B33C-0268 1340h
Osmium-isotope Evidence for a Projectile Component in Impact-melt Rocks, Chesapeake Bay Impact Structure, Virginia, USA
The late Eocene Chesapeake Bay impact structure (CBIS) is preserved beneath post-impact sediments on the Atlantic margin of Virginia. This 85-km-diameter complex crater formed on the continental shelf of a passive margin in a layered target consisting of ocean water, Cretaceous and Tertiary sediments (mainly siliciclastic), and crystalline basement rocks. The basement rocks include Neoproterozoic granitoids and felsite as well as gneiss of undetermined age. In May, 2004, the USGS drilled an 823-m test hole in the central uplift of the CBIS at Cape Charles, Va., providing drill cuttings and limited core. The core from 744 to 823 m depth contains crystalline-clast breccia and brecciated gneiss that are distinct from sediment-clast breccias recovered from coreholes in the annular trough of the CBIS. Rocks interpreted to be impact-melt clasts and dikes in the crystalline-clast breccia were sampled for analyses of osmium (Os) concentrations and $^{187}$Os/$^{188}$Os ratios to test for evidence of the projectile. These analyses were conducted on samples from a dike (aphanitic to partly hyaline, ST2440.8C) within a gneissic block, from a block of holocrystalline mafic rock (aphanitic, ST2453.3C), and from a flow-laminated bomb (aphanitic to partly hyaline, ST2570.0C). The Os concentrations and $^{187}$Os/$^{188}$Os ratios for samples ST2440.8C, ST2453.3C and ST2570.0C are 0.928, 0.711 and 0.312 ppb, and 0.15205, 0.15545 and 0.22345, respectively. These values are much higher (Os) or lower ($^{187}$Os/$^{188}$Os) than those reported for rocks of the upper continental crust, suggesting a significant contribution of osmium from the projectile in these impact-melt rocks. Moreover, a strong negative correlation between $^{187}$Os/$^{188}$Os and Os for these samples suggests that it may be possible to use mixing curves to calculate the proportions of projectile and target-rock components. Our results from the CBIS contrast with those from the Chicxulub crater, where there is little or no evidence for the incorporation of meteoritic material in samples of impact melt. The CBIS and the Popigai impact structure in Siberia have similar 35-36 Ma ages, leading to suggestions that they were produced by projectiles of similar composition (comet or asteroid?) from a late Eocene meteor shower. The CBIS is also proposed, based on age and location, to be the source of tektites in the North American strewn field. Analyses of additional impact melts and target rocks from the CBIS, and comparisons with data from Popigai melt rocks and North American tektites, could test these hypotheses.
B33C-0269 1340h
Bedout basement rise, offshore northwestern Australia: evidence of an unshocked mafic volcanic hyaloclastite volcanic breccia
Core samples from Bedout-1 (3035.8-3044.95 m.), Bedout basement rise, offshore northwestern Australia, were examined by optical microscopy, SEM, EDS and WDS spectrometry. At this stratigraphic depth level Becker et al. (2004) interpret cryptocrystalline alteration zones around and within plagioclase in terms of shock-induced transformation of feldspar into diaplectic maskelynite glass _u postulating a ~200 km-large impact structure and thereby an impact connection of the Permian-Triassic boundary mass extinction. However, the breccia is dominated by fragments of microlitic basalt and ophitic-textured dolerite with well preserved igneous textures, showing no evidence of shock metamorphism. Euhedral pseudomorphs of chlorite and amphibole, probably after pyroxene, protrude into or are enveloped by euhedral albite-twinned calcic plagioclase (andesine to bytownite). Minor phases include euhedral ilmenite needles and subhedral magnetite grains. Plagioclase is altered by cryptocrystalline albite and microcrystalline albite-chlorite matrix along crystal boundaries, along twin lamella and within internal oscillatory crystal zones, consistent with burial metamorphosed hydrovolcanic basalts and spilites (e.g. Amstutz, 1974). The volcanic fragments are set within, and injected by, microcrystalline intergranular mesostasis of mixed mineral fragments and volcanic meta-glass. Becker et al. (2004) refer to the breccia in part as product of Mg-rich sediments (e.g. dolomites). However, apart from the pristine igneous textures of the breccia, the transition element levels (chlorite in dolerite fragment "C Ni 97-160 ppm; Co 75-152 ppm; Cu 69-204 ppm; mesostasis "C Ni 29-45 ppm; Co 18-52 ppm; Cu 26-110 ppm) are consistent with Fe-rich basalts but exceed common abundances in carbonates and marls (BVTP, 1981; Wedepohl, 1978). No shock metamorphic features, such as planar deformation features (PDF), are observed in the feldspar or in any other phases. No criteria for discriminating maskelynite and volcanic meta-glass are indicated by Becker et al. (2004). As PDF formation (10-35 GPa) necessarily precedes diaplectic transformation into maskelynite (35-45 GPa) (French, 1998), a presence of maskelynite is inconsistent with the absence of PDF in the plagioclase _u a phase prone to the development of shock effects (e.g. Mory, 2000). Little evidence exists for the hydrothermal activity which typically follows impact events. However, thanks to a partial development of a rim syncline-like structure in Triassic sediments around the Bedout rise, further testing of the origin of this remarkable structure is warranted. Amstutz, G.C., 1974, Spilites and Spilitic Rocks, Springer-Verlag, Berlin; Becker, L. et al., 2004, Science Express, 13.5.04; BVSP - Basaltic Volcanism Study Project, 1981, Pergamon; French, B.M., 1998, Traces of Catastrophe, Lunar and Planetary Contributions 954; Mory et al., 2000, Earth and Planetary Science, 177, 119-128; Wedepohl, K.H., Handbook of Geochemistry, Springer-Verlag, 1978.
