Eltanin impact



The Eltanin impact is thought to be an asteroid impact in the eastern part of the South Pacific Ocean that occurred around the Pliocene-Pleistocene boundary approximately 2.51 ± 0.07  million years ago. The impact occurred at the north edge of the Bellingshausen Sea 1500 km southwest of Chile, where the sea floor is approximately 4 - 5 km deep. The asteroid was estimated to be about 1 to 4 km in diameter. No crater associated with the impact has been discovered. The impact likely evaporated 150 km3 of water, generating large tsunami waves hundreds of metres high.

Description
The possible impact site was first discovered in 1981 as an iridium anomaly in sediment cores collected by the research vessel Eltanin, after which the site and impactor are named. Later studies were done by the vessel Polarstern. Sediment at the bottom of the 5 km deep ocean in the area had an iridium enrichment, a strong sign of extraterrestrial contamination. Possible debris from the asteroid is spread over an area of 500 sqkm. Sediments from the Eocene and Paleocene were jumbled and deposited again chaotically. Also mixed in were melted and fragmented meteorite matter. The area near the Freeden Seamounts over 20000 km2 has a meteorite material surface density of 10 - 60 kg/m2. Of this, 87% is melted and 13% only fragmented. This area is the region of the Earth's surface with the highest known density of meteorite material coverage.

The disturbed sediment had three layers. The lowermost layer SU IV is a chaotic mixture of crumbled sediments in the form of a breccia. Above this is layer SU III consisting of layered sand, consistent with having been deposited from turbulently flowing water. Above this is SU II layer with meteorite fragments and graded silt and clay that plausibly settled out of still but dirty water.

Asteroid
The supposed impacting body, the Eltanin asteroid, is estimated to have been between 1 and 4 km in diameter and traveling with a speed of 20 km/s. The possible size of the asteroid was calculated by the amount of iridium found in the disturbed sediments. Assuming that there were 187 parts per billion of iridium in the asteroid, the known distribution of the metal leads to estimates that the body was over 1 km in size. Based on a diameter of one km, it is estimated it would have left a crater about 35 km across.

The composition of suspected asteroid remnants has been classified as low metal mesosiderites. The bolide explosion would also have produced microspherules under half a millimeter in diameter. Some of these are glass, and others have spinel and pyroxene. Elements enriched include calcium, aluminium and titanium.

Tsunami
On the shorelines of the Pacific Ocean there are erosional features that are indicative of a very large tsunami. These include an erosional surface and chaotic deposits of mixed terrestrial and ocean-derived sediment. Boulders as big as buses are mixed with marine fossils and mud. The most well-characterised tsunami deposits are near the coast of Chile. Off the coast of Antarctica there are mudslides into the deep ocean from this age.

The size of a possible tsunami has been calculated. An asteroid that was 4 km in diameter falling onto the 5 km deep ocean would have blasted the water off the ocean floor for at least 60 km, and made a wave over 200 m high on the southern end of Chile and the Antarctic Peninsula. After ten hours, waves around 35 m would reach Tasmania, Fiji and Central America, and the New Zealand east coast would have been washed with 60 m high waves. If the impact object was 1 km in diameter, the wave heights would have been one-fifth as great.

Ice age trigger
At the time of the impact in the Late Pliocene, the Earth was cooling. The impact and disruption to the weather could have triggered the start of ice cap formation in the Northern Hemisphere. The impact would have put a large amount of water and salt into the atmosphere, disrupted ice shelves, depleted the ozone layer, caused surface acidification, and increased the Earth's albedo.