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Project Progress

The project being reported on here deals with various aspects of asteroid or comet impact on the Earth, and the Astrobiological consequences (including extinction).

During Year 4 our group undertook several field trips for sample acquisition, continued with profitable laboratory research, and began to see an increasing stream of publications. This project is now running smoothly after a long start-up period.

Field trips (Ward): In June of 2004, Ward, accompanied by NAI NRC post doc Geoff Garrison, UW Astrobiology grad student Ken Williford, an undergraduate field assistant from Cal Tech, and SUNY bivalve specialist Chris McRoberts, and Geological Survey of Canada researcher Jim Haggart (an ex-Ward student) spent a week at the Triassic/Jurassic boundary site in the Queen Charlotte Islands, British Columbia. This is the same site that yielded the first recognizable carbon isotope signature across the T/J boundary (Ward et al, 2001). On this trip we sampled both higher and stratigraphically lower than on previous trips, and also sampled extensively for paleomagnetism with oriented cores.

In April of 2005 Ward, Williford, McRoberts, and Garrison returned to the classic Triassic/Jurassic site at Muller Canyon, Nevada, and sampled both higher and lower in the sections, as well as at other nearby sections.

Laboratory analysis, stable carbon isotopes (Ward). During year 4 we continued analyzing samples obtained this and in previous years for both organic and inorganic carbon isotope ratios. The sections analyzed included the Queen Charlotte Islands (T/J), Muller Canyon (T/J), New Zealand (T/J), Italy (T/J), Opal Creek, Alberta (Permian/Triassic), and the Karoo of South Africa (Permian and Triassic). Fossils from several of these sites were also prepared and photographed. Williford performed micro fossil extraction as well.

We obtained highly significant results from several of our T/J sections, all of which show a self similar pattern of isotopes across the Triassic/Jurassic.

Laboratory analysis (Farley): Helium as an Indicator of Extraterrestrial Impacts
Over the last few years helium concentrations and isotopic compositions of sedimentary rocks have been used to detect important solar system events including catastrophic bolide impacts with Earth at several different extinction horizons. One advantage of using ³He as an impact tracer is that elevated levels associated with major solar system events can last for a few million years making detection far easier than the location of a single ejecta layer in a long stratigraphic sequence. In 2003-2004 we completed detailed helium isotopic investigations of two possibly impact-induced extinction boundaries: the Permian-Triassic and the Triassic-Jurassic.

1. Permian-Triassic Boundary

Helium concentration and isotopic composition were measured in a suite of 33 cherts, siltstones and shales across the PT boundary at Opal Creek, Canada (Henderson, 1997). No extraterrestrial ³He was detected, implying that neither fullerene hosted nor IDP hosted He is present at or near the boundary. This observation is consistent with similar studies of other PT sections, but contrasts sharply with reports from a single group of both fullerene and IDP hosted extraterrestrial ³He at some other PT sections. If extraterrestrial ³He is present at the PT boundary, it must be very heterogeneously distributed. As such, we conclude that the ³He-based evidence for impact at PT time is uncompelling.

While no extraterrestrial ³He was detected, there is a sharp increase in nucleogenic ³He very close to or at the PT boundary. This presumably arises from the major lithologic change at this time, from cherts in the Permian to shales and siltstones in the Triassic. Increased nucleogenic ³He is associated with increases in both lithium and organic carbon content into the Triassic. Either the production rate or the retention of this ³He is higher in the shales and silstones than in the cherts. One important conclusion of this new work is that care must be taken to eliminate such artifacts before interpreting changes in ³He concentration in terms of fluctuations in the delivery of ³He from space.
A manuscript has been completed on this work and should be submitted shortly.

2. Triassic-Jurassic Boundary

To assess whether ³He provides any insight to the cause of this extinction we analyzed thirty silty limestones spanning 40 meters centered on the TJ boundary, from Muller Canyon, Nevada (Hallam and Wignall, 2000).

The preliminary results are shown in Figure 1. Just as at the PT boundary, these data provide no support for extraterrestrial ³He at or near the boundary – no evidence for either fullerene-hosted ³He or enhanced interplanetary dust flux. This conclusion is consistent with work undertaken at several other TJ sections in eastern North America (Farley and Olson, unpublished). Further work is required to conclusively establish the origin of the ³He in this section, but it is probably nucleogenic.

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Kring- Shocked minerals

Co-I Kring continued examination for shocked quartz from two of our Triassic/Jurassic target sections: Muller Canyon, Nevada and the Kennecott Point section in the Queen Charlotte Islands. This involved making numerous thin sections and then examining individual grains for shock potential shock lamellae. No evidence of impact was found at either site, which is consistent with the negative results of Farley, and the faunal and isotopic patterns observed by Ward.

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