B33C-0270 1340h
How Big is Bedout?
Based on analyses of recovered core material, the Bedout High, located on the Northwest Shelf of Australia, has been proposed as the central uplift of a large, buried complex impact crater of end-Permian age. A major factor in assessing its possible contribution to the Permian-Triassic mass extinction and other end-Permian events is its estimated size. Although geophysical data alone cannot resolve its origin, initial analyses of regional gravity and magnetic data, together with seismic reflection and refraction surveys, indicate that the Bedout structure is at least as large as Chicxulub, or about 200 km in diameter. Comparison of the diameter of the central uplift (40$-$60 km), the vertical extent (or structural relief) of the central uplift (6$-$7 km), and the diameter of the inferred primary collapse (or transient) crater ($\sim$90$-$100 km) are all of equal or larger dimensions. Further details of crater morphology and maximum crater diameter are less well resolved at Bedout owing to its greater depth of burial ($<$3 km), its greater age ($\sim$250 Ma) and subsequent overprint, and the resolution capability of the currently available data. The estimated size, age and location of the Bedout structure, however, are consistent with the known distribution of end-Permian impact debris found worldwide. Additional multidisciplinary studies, including aeromagnetic and gravity surveys, 3D wide-angle reflection and refraction experiments, drilling, and additional core analyses are needed to further verify the inferred impact origin of Bedout, and to better refine estimates of crater size and 3D geometry.
http://beckeraustralia.crustal.ucsb.edu
B33C-0271 1340h
Profound 62 Myr Cycle in Fossil Diversity
By combining the Sepkoski Compendium of Marine Fossil Genera with the new ICS 2004 geologic time scale, we have shown that the fossil record contains a 62 +/- 3 Myr cycle in the diversity of genera. This cycle has a very high statistical significance and while the associated changes in diversity are frequently gradual, all of the sharp drops known as major mass extinctions have occurred during declining phases of this cycle. This suggests that the timing or magnitude of these extinctions has been influenced by this periodic process; however, it remains unclear whether extinction events actually cause the cycle. While, we cannot explain the origin of the 62 Myr cycle, we believe it indicates a profound influence of some periodic physical process on Earth's environment throughout at least the last 540 Myr. In addition, the diversity data contain a statistically ambiguous 140 +/- 15 Myr cycle which could be due to changes of the same frequency reported in climate and cosmic rays. While all major mass extinctions seem to bear some relation to this cycle, we also find that the Permian-Triassic extinction was qualitatively unique. This extinction, the most severe in Earth's history, had substantial impact on classes of organisms that were historically resistant to the 62 Myr changes. This suggests that the processes leading to the P-T extinction may have included factors that were unique to that point in the Phanerozoic. It should be noted that the 62 Myr hypothesis is originally due to Thomson (Thomson KS, Nature 261, 578-580 (1976); Devs. in Palaeo. and Strat. 5, 377-404 (1977)) and Ager (Ager DV, Proc. Geologists' Assoc. 87, 131-159 (1977)), though their work has largely been forgotten. At the time of this submission, our revival of the 62 Myr hypothesis and related analysis is currently undergoing peer review at Nature.
