Biology
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.b13b1088k&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #B13B-1088
Biology
0406 Astrobiology And Extraterrestrial Materials, 0408 Benthic Processes (4804), 0456 Life In Extreme Environments, 5420 Impact Phenomena, Cratering (6022, 8136), 6205 Asteroids
Scientific paper
Introduction: We report the unique occurrence of microfossils composed largely of meteoritic ejecta particles from the late Pliocene (2.5 Ma) Eltanin impact event. These deposits are unique, recording the only known km- sized asteroid impact into a deep-ocean (5 km) basin. First discovered as in Ir anomaly in sediment cores that were collected in 1965, the deposits contain mm-sized shock-melted asteroidal material, unmelted meteorite fragments (named the Eltanin meteorite), and trace impact spherules. Two oceanographic expeditions by the FS Polarstern in 1995 and 2001 explored 80,000 square km of the impact region, mapping the distribution of meteoritic ejecta, disturbance of seafloor sediments by the impact, and collected 20 new cores with impact deposits in the vicinity of the Freeden Seamounts (57.3S, 90.5W). Analyses of sediment cores show that the impact disrupted sediments on the ocean floor, redepositing them as a chaotic jumble of sediment fragments overlain by a sequence of laminated sands, silts and clays deposited from the water column. Overprinted on this is a pulse of meteoritic ejecta, likely transported ballistically, then settled through the water column. At some localities, meteoritic ejecta was as much as 5 to 50 kg per square meter. This is the most meteorite-rich locality known on Earth. Results: Two cores were taken in a basin near the top of the Freeden Seamounts at a water depth of 2.7 km. Sediments in this shallow basin are compositionally different than those at all other sites as they contain abundant calcareous microfossils. In deeper water sites (4 to 5 km depth), higher pressures and CO2 concentrations cause dissolution of calcite and sediments contain siliceous (opal) microfossils or are barren. An exception to this is a few sites in the immediate vicinity of the seamounts that contain calcareous sediments that flowed off the seamounts after being disturbed by the impact. At the top of the seamounts, sediments with meteoritic ejecta are bioturbated by burrowing organisms. Microfossils of some of these organisms are truly remarkable, agglutinated foraminiferans of the genus Hyperammina, perhaps also Rhabdammina. Agglutinated foraminifera, especially such species with very simple tests, are well-known important recolonizers of disturbed areas like the HEBBLE site or, for instance, following the 1991 Mt Pinatubo eruption. These foraminiferans are unicellular, live in the sediment, and construct their shells from mineral grains in the sediment, which they bind together with an organic cement. In the impact deposits they occur as tubes (up to 7 mm x 3 mm), with a central cavity that the foraminifer inhabited. On the Freeden seamounts, the most abundant "mineral" grains in these sediments, other than calcareous microfossils, are fragments of meteoritic ejecta. Point counting of 283 grains (25 to 250 microns in size) examined by microprobe EDS and WDS in one specimen revealed that 70 percent of the particles are meteoritic ejecta, primarily vesicular melt rock with lesser mineral and rock fragments from the Eltanin meteorite. The other 30 percent is terrestrial quartz, feldspars, unclassified grains (probably mostly basaltic glass fragments), and a few lithic grains and microfossils. We are uncertain as to why these microfossils are found only at the top of the seamounts. Perhaps the forams also inhabited the deeper localities which were also devastated by the impact, but the organic cement binding the grains might have been dissolved in more oxic environments.
Gersonde Rainer
Kuhn Genevieve
Kyte Frank T.
Wollenburg J.
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