In the world’s only systematic search for fossil meteorites that fell on an ancient sea floor we have recovered more than 100 fossil meteorites, representing almost all such meteorites known to science. The meteorites have been recovered in the Thorsberg quarry in southern Sweden, where the so called Orthoceratite Limestone is quarried for the production mainly of floor tiles. The project is pursued together with Mario Tassinari and the owners of the quarry, Gustav, Göran, Sören and Stig Thor. Since 1993 we have searched a circa five-meter-thick limestone succession distributed over circa 40,000 square meters for meteorites that were embedded in the limestone when it formed ca. 466 Myr ago. The limestone accumulated slowly on the sea floor, 2-4 mm of sediment per thousand years, thus we know that our >100 meteorites fell over the quarried area during 1-2 Myr. The systematic quarrying of these limestone beds provide a near ideal setting for a detailed reconstruction of the distribution of meteorites on an ancient sea floor.
Our studies of the fossil meteorites have shown that they represent the signature of the perhaps most dramatic event in the solar system the past 500 Myr. There are more than 50,000 meteorites documented to have fallen on Earth in recent times. Most of these meteorites have been found in Sahara or on Antarctica. Almost a quarter of the recent meteorites, the so-called L chondrites, originate from the largest asteroid breakup event that we know of from the past three billion years. The L chondrites represent one of the two most common types of stony meteorites that fall on Earth today. By measurements of the argon isotopic composition of the recent L chondrites we know that they all were liberated as small bodies when the 100-200 km large parent body of the L chondrites broke up in a major collision ca. 470 Myr ago. All except one of the >100 fossil meteorites that we have found in the Ordovician sediments are L chondrites. We also see from the high abundance of fossil L chondrites on the ancient sea floor that the flux of meteorites to Earth was enhanced by a factor of circa 100. With all likelihood the fossil meteorites that we have found represent the meteorites that headed to Earth shortly after the major collisional event in the asteroid belt ca. 470 Myr ago.
One of our fossil meteorites has a composition that does not resemble any of the 50,000 documented meteorites that have fallen on Earth in recent times. This meteorite, named Österplana 065, likely represents a fragment of the smaller body that hit and broke up the large L-chondrite parent body.