Dust from the breakup of a 150-kilometer- (93 mile) diameter asteroid may have caused — or at least intensified — an ice age half a billion years ago, providing the impetus for a sweeping array of aquatic animal adaptations that shaped today's spectacularly diverse ocean ecosystems, according to a new study published in the September 20 issue of Science Advances.
The authors uncovered extraterrestrial material in sediments that correlate the timing of asteroid breakup with a major dip in sea level frequently attributed to the onset of the Mid-Ordovician ice age. Their findings suggest that asteroid dust may have settled in Earth's atmosphere, shading the planet from the sun's radiation and cooling global temperatures.
While extraterrestrial dust only accounts for about one percent of the modern atmosphere and does not impact the climate, large quantities of dust lingering for several hundred thousand years or more would be expected to cause global cooling.
"This is the first time anyone has shown that asteroid breakups and asteroid dust can lead to ice ages," said Birger Schmitz, a professor of geology at Lund University in Sweden and the first author of the study. "This is also the first time since the discovery of the asteroid impact that killed the dinosaurs that an important event in the history of life has been tied to an astronomical event."
The findings suggest a chance collision in the far reaches of the asteroid belt launched an ecological saga that changed life on Earth forever. The asteroid breakup flooded the inner solar system with fine-grained dust, increasing the quantity of dust in the atmosphere by three to four orders of magnitude, triggering a climatic shift.
"The blocking effect of the dust partly stopped sunlight from reaching the Earth and a peculiar ice age began," said Schmitz. "The climate changed from being more or less homogeneous to becoming divided into climate zones — from Arctic conditions at the poles to tropical conditions at the equator. The high diversity among invertebrates came as an adaptation to the new climate, triggered by the exploded asteroid."
The Cambrian Explosion about 541 million years ago may have given rise to many modern marine taxonomic groups, but it was during a 30-million-year evolutionary frenzy called The Great Ordovician Biodiversification Event that these groups filled out their membership. The number of taxonomic families tripled, shepherding in an otherworldly assortment of colorful corals, brainy mollusks, abundant gastropods, and reef-building brachiopods. As biodiversity blossomed, the dynamics of life beneath the waves grew complex, with food webs becoming ever more intricate and ecosystems differentiating from one region to the next.
Scientists have struggled to identify the root of this ecological boom. While there is accumulating evidence for an increase in asteroid impacts during the time of the boom, there has been a lack of data pinpointing the timing of asteroid breakup in relation to changes in climate and species diversity on Earth.
To resolve whether dust from the asteroid breakup directly affected Earth's climate and biodiversity, Schmitz and his team analyzed neon, helium, and osmium isotopes on extraterrestrial chrome-spinel mineral grains in marine limestone at Kinnekulle in southern Sweden and in the Lynna River near St. Petersburg in Russia. Neon and helium isotopes flowed from the sun and became embedded in the chromite grains as they traveled through outer space, indicating these minerals come from meteorites, while plunging osmium isotope concentrations reflect higher levels of extraterrestrial material in the sediment.
Fossils found in the rock layers provided dates for the dust, by comparing them with ages for different biozones characterized in the official geological time scale dating system. The researchers then compared their findings with previous noble gas data for chromite grains from stony meteorites, finding consistencies suggesting an extraterrestrial source for the grains in the mid-Ordovician limestone.
"The challenging part was to locate the precise level in the geological record that corresponds to the breakup because different size fractions arrive at Earth at different times," said Schmitz. The fine dust arrives at Earth shortly after a breakup and it picks up helium-3 from the solar wind on its way to Earth. With the distribution of the helium-3 isotope in the strata of petrified sea floor sediment we could locate the precise level that corresponds to the breakup."
Schmitz and colleagues found that the first signal of the asteroid breakup is recorded in limestone one meter (about 3.3. feet) below the base of the Arkeologen bed at the Hällekis quarry in Sweden, where they first began to see a gradual onset in the number of extraterrestrial chromite grains. They said that this sudden increase in space matter cannot be dismissed as a gap in the geological record — rather, it marks the first dust to reach Earth from the demolished asteroid. The team's new helium-3 data for Ordovician sediments shows that the breakup took place just at the onset of a major sea level fall previously attributed to an Ordovician ice age.
The team plans to continue combing the sedimentary record for clues that link Earth's biological history with the seemingly remote happenings beyond its atmosphere.
"We are now working on [sedimentary] sections at other places in the world, focusing on a section in the Hubei province of China," said Schmitz. "The obvious continuation of this study would be to see if other major ice ages in Earth's history were also triggered by asteroid dust."
[Credit for associated photo: Fredrik Terfelt]