SUPPLEMENTARY CALCULATIONS AND INFORMATION:

(Note: paragraph numbering corresponds to that of the GSA Today response)

1. MICROSPHERULES. To compare the local ET accretion rate for the Younger Dryas impact layer (YDB) to the global accretion rate from Karner et al. (2003) of 2.5 x 10⁹ g yr^-1, it is necessary to convert the YDB rate to a global value. At 14 sites tested for YDB microspherules, the average is 389 per 100 cm² with a range of 6 to 2144 spherules kg^-1 (Firestone et al. 2007, Table 4), which, we propose, accumulated within one year or less. For the density of microspherules, we adopted a value of 5.18 g cm^-3, the density of magnetite. Sizes ranged from 10 to 250 microns, averaging ~100 microns in diameter. Using the above, we calculated:

Volume of one avg. YDB spherule = 5.24 x 10^-13 m³

Weight of one avg. YDB spherule = 2.71 x 10^-6 g

Number of YDB spherules (if global) = 1.98 x 10¹⁹ yr^-1

Total weight of global YDB spherules = 5.37 x 10¹³ g yr^-1

However, we counted only microspherules, whereas Karner reported values for the accretion of total ET particles, some of which are not spherules. Yada et al. (2004, Fig. 4) found that microspherules make up an average of 38% of total ET material extracted from Antarctic ice. To normalize the global YDB ET material to account for 38% microspherules, we get:

Total global YDB material = (5.37 x 10¹³ g yr^-1)/(0.38) = 14.13 x 10¹³ g yr^-1

Comparing the YDB value to Karner’s value for ET accretion:

(14.13 x 10¹³ g yr^-1)/( 2.5 x 10⁹ g yr^-1) = 56,500 times annual accretion

2. IRIDIUM. Rudnick and Gao (2003) reported iridium concentrations of 0.022 ng g^-1. Firestone et al. (2007, Table 3) reported measurable YDB iridium values ranging from 0.04 to 3.8 ng g^-1 and averaging 1.94 ng g^-1. Comparing the YDB value to Rudnick’s iridium values:

(1.94 ng g^-1)/(0.022 ng g^-1) = 88 times higher

3. NANODIAMONDS. Firestone et al. (2007, Fig. 11) show a ¹³C-NMR plot of nanodiamonds found in glass-like carbon. The glass-like carbon at that site comprises 1.6% of the sediment, and in the samples tested, the diamonds comprise ~3% by volume. None were found above or below the YDB. Testing is underway to fully quantify the nanodiamonds, which are a widely accepted impact marker. Preliminary results indicate that at 14 sites in 5 countries on 2 continents, they exist only in the YD impact layer in significant quantities that appear comparable to K/T values reported.

4. STRATIGRAPHIC CONTINUITY. At Blackwater Draw, NM, the Clovis type-site, Haynes (1995) reported 60 radiocarbon dates, of which 20 closely span the time of the YD impact event. Based on this sequence of dates, Haynes et al. (1999) concluded that at the Clovis surface, which we call the YDB, any break in sedimentation lasted “no more than a decade and possibly much less.” This duration was much too short to produce the high iridium levels or magnetic microspherule concentrations we have reported that average 56,500 years of ET accretion.

In addition, radiocarbon dates of sufficient chronologic resolution from other YDB sites, such as Murray Springs, AZ, support the conclusion that at the YDB, there was either continuous deposition or only very brief interruption in sedimentation. Given this evidence, it is implausible that a “normal rain of ET material” would have produced the reported large concentrations of markers.

5. IMPACT MARKERS. The following YDB markers peak at the K/T and YD impact events (for discussion and references, see Firestone et al., 2007):

1) Magnetic microspherules

2) Magnetic grains

3) Iridium

4) Grape-cluster soot

5) PAHs (polycyclic aromatic hydrocarbons)

6) Fullerenes

7) Helium-3

8) Nanodiamonds

9) Extinctions

The following YDB markers can be produced by intense wildfires (see discussion and references in Firestone et al., 2007), such as are associated with the K/T impact:

10) Charcoal

11) Glass-like carbon

12) Carbon spherules

13) Black layer (containing charcoal and wood fragments)

14) Ammonium, nitrates, nitrites, and oxalates.

While some of these markers exist at low levels in non-impact layers as well, due to redeposition and normal ET accretion rates, the crucial distinction is that the concentrations of markers rise to substantial, synchronous peaks only in impact layers, including the YDB layer.

REFERENCES:

Carlisle, B.C. & Braman, R.B., 1991, Nanometre-size diamonds in the Cretaceous/Tertiary boundary clay of Alberta, Nature, v. 352, p. 708–709.

Firestone, R. B., West, A., Kennett, J. P., Becker, L., Bunch, T. E., Revay, Z. S., Schultz, P. H., Belgya, T., Kennett, D. J., Erlandson, J. M., Dickenson, O. J., Goodyear, A. C., Harris, R. S., Howard, G. A., Kloosterman, J. B., Lechler, P., Mayewski, P. A., Montgomery, J., Poreda, R., Darrah, T., Hee, S. S. Que, Smith, A. R., Stich, A., Topping, W., Wittke, J. H., Wolbach, W. S., 2007, Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling, Proceedings of the National Academy of Sciences, v. 104, p. 16,016–16,021.

Haynes C.V., Jr., 1995, Geochronology of paleoenvironmental change, Clovis type site, Blackwater Draw, New Mexico, Geoarchaeology 10 (5):317–388.

Haynes, C.V., Jr., Stanford, D.J., Jodry, M., Dickenson, J., Montgomery, J.L., Shelley, P.H., Rovner, I, Agogino, G.A., 1999, A Clovis well at the type site 11,500 B.C.: The oldest prehistoric well in America, Geoarchaeology, v. 14, no. 5, p. 455–470.

Karner, D. B., Muller, R. A., Levine, J., Asaro, F., Ram, M., and Stolz, M. R., 2003, Extraterrestrial accretion from the GISP2 ice core, Geochimica et Cosmochimica Acta, v. 67, no. 4, p. 751–763.

Rudnick, R.L. & Gao, S., 2003, “Composition of the Continental Crust, “ in Holland, H.D. & Turekian, K.K., eds., Treatise on Geochemistry: Oxford, UK, Elsevier, v. 3, 50 p. 1–64.

Yada, T., Nakamura, T., Takaoka, N., Noguchi, T., Terada, K., Yano, H., Nakazawa, T., and Kojima, H., 2004, The global accretion rate of extraterrestrial materials in the last glacial period estimated from the abundance of micrometeorites in Antarctic glacier ice, Earth, Planets and Space, 56, 67-79.