IMPACT-AFFECTED MARINE MICROFOSSILS FROM THE GULF OF CARPENTARIA, AUSTRALIA

We report here additional evidence supporting 1-2 ca. 1450 yrs. BP impacts in the Gulf of Carpentaria, Australia [1, 2, 3, 4]. Impact effects on marine microfossils have been noted in numerous events, such as the Chesapeake Bay, Eltanin, Ewing, and Alamo impacts [5, 6, 7, 8]. Impact-damaged microfossils may be useful in identifying target formations in marine impacts, as well as investigating impact-related processes such as microspherule and tektite formation.

We selected samples primarily from core GC04 (9.83°S, 135.35°E). We measured the magnetic susceptibility of the core, sieved samples into >150 μm, >63 μm, and >38 μm fractions, and examined the >150 μm samples under a binocular microscope. Samples of obvious microfossils were picked based on a melted appearance, and analyzed with EDAX as CaCO₃. We found impact-related materials to 60 cm down, contrary to our calculations of a millimeters-thick ejecta layer. We examined the grains under an SEM for characteristic textures. A few samples were teardrop-shaped or elliptical, transparent, and vitreous in visible light, but analyzed as CaCO₃. Splashes of native Fe and TiO₂ were observed, some with a stacked appearance. Grains of rutile, melted apatite and ilmenite, zircon, SiO₂, tin-rich BaSO₄, and FeS were also found on various samples. Three microfossils exhibited quench textures. We identified one of these microfossils as G. ruber [10]; SEM imaging revealed melted on coccoliths, Fe splashes, and grooves. We also noted CaO in a separate sample, a breakdown product of CaCO₃.

In addition to melted microfossils in GC04, we found SiO₂ with triangular fractures, a candidate for shocked quartz. Microspherules were not abundant in the core, supporting our hypothesis of two low-angle impacts, which produce more distal ejecta [9]. It is our opinion that melted microfossils in the Gulf of Carpentaria are best explained as formed in impact-related processes.

1. Shulmeister & Lees, 1992. Geomo. (5): 521-534 2. Martos et al., 2006. GSA Abs. (38, #7): 299 3. Abbott et al., 2006. GSA Abs. (38, #7): 299 4. Elkinton et al., 2006. GSA Abs. (38, #7): 299 5. Nunes et al., 2002 GSA Abs. (34): 543 6. Edwards & Powars, 2003. Palaios (18): 275-285 7. Morrow et al., 2005. GSA S.P. 384 8. Kyte et al., 2006. AGU Fall Meet., Abs. B13B-1088 9. Tester et al., 2007 (this vol.) 10. Parker, 1962. Micropal. (8, #2): 219-254