Bostwick J.A. et al. (1995) Asteroid sample return mission II: Eltanin recovered (abs. Meteoritics 30, 490 ):

The Late Pliocene impact of the Eltanin asteroid produced a truly unique deposit in the known sedimentary record. It is the only known impact into a deep-ocean basin. Evidence of this impact was first discovered as an Ir anomaly in sediments from core El 3-3 recovered by the USNS Eltanin in 1964. Further examination of these sediments revealed that Ir was largely contained (ca. 50%) within mm-sized grains of vesicular impact melt and that several % of the impact debris consists of unmelted meteorite fragments. The unmelted fragments are from a meteoritic basalt similar to howardites and mesosiderites and have been named the Eltanin meteorite.

Trace components of the impact debris include rare metal grains and glassy spherules containing megnesioferrite spinel. The spherules may be impact vapor condensates. Thus the impact debris contains materials derived from the unaltered projectile, an impact melt (a simple mixture of projectile and salts from the seawater target), and an impact vapor cloud. Assuming that the impact site was near core El 3-4 (57° 47.2' S and 90° 47.6' W), the site with the most debris, a conservative estimate of the minimum asteroid diameter was about 500 m, although it could have been as large as 2 km.

Collision With Earth: An Impact on the Weather

Frank Kyte, who discovered the impact of the Eltanin asteroid in 1981 when he was a graduate student, now believes that the planetoid may have been large enough to have devastated Earth's climate.

Writing in the November 27, 1997, issue of Nature, Kyte and his 12
co-authors report that additional ocean-bottom sampling completed recently suggests that instead of being an estimated 0.5 kilometer in diameter, the asteroid was at least 1 km and possibly as large as 4 km across. That's big enough to have caused global "devastating megatsunamis," or 120-foot-tall tidal waves, after the object struck the Southern Ocean about 2.15 million years ago. "This was right before a significant cooling event, " said Kyte, a geochemist with UCLA's Instituteof Geophysical and Planetary Physics. "Whether this impact was just a coincidence we can't say,but no one has looked."

There are about 140 known terrestrial impacts of asteroids, but 60 percent of Earth is covered by oceans. Water impacts would leave little evidence behind other than pulverized pieces of the asteroid buried under more recent sediments, terrestrial evidence of ancient tsunamis, or other subtle clues.

In the mid-1960s, the crew of the USNS Eltanin, an American research
ship looking for evidence of ancient glacier activity, punched one of many 20-meter "piston cores" into the floor of the Southern Ocean about 1,400 km (900 miles) west of the southern tip of South America. The ocean bottom often is deeply covered with recent sediment but, fortunately, the ship passed above an area with little sediment when the fateful core sample was taken. The sample extended deep enough into the sea bottom to pierce a layer of asteroid debris. However, that core sample sat undisturbed on a shelf until Kyte examined it and discovered that it contained a region rich in iridium, a signature element of asteroids.

"I just got lucky to find this one," said Kyte. "The labs of sedimentologists and paleontologists all over the world have processed sediments from deep-sea cores, and I wonder how many (asteroid) impacts are sitting on the back of people's shelves."