtektite and microkrystite major oxide chemistry from Wilson Lake B, Millville, Medford, and Hole 1051B using EDS (modified from Schaller et al. [18]) and WDS (this study). Closed symbols represent microtektites, and open symbols denote microkrystites. WDS measurements are shown in black, with a black rim for microtektites and a black interior for microkrystites.

Figure 5.

(a) Background corrected Raman spectra of representative lechatelierite inclusions found in PE spherules compared with the spherule matrix, lechatelierite from fulgurite, SiO₂ glass, and quartz (modified from Schaller et al. [18]). (b) Raman spectra of representative clinopyroxene crystallites compared with augite, diopside and ferrosilite standards [18]. (c) Micro-laser Raman spectra of representative microtektite matrices. Spectra are collected using a Bruker 532 nm green laser system at the Rensselaer Polytechnic Institute, Troy, NY, and are background corrected [18].

Figure 6.

(a) Distribution of the shocked grains with depth, compared with the spherule peak and the CIE. At present only 200 quartz grains from each sample at Millville were analysed for shock metamorphism. (b) Background corrected Raman spectrum of a polished microkrystite containing crystalline quartz (Millville 898.5 du) and an unshocked quartz standard [18]. Black dashed lines note shifts in various vibrational modes between an unshocked quartz grain and the quartz inclusion in the microkrystite. These measurements are compared with spectra for quartz shocked to peak pressures of 25.8 GPa [49], which are remarkably consistent with those from the Millville inclusion. Inset shows characteristic 464 to 450 wavenumber vibrational relaxation associated with shocked quartz. (c) Electron backscatter image of a polished microkrystite from Millville containing the shocked quartz grain analysed in (b). Scale bar = 50 µm [18]. L, lechatelierite; M, matrix; cpx, clinopyroxene microlites. (d) Raman spectra of the shocked grain at 898.8 du (pink) compared with synthetically shocked quartz from McMillan et al. [49] (purple).

Figure 7.

Age estimates for the onset of the Palaeocene–Eocene CIE (modified from Westerhold et al. [57]). The different ages of the Fish Canyon Tuff Ar-Ar standard yield different ages for the P-E boundary.

Figure 8.

(a) Electron backscatter (15 kV) images of the spherules containing Fe-rich inclusions measured in table 2. Scale bar = 200 µm. (a) Millville 898.8 AJ. (b) Millville 898.8 AM. (c) Millville 898.8 AN. (d) Raman spectra of an Fe-rich inclusion at Millville 898.8 AJ, compared with standard of magnetite and a reference spectrum of magnesiochromite (RRUFF ID: R050399).

Figure 9.

An expanded view of the CIE onset on the palaeo-continental shelf at sites Wilson Lake and Millville demonstrating the order of events at the P-E boundary. Spherule abundance at Wilson Lake B and Millville [18], bulk carbonate δ¹³C at Wilson Lake B and Millville [19], bulk carbonate and foraminiferal δ¹³C at Wilson Lake A [92], foraminiferal isotopes at Millville [93] and magnetics from Wilson Lake B [93] are shown for comparison. Arrow indicates the depth of the peak in charcoal abundance at Wilson Lake B [94]. The horizontal dashed grey lines identify the apparent offset between an initial decrease in δ¹³C measured in bulk carbonate and the spherule peak, which coincides with the base of the Marlboro Clay and defines the start of the CIE onset and P-E boundary (shown in excursion foraminiferal and bulk δ¹³C values above). A 5 du depth adjustment is applied to the Wilson Lake A record for comparison with Wilson Lake B. Ratio of saturation remanence to saturation magnetization (Mr/Ms), low-field magnetic susceptibility (XLF) and volume-weighted SD fraction (f_SD) at Wilson Lake B from Kent et al. [93] illustrates the abrupt rise in magnetization coincident with the peak in charcoal abundance. The foraminiferal isotopes at Millville are not corrected for a δ¹³C size offset and samples containing specimens greater than 212 µm are outlined in black. The samples through the onset at Millville are assembled from a number of different sampling campaigns and we note that samples could be shifted up or down ca 10 cm due to depletion of the core and shrinkage from their absolute positions.