The interval has been studied in particular detail in Baltoscandia where many well-preserved sedimentary sections are exposed. Here, we focus on an interval of the geological record that has been proposed to represent the onset of the Ordovician ice age and where important faunal turnovers occurred worldwide ( 4, 8). Although much of Earth’s short-term climate variability is astronomically paced, as expressed by the Milankovitch cycles, the fluctuations on a 10- to 100-Ma scale between greenhouse and icehouse climates are generally explained in terms of Earth-bound causes, such as the closing or opening of seaways, uplift of mountain chains, or changes in atmospheric CO 2 concentrations ( 1). The oldest major Phanerozoic ice age peaked in the Late Ordovician, as indicated by glacial deposits in, e.g., North and South Africa and South America ( 2, 3), but sea level records indicate that ice age conditions may have started already in the mid-Ordovician ( 4– 7). The preceding major ice age lasted from the Late Devonian to the mid-Permian, leaving behind extensive glacial deposits over ancient Gondwanaland. We live in the latest ice age that began in the Late Eocene, ~35 Ma ago, after more than 230 Ma of ice-free high-latitude continental regions. Lynna River section-chrome-spinel chemical results.ĭuring the past 500 million years (Ma), Earth has experienced three major ice ages ( 1). Spinels in Antarctic micrometeorite.ĭata file S5. Hällekis-Thorsberg section-chrome-spinel chemical results.ĭata file S4. Poynting-Robertson transfer times (Ma) from the outer solar system to Earth.ĭata file S1. Published abundances of micrometeorite types from different collections. Extraterrestrial chromite division above reference level in Thorsberg section. Extraterrestrial chromite division below reference level in Hällekis section. Chrome-spinel distribution through the Hällekis-Thorsberg section. Size distribution of Antarctic micrometeorites. Back-scattered electron images of Antarctic micrometeorites.įig. Map of Antarctic micrometeorite localities.įig. Distribution of extraterrestrial chromite across the Lynna River section.įig. Cystoid echinoderms in the Likhall bed of the Täljsten.įig. The gray Täljsten in the Degerhamn Quarry, southern Öland.įig.

High-resolution 187Os/ 188Os isotope profile across a proposed discontinuity surface.įig. Distribution of equilibrated ordinary chondritic chromite (EC) grains through the Hällekis-Thorsberg section.įig.

Supplementary material for this article is available at įig.

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