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Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis

  1. Wendy S. Wolbachn
  1. aDepartamento de Geología y Mineralogía, Edif. U-4. Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicólas de Hidalgo, C. P. 58060, Morelia, Michoacán, México;
  2. bUS Geological Survey, Menlo Park, CA, 94025;
  3. cFacultad de Biología, Universidad Michoacana de San Nicólas Hidalgo C. P. 58060, Morelia, Michoacán, México;
  4. dDepartment of Geosciences, National Taiwan University, Taipei 106, Taiwan, Republic of China;
  5. eSRI International, Menlo Park, CA 94025;
  6. fGeology Program, School of Earth Science and Environmental Sustainability, Northern Arizona University, Flagstaff AZ 86011;
  7. gWyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 02138;
  8. hLawrence Berkeley National Laboratory, Berkeley, CA 94720;
  9. IGeoScience Consulting, Dewey, AZ 86327;
  10. jDepartment of Earth Science and Marine Science Institute, University of California, Santa Barbara, CA 93106;
  11. kNational Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan;
  12. lCAMCOR High Resolution and MicroAnalytical Facilities, University of Oregon, Eugene, OR 97403;
  13. mMaterials Science Institute, University of Oregon, Eugene, Oregon 97403; and
  14. nDepartment of Chemistry, DePaul University, Chicago, IL 60614
  1. Edited by* Steven M. Stanley, University of Hawaii, Honolulu, HI, and approved January 31, 2012 (received for review July 13, 2011)

  1. Fig. 1.

    (Left). Lake Cuitzeo lithostratigraphy from 4.0 to 2.0?m. Red brackets indicate the carbon-rich layer corresponding to the YD. Blue tick marks at left indicate sample depths. (Right) Graph of calibrated 14C dates. A regression polynomial (black line) of accepted dates (red circles) and tephra date (black dot); blue circles are excluded dates. Error bars are less than circle widths. Dark gray band denotes YD interval; lighter gray band corresponds to interval between 4.0 and 2.0?m. Cal ka BP, calibrated kiloannum before present; char, charcoal.

  2. Fig. 2.

    Graphs for pollen, Cariaco Basin proxies, and GISP2 temperatures. (AC) compare Lake Cuitzeo pollen abundances to two regional lakes. Warmer B?lling–Aller?d (BA) in light gray, and YD in dark gray. D and E show that Lake Cuitzeo Quercus abundances are similar to those of Lake Petén Itzá. All lake plots correspond well to graph F from a Cariaco Basin core displaying ppm abundances of titanium (orange; smoothed 30×) and molybdenum (red; smoothed 3×) (23, 24). Graph G is a GISP2 temperature proxy plot (‰ δ18O; smoothed 10 × ) (22). Black diamonds are depth of 14C dates. All graphs are similar, demonstrating that the YD onset is consistent at all sites.

  3. Fig. 3.

    Changes in carbon for the upper 6?m of the Lake Cuitzeo sequence. There YD onset peaks in TOC wt%, C/P, and δ13C. The dark gray band denotes the YD interval, and the light gray band is the interval between 4.0 and 2.0?m.

  4. Fig. 4.

    Markers over the interval between 3.6 and 2.2?m. The YD episode (12.9 to 11.5?ka) is represented by dark band. YDB layer is at 2.8?m. NDs and magnetic impact spherules both peak at the YD onset, whereas framboidal spherules, CSps, and charcoal peak higher in the sequence. Magnetic grains peak just prior to the YD onset. NDs are in ppb; Msps, framboidal spherules, CSps, and charcoal are in no./kg; magnetic grains in g/kg.

  5. Fig. 5.

    SEM images of magnetic impact spherules. (AB) Magnetic impact spherules with dendritic surface pattern. (C) Framboidal pyrite spherule. (D) Collisional magnetic impact spherules. (E) Light micrograph of same magnetic impact spherules. (F) Teardrop-shaped spherule with dendritic pattern. (G) Photomicrograph of same MSps. For labels such as “2.80 #3,” “2.80” represents depth of sample in meters and “#3” is the magnetic impact spherule number as listed in SI Appendix, Table?4.

  6. Fig. 6.

    Ternary geochemical diagrams: (A) Cuitzeo magnetic impact spherules compared to volcanogenic titanomagnetite and glassy grains, as well as framboidal spherules. Cuitzeo magnetic impact spherules are nonvolcanogenic. Of two framboidal spherules analyzed, one overlaps the magnetic impact spherules and one does not. Neither exhibits quench melting. (B) Cosmic particles compared to Lake Cuitzeo magnetic impact spherules, indicating they are noncosmic. (C) Terrestrial impact materials compared to Lake Cuitzeo magnetic impact spherules, showing close geochemical match.

  7. Fig. 7.

    SAD patterns of NDs from 2.7?m. (A) D-spacings indicative of n-diamonds. (B) D-spacings indicative of i-carbon.

  8. Fig. 8.

    Crystallographic data for NDs from 2.8?m. (A) HRTEM image of monocrystalline nanoparticle identified as lonsdaleite. The (101) and (110) planes are visible with d-spacings of 1.93 and 2.18??), respectively. (B) FFT of same lonsdaleite crystal above. The values adjacent to each spot indicate the reciprocal lattice vector. Image reveals (101) planes with lattice spacing of 1.93??, consistent with lonsdaleite. (C) HRTEM image displaying typical lattice spacing of n-diamond. The 1.78?? measurement represents the (200) planes, consistent with n-diamond. (D) FFT of same n-diamond shown above.

  9. Fig. 9.

    EELS spectra for NDs from 2.8?m. A typical carbon peak of approximately 295–300?eV shows that the particle is carbon. Published n-diamond and i-carbon spectra (dotted lines) are shown for comparison (4, 32, 36, 37).

  10. Fig. 10.

    EFTEM maps of NDs. (A) An inverted “zero-loss” image displaying brighter nanoparticles (numbered). Inset shows 4-nm-wide nanoparticle number 1 that exhibits crystalline lattice spacings of 1.30 and 2.12??, consistent with i-carbon. Resolution is low due to surrounding amorphous carbon. (B) A map of particles detected with characteristic carbon signature at 299?±?5?eV. Brighter particles at numbers 1 through 4 correspond to the Left panel, indicating sp3 bonding typical of NDs. Darker area near number 3 is due to sp2 bonding in the amorphous carbon film.

  11. Fig. 11.

    HRTEM images of twinned NDs from the 2.8?m layer. Double yellow lines represent lattice planes and the numbers indicate d-spacings in ?. Arrows are parallel to common twinning plane. (A) Star-twin ND with fivefold star-like morphology. (B) Accordion twin lonsdaleite with pleated morphology. (C) Twin with multiple folds. (D) “Scalloped” twin.

  12. Fig. 12.

    Carbon onions from 2.75?m. (A) HRTEM image displays 10?nm carbon onion. Parallel strands nearby appear to be carbon ribbons. (B) Drawing illustrates shells of carbon onion and nanoparticle shown in A. (C) FFT of enclosed crystal with d-spacings generally consistent with i-carbon but with insufficient resolution to be definitive.

  13. Fig. P1.

    Images of (A) a nanodiamond, (B) magnetic spherule, and (C) carbon spherule. The graphs below show peak abundances in nanodiamonds (ppb), magnetic spherules (no./kg), carbon spherules (no./kg), and charcoal (no./kg) at or close to onset of the YD (2.8?m; 12,900?BP). Younger Dryas episode marked by gray band and arrows.

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