Meteorite impacts have long been one of my favorite subjects. Years ago, myself and a former colleague (who recently passed away) determined the age of the Marquez impact structure in Texas from analyses of mineral samples from the impact site. It was previously believed simply to be a salt dome. Alas, the age was 58 Ma, not the 66 Ma (in)famous impact killer of the dinosaurs! This edition of Core Elements shares some interesting studies of other impact events.
Rasoul Sorkhabi
Editor, Core Elements
News from the End-Cretaceous Extinction
Peter Zajda/ Shutterstock.com
The Cretaceous-Paleogene (K-Pg) boundary impact killed off about 75 percent of species on land and in the seas. The impact crater at Chicxulub in southern Mexico, now buried under 2 km of sediments, is widely considered to have been responsible for the 66 Ma extinction. Two new studies elaborate on this idea.
End-Cretaceous impact winter: Vajda and colleagues, published in Global and Planetary Change, analyzed spherules from the K-Pg boundary layer at Gorgona Island in offshore Colombia. Their nano-XRF analyses revealed some interesting geochemical results:
The samples showed a high concentration of platinum group elements, (PGEs) including platinum, iridium, osmium, and ruthenium that originated from the vaporized meteorite.
The PGEs occur in cubed or needle-shape nanocrystals, co-localizing with metallic elements such as cobalt, nickel, copper, and zinc.
The nanocrystals are embedded in a matrix of iron-rich calcium and silica glass, indicating the mix of vaporized meteorite and target rock.
Iridium from the meteorite that is found in K-Pg boundaries globally was carried by other metallic components in the form of new minerals.
The researchers also suggest that nano-shards of unreactive elements, such as platinum and iridium, acted as nuclei for aerosol formation in the atmosphere. They believe these may have contributed to a prolonged, impact-caused cold and dark winter across the globe, which was detrimental to photosynthesis and many life-forms.
Meteorite makeup: Meteorites may be chondrite (stony), metallic, or stony-metallic. About 85 percent of meteorites are chondrites, containing once molten silicate grains (chondrules) that accreted and formed this type of asteroids in the early Solar System.
The ancestry of the Chicxulub meteorite: Mario Fischer-Gödde and colleagues, published in Sciencein summer 2024, reported ruthenium isotope data from spherule samples collected at the Chicxulub impact site, as well as those from five Phanerozoic sites (ranging in age from 470 to 36 Ma) and three Archean impact sites (3.5 to 3.2 billion years).
Their results indicate that:
The Chicxulub meteorite was a carbonaceous chondrite belonging to the C-type asteroids at the outer edge of the asteroid belt.
The five other Phanerozoic meteorites were iron meteorites coming from siliceous-type parent asteroids.
The Archean meteorites come from carbonaceous-type asteroids.
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Eight impact events (termed S1 through S8) are recorded in the Barberton Greenstone Belt of Kaapvaal Craton in South Africa. They date from 3.6–3.2 billion years ago.
These impact events are identified by millimeter-scale spherules, iridium anomalies, and extraterrestrial chromium isotope signatures.
The S2 meteorite was a carbonaceous chondrite—measuring 37–58 km—which hit Earth about 3.26 billion years ago. This is evidenced by the volcanic tuff of the same age immediately above the impact structure.
Researchers studied the petrography and carbon stable geochemistry of sedimentary rocks across the S2 impact event.
Major findings:
The impact created a giant tsunami that mixed iron-rich bottom waters with iron-poor shallow waters and deposited massive debris along coastal areas.
Impact heating locally evaporated ocean waters and induced short-term increases in weathering and erosion on land.
The vaporization of the meteor in the atmosphere before hitting Earth would spread large amount of phosphorus into the atmosphere.
The tsunami, atmospheric heating, and cloud darkness locally destroyed large populations of phototrophic microbes in shallow waters.
Marine strata immediately above the S2 spherule bed contain abundant siderite (iron carbonate) minerals associated with organic matter and light delta carbon-13 values. These indicate massive activity of iron-cycling microbes soon after the impact.
Why it matters: This study shed light on the specific influences of an Archean impact on Earth’s early life and surface environment—a little-investigated topic.
Similar studies of other Archean impacts will reveal much more information about ecological and biotic history of early Earth.
Last week’s quiz was: What and when was the Earth’s earliest known glacial period? How did it start and end?
Reader Pete Webb has responded that the Huronia glaciation (named after glacial deposits near Lake Huron in Canada) between 2.5 and 2.2 million years ago was the earliest on record. I should add that this early Proterozoic glaciation was caused by the Great Oxygenation of the atmosphere as photosynthetic cyanobacteria spread globally.
Some geologists believe that there was an even earlier Pongola glaciation some 2.9 billion years ago based on glacial deposits from Eswatini in southern Africa. Thanks for writing in, Pete!
Now for this week’s quiz: The K-Pg extinction was one of the “Big Five” extinctions during the Phanerozoic (the period covering the past 540 million years). What is the Earth’s earliest known mass extinction? (Think of the Precambrian.)
Please send your response by January 16 to editorial@aapg.org (subject line: Core Elements Quiz).
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