B33C-0272 1340h
Primary Mineralogical and Chemical Characteristics of the Major K/T and Late Eocene Impact Deposits
Three well-characterized, distal impact deposits at the K/T boundary and in upper Eocene sediments serve as a baseline for understanding other proposed impact deposits. All contain abundant spherules, evidence of shock metamorphism, and the largest have significant extraterrestrial components (ETCs). The K/T and the Eocene cpx-spherule (cpxS) deposits are global - likely from the events that produced the 180 km Chicxulub and 100 km Popigai craters. The Eocene North American microtektite (NAM) deposit is regional and likely from the event that produced the 45 km Chesapeake Bay crater. These deposits all contain abundant spherules formed from both shock-melted target and mixtures of target and projectile in the ejecta plume. Spherules constitute most of the mass of the distal ejecta. K/T spherules in regional deposits around the Gulf of Mexico are from low-velocity, target-rich ejecta. These can be a few mm in size and form deposits 10s of cm thick. Globally deposited K/T spherules from the plume (typically a few hundred micron size) are both target- and projectile- rich. When well preserved, the global deposits are 3 mm thick. Eocene cpxS deposits are similar to distal K/T with both target- and projectile-rich varieties (i.e., glassy microtektite, and cpx spherules). They are smaller on average than K/T spherules, concentrated in the 125-250 micron and smaller fractions. They are invariably bioturbated, but the initial deposit was probably less than 1 mm thick. The NAM are composed entirely of target-rich glass. They are similar in size to the cpxS. Size is an important criterion for distal ejecta because droplet size in the impact plume is proportional to the energy of the impact. Both the K/T and cpxS deposits are characterized by well-defined ETCs, commonly measured by Ir. The total Ir deposited is about 55 ng per square cm in K/T sediments, and about 11 ng for the cpxS layer. This 5/1 proportion in Ir is generally consistent with the ~1.8/1 ratio in crater diameters. The NAM have no significant ETC. This may be a function of the smaller impact. It indicates there was no significant projectile-rich plume deposit. All three deposits also contain evidence of shock metamorphism, including quartz with planar deformation features, and coesite. K/T and NAM deposits are also known to contain shocked feldspar and zircon. Shocked minerals are not as ubiquitous as spherules, although in K/T deposits they are found in the Pacific, North America, and in trace amounts in Europe. Shocked minerals are only a small fraction of the total mass (typically less than 1 mg/g). These diagnostic criteria are clearly demonstrated by numerous labs on samples from a large number of K/T and Eocene sites. At present, such evidence of impact is not ubiquitous in P/T or T/J boundary sediments. Scattered reports of very small spherules (less than 100 microns) in P/T boundaries do not include abundance data. There are no convincing Ir anomalies that would represent a large ETC. Reported traces of meteorite fragments or anomalous noble gases, while intriguing, could be derived from non-impact sources (e.g., interplanetary dust particles). A few reports of shocked quartz in P/T boundaries are also intriguing, but this author won't be convinced of their accuracy until confirmed by TEM analysis. A problem with searching for evidence of impact at the P/T and T/J boundaries is the paucity of good localities with continuous sediment records and the fact that they are unavailable to most researchers. Those who wish to advance impact at the T/J and P/T need to work to get key samples distributed the broader impact community.
B33C-0273 1340h
Repeated Carbon-Cycle Disturbances at the Permian-Triassic Boundary Separate two Mass Extinctions
Non-marine organic matter in Permian-Triassic sediments from the Blue Mountains, eastern Australia shows seven negative δ13C excursions of up to 7%, terminating with a positive excursion of 4%. Fluctuations start at the late Permian Glossopteris floral extinction and continue until just above the palynological Permian-Triassic boundary, correlated with the peak of marine mass extinction. The isotopic fluctuations are not linked to changes in depositional setting, kerogen composition or plant community, so they evidently resulted from global perturbations in atmospheric δ13C and/or CO2. The pattern was not produced by a single catastrophe such as a meteorite impact, and carbon-cycle calculations indicate that gas release during flood-basalt volcanism was insufficient. Methane-hydrate melting can generate a single -7% shift, but cannot produce rapid multiple excursions without repeated reservoir regeneration and release. However, the data are consistent with repeated overturning of a stratified ocean, expelling toxic gases that promoted sequential mass extinctions in the terrestrial and marine realms.
B33C-0274 1340h
Cycles in Fossil Diversity: Causes and Considerations
Improvements in paleontology and geochronology have made it clear that the changes in biodiversity during the Phanerozoic (0-542 Ma) are not simply random but also contain strikingly periodic features. The most prominent and compelling of these is a 62 +/- 3 Myr cycle whose declines incorporate many of the mass extinctions in Earth's history. A second, statistically ambiguous cycle with a period of 140 +/- 15 Myr also occurs. Such periodicities almost certainly indicate that an as yet unknown cyclic physical process has been having a significant influence on the Earth's environment. Having searched records of sea level, climate, glaciation, isotopic shifts, volcanism, and other environmental changes, we have found no strong evidence of a matching 62 Myr cycle, though some similarities between the changes in diversity and episodes of volcanism and/or sea level change cannot be entirely ignored. By contrast, the 140 Myr cycle matches changes at the same frequency previously reported in climate, glaciation, and cosmic ray flux. The causes of these cycles are unknown though one can speculate on the kinds of astrophysical or geophysical processes that might be capable of maintaining periodic behavior on such long time scales. The leading candidates appear to be encounters with large scale galactic structure (e.g. spiral arms) and periodic pulses of plume formation from the core-mantle boundary. Encounters with spiral arms have previously been invoked to explain the 140 Myr changes in climate but the timing of such encounters is still very uncertain. In simulations and experimental models, plume formation can exhibit periodic modes and could thus lead to periodic episodes of major volcanism of the form frequently implicated in extinctions, though as yet no compelling evidence of periodic volcanism exists. Other scenarios, such as oscillations about the galactic plane or the presence of a solar companion star, are considered, but they are regarded as less likely. However, regardless of the mechanisms, the substantial changes in the biosphere accomplished by the 62 Myr cycle must have left environmental clues that will ultimately lead us to determining its origin.