Research ArticlePLANETARY SCIENCE

An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body

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Science Advances  18 Sep 2019:
Vol. 5, no. 9, eaax4184
DOI: 10.1126/sciadv.aax4184
  • Fig. 1 The mid-Ordovician Hällekis section in southern Sweden.

    The red line represents the stratigraphic level (at −1 m in this study) that corresponds to the time of the breakup of the LCPB in the asteroid belt. At this level, there is a change in the strata in abundance and types of extraterrestrial chrome-spinel grains. A low-abundance, mixed micrometeorite assemblage is replaced by a high-abundance assemblage completely dominated by L-chondritic grains. At the same level, the grain size of bioclastic limestone fragments begins to increase, indicating onset of a gradual sea level fall that culminates with the conspicuous Täljsten lowstand deposit traceable over most of Baltoscandia and likely also globally. Asteroid breakup artwork by Don Davis. (Photo credit: Birger Schmitz, Lund University)

  • Fig. 2 Stratigraphic scheme for the composite Hällekis-Thorsberg section and distribution of equilibrated ordinary chondritic chromite (EC) grains through the section.

    The stratigraphic interval is marked over which >130 fossil meteorites have been found in the Thorsberg Quarry. In 18 samples studied representing 791 kg of rock spanning the interval from −9 to −1 m relative to the base of the Arkeologen bed, we found only 15 EC grains >63 μm, i.e., 2 grains per 100 kg, which is about the same number of grains that we find per kilogram in samples in the overlying ca. 7 m of section. At −1 m, we also see a change in the abundance ratios of H, L, and LL chondritic grains from an evenly mixed assemblage to one completely dominated by L-chondritic grains. The results in the figure build on a total of 1320 kg of limestone dissolved in acids and searched for chrome spinels (see also fig. S1). TS, Täljsten lowstand deposit.

  • Fig. 3 The 21Ne CRE ages (T21) of fossil L chondrites from the Thorsberg Quarry inversely correlate with sediment ages.

    Solid symbols are ages with cosmogenic 21Ne production rates from Heck et al. (20), and open symbols are ages with production rates determined by Heck et al. (21) (see Supplementary Text). The interval between linear regressions on the lower and upper limits, respectively, of the two data sets is shaded orange and sets the LCPB breakup at T21 = 0 Ma between −0.4 and –1.2 m relative to the base of the Arkeologen bed.

  • Fig. 4 The lower part of the Hällekis section with plots of bulk-rock concentrations of equilibrated ordinary chondritic chromite (EC) grains, 3He and Al2O3, and 187Os/188Os ratios.

    In the far-right column, skeletal grain abundance according to (41, 42) is shown. The chromite and He and Os isotopes indicate a sudden increase in extraterrestrial material in the sediment at −1 m, whereas the Al2O3 and skeletal grain abundances illustrate the change to a more clean and coarse-grained limestone that can be used for production of industrial limestone slabs. The coarsening of the sediment reflects stronger hydrodynamic forcing with shallowing, leading to winnowing of the fine fraction.

  • Fig. 5 A chromite grain in an Antarctic micrometeorite.

    Back-scattered electron image of a porphyritic olivine spherule with large (>63 μm) chromite relict grain (light gray). This grain is not included in the present study but is shown here to illustrate the distribution of relict grains in cosmic spherules, often found near the particle edge. This relict chromite grain is angular, with limited indication of melting during atmospheric passage.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/9/eaax4184/DC1

    Supplementary Text

    Fig. S1. Distribution of equilibrated ordinary chondritic chromite (EC) grains through the Hällekis-Thorsberg section.

    Fig. S2. High-resolution 187Os/188Os isotope profile across a proposed discontinuity surface.

    Fig. S3. The gray Täljsten in the Degerhamn Quarry, southern Öland.

    Fig. S4. Cystoid echinoderms in the Likhall bed of the Täljsten.

    Fig. S5. Distribution of extraterrestrial chromite across the Lynna River section.

    Fig. S6. Map of Antarctic micrometeorite localities.

    Fig. S7. Back-scattered electron images of Antarctic micrometeorites.

    Fig. S8. Size distribution of Antarctic micrometeorites.

    Table S1. Chrome-spinel distribution through the Hällekis-Thorsberg section.

    Table S2. Extraterrestrial chromite division below reference level in Hällekis section.

    Table S3. Extraterrestrial chromite division above reference level in Thorsberg section.

    Table S4. Published abundances of micrometeorite types from different collections.

    Table S5. Poynting-Robertson transfer times (Ma) from the outer solar system to Earth.

    Data file S1. Hällekis-Thorsberg section—chrome-spinel chemical results.

    Data file S2. Helium isotope data.

    Data file S3. Osmium isotope data.

    Data file S4. Spinels in Antarctic micrometeorite.

    Data file S5. Lynna River section—chrome-spinel chemical results.

    References (6088)

  • Supplementary Materials

    The PDF file includes:

    • Supplementary Text
    • Fig. S1. Distribution of equilibrated ordinary chondritic chromite (EC) grains through the Hällekis-Thorsberg section.
    • Fig. S2. High-resolution 187Os/188Os isotope profile across a proposed discontinuity surface.
    • Fig. S3. The gray Täljsten in the Degerhamn Quarry, southern Öland.
    • Fig. S4. Cystoid echinoderms in the Likhall bed of the Täljsten.
    • Fig. S5. Distribution of extraterrestrial chromite across the Lynna River section.
    • Fig. S6. Map of Antarctic micrometeorite localities.
    • Fig. S7. Back-scattered electron images of Antarctic micrometeorites.
    • Fig. S8. Size distribution of Antarctic micrometeorites.
    • Table S1. Chrome-spinel distribution through the Hällekis-Thorsberg section.
    • Table S2. Extraterrestrial chromite division below reference level in Hällekis section.
    • Table S3. Extraterrestrial chromite division above reference level in Thorsberg section.
    • Table S4. Published abundances of micrometeorite types from different collections.
    • Table S5. Poynting-Robertson transfer times (Ma) from the outer solar system to Earth.
    • References (6088)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Data file S1 (.pdf format). Hällekis-Thorsberg section—chrome-spinel chemical results.
    • Data file S2 (Microsoft Excel format). Helium isotope data.
    • Data file S3 (Microsoft Excel format). Osmium isotope data.
    • Data file S4 (Microsoft Excel format). Spinels in Antarctic micrometeorite.
    • Data file S5 (.pdf format). Lynna River section—chrome-spinel chemical results.

    Files in this Data Supplement:

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