Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmos22c0287k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #OS22C-0287
Mathematics
Logic
1065 Trace Elements (3670), 5420 Impact Phenomena (Includes Cratering), 6205 Asteroids And Meteoroids, 6215 Extraterrestrial Materials
Scientific paper
The late Pliocene impact of the Eltanin asteroid is the only known asteroid impact in a deep-ocean (~5 km) basin . This was first discovered in 1981 as an Ir anomaly in sediment cores collected by the USNS Eltanin in 1965. In 1995, Polarstern expedition ANT XII/4 made the first geological survey of the suspected impact region. Three sediment cores sampled around the San Martin seamounts (~57.5S, 91W) contained well-preserved impact deposits that include disturbed ocean sediments and meteoritic impact ejecta. The latter is composed of shock-melted asteroidal materials and unmelted meteorites. In 2001, the FS Polarstern returned to the impact area during expedition ANT XVIII/5a. At least 16 cores were recovered that contain ejecta deposits. These cores and geophysical data from the expedition can be used to map the effects of the impact over a region of about 80,000 square km. One of the most remarkable characteristics of this impact deposit is the high concentrations of melted and unmelted meteoritic material distributed across a large area of ocean floor. We estimate the total amount of meteoritic materials by measuring bulk Ir concentrations in the sediment (using 187 ng/g as the concentration of Ir in the meltrock and the precursor asteroid), and by measuring the amount of meteoritic ejecta in the coarse fractions of the sediment. Such measurements of the original USNS Eltanin cores provided a 500 m estimate for the minimum asteroid diameter. Using data from the 1995 Polarstern expedition, Gersonde et al. (1997) found this was too conservative and recommended a minimum diameter of 1 km. With the much greater areal coverage and large number of sediment cores from the recent expedition, we will be able to greatly refine the model for ejecta distribution. To date we have measured Ir concentrations in sediments from seven of the new cores and preliminary data should be available for a few more by the time of the meeting. Our initial interpretation of these data is that there is a region in the vicinity of the San Martin Seamounts comprising at least 20,000 square km in which the average amount of meteoritic material deposited was more than 1 g per square cm. This alone is enough material to support a 500 m astteroid. Beyond this is a region of about 60,000 square km, mostly to the north and west, where the amount of ejecta probably averages about 0.2 g per square cm. Another 400 km to the east, USNS Eltanin core E10-2 has about 0.05 g per square cm, so we know that ejecta probably occurs across more than a million square km of ocean floor. A key to future exploration of this impact is to find evidence of the ejecta at more sites distant from the seamounts. We currently have almost no data from regions to the west or south of the San Martin seamounts. At least one site (PS58-281, just north of the seamounts) has more than 8 g of meteoritic ejecta per square cm. Near the top of 8.5 m of impact deposit in this core, this ejecta is interbedded with disturbed sediments in a graded unit more than 50 cm thick. One 2.5 cm interval has 150 ng/g Ir, or about 80% meteoritic ejecta, by weight. One 2.5 cm meteorite has been recovered and 10 g of meteoritic ejecta (0.5 to 8 mm) has been examined. Of this, 87% is asteroidal meltrock and 13% is meteorite fragments. Unmelted meteorites are most concentrated in the lower part of the ejecta-bearing sediments, consistent with their greater density, and thus more rapid settling time through the 5 km water column, relative to the vesicular meltrock.
Gersonde Rainer
Kuhn Genevieve
Kyte Frank T.
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