http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Ocean splashdown by Henry Gee An asteroid between one and four km in diameter that splashed into the Southern Ocean, 1500 km SW of Chile, just over two million years ago, may have worsened a period of global cooling that saw the emergence of modern humans. The asteroid impact explains several other puzzles, too, as Rainer Gersonde of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven and colleagues discuss in the 27 November 1997 /Nature/. The impact in question was first discovered during a cruise of the /Eltanin/ in the 1960s: betrayed by anomalously high amounts of iridium in ocean-bed cores. The Eltanin impact is the only example of the 140 known on Earth to have occurred in the deep ocean. Given that 70% of the Earth's surface is underwater, and most of that is ocean, we would expect most incoming asteroids and comets to meet watery graves. Statistics suggests that around 500 bodies the size of the Eltanin asteroid hit the oceans in the past 540 m.y.: almost one every million years, on average. This is why the Eltanin impact site is so valuable, and why Gersonde and his colleagues have taken another look, their results coming from a cruise in 1995 by the research ship /Polarstern/. The impact left a distinctive 'signature' of geological layers, very like that of the Chicxulub impact. Lowest in the 'impact' sequence is a thick layer of disordered rubble, full of chunks of rock up to 50 cm across: this layer represents the large-scale disturbance immediately after the impact as the ten-megaton blast ripped up the ocean floor. This layer took around four hours to settle after the blast. Smaller particles, such as grains of sand, took longer to settle, explaining why this layer was found immediately above the rubble layer. Capping the whole sequence is a thin layer of very fine sediment, dispersed over a wide area. This would have contained fine-grained material (including vaporized asteroid) flung high into the air and which took days or months to settle out. This layer contained the iridium. The Eltanin asteroid was much smaller, less than half the diameter of the asteroid of the Chicxulub impact. There is no trace of a submarine Eltanin crater; indeed, most of the asteroid probably vaporized before it could leave much of a scar on the ocean floor. Nevertheless, Eltanin was still an impact to be reckoned with, being just large enough to disturb the global ecosystem. The impact would have blasted sediment high into the atmosphere, explaining why fossils of microscopic deep-sea diatoms made their way to the mountainous interior of Antarctica. It would also have sent terrifyingly high tsunamis to broach on coasts all over the South Pacific and Southern Oceans. These would have washed hundreds of metres inland, explaining the strange jumble of terrestrial and marine animals from contemporary deposits in Peru. The impact would also have sent enormous quantities of water and dust into the atmosphere, which would have influenced the climate. Although polar ice-sheets had become well-established by that time, the Earth was yet to settle into the pronounced cycle of continental glaciation that has characterized the past 1-2 million years. At the time of the Eltanin impact, /Homo erectus/ was emerging in Africa, and beginning to spread into cooler Eurasia. It is tempting to speculate that an impact in the South Pacific could have been a stimulus for humanity's first great migration out of Africa. ©Macmillan Magazines Ltd 1997 - NATURE NEWS SERVICE Note: This item from the Nature News Service is mounted on this Web page by special permission of /Nature/ while the Nature site is being revised. [Return to Chapter 18 ] [Return to UC Davis Geology Department Home Page]