Investigations into an unknown organism on the martian meteorite Allan Hills 84001

Abstract— Examination of fracture surfaces near the fusion crust of the martian meteorite Allan Hills (ALH) 84001 have been conducted using scanning electron microscopy (SEM) and atomic force microscopy (AFM) and has revealed structures strongly resembling mycelium. These structures were compared with similar structures found in Antarctic cryptoendolithic communities. On morphology alone, we conclude that these features are not only terrestrial in origin but probably belong to a member of the Actinomycetales, which we consider was introduced during the Antarctic residency of this meteorite. If true, this is the first documented account of terrestrial microbial activity within a meteorite from the Antarctic blue ice fields. These structures, however, do not bear any resemblance to those postulated to be martian biota, although they are a probable source of the organic contaminants previously reported in this meteorite.     

Origin of a late Eocene to pre‐Miocene buried crater and breccia lens at Fohn‐1, North Bonaparte Basin,Timor Sea: A probable extraterrestrial connection

Abstract— Seismic reflection data and an at least 350 m thick, PGE‐rich carbonate breccia lens intersected by the Fohn‐1 exploration well in the Timor Sea off northern Australia, are interpreted in terms of a buried 4.8 km‐diameter impact crater of late Eocene to pre‐Miocene age. The crater displays the classic elements of impact structures, including a central uplift, ring syncline, and upraised rims. The presence in the breccia of redeposited Campanian and Maastrichtian microfossils suggests rebound of strata from levels deeper than 1250 m below the pre‐Miocene unconformity. Morphometric modelling suggests an original crater at least 1400 m deep, which is consistent with the excavation of Cretaceous strata. Stratigraphic and palaeontological evidence suggests that the impact occurred between 36 and 24.6 Ma. The breccia contains a pseudotachylite component enriched in the inert Pt group elements (PGE) (Ir, Ru) by factors of 5–12 above the values of common sediments. The more mobile PGE (Os, Pt, Pd) show a wide scatter and terrestrial‐type values. Opposite geochemical/stratigraphic trends pertain to different PGE species—the relatively inert Ir‐Ru group shows an overall concentration at the base of the section, whereas the more mobile Os shows peaks at median levels of the section—suggesting upward diagenetic leaching. The near‐chondritic PGE patterns at the base of the breccia pile are accompanied by near‐chondritic Ni/Cr, Co/Cr, Ni/Ir, Ni/Pt, and Cu/Pd ratios. Departure from these values related to alteration at higher levels in the breccia pile is accompanied with high S levels (～1%).     

Short-lived thorium isotopes (Th, Th) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean

Repeated measurements of depth profiles of ²³⁴Th (dissolved, 1–70 and >70 μm particulate) at three stations (Orca, Minke, Sei) in the Ross Sea have been used to estimate the export of Th and particulate organic carbon (POC) from the euphotic zone. Sampling was carried out on three JGOFS cruises covering the period from October 1996 (austral early spring) to April 1997 (austral fall). Deficiencies of ²³⁴Th relative to its parent ²³⁸U in the upper 100 m are small during the early spring cruise, increase to maximum values during the summer, and decrease over the course of the fall. Application of a non-steady-state model to the ²³⁴Th data shows that the flux of Th from the euphotic zone occurs principally during the summer cruise and in the interval between summer and fall. Station Minke in the southwestern Ross Sea appears to sustain significant ²³⁴Th removal for a longer period than is evident at Orca or Sei. Particulate ²³⁴Th activities and POC are greater in the 1–70 μm size fraction, except late in the summer cruise, when the >70 μm POC fraction exceeds that of the 1–70 μm fraction. The POC/²³⁴Th ratio in the >70 μm fraction exceeds that in the 1–70 μm fraction, likely due in part to the greater availability of surface sites for Th adsorption in the latter. Particulate ²³⁴Th fluxes are converted to POC fluxes by multiplying by the POC/²³⁴Th ratio of the >70 μm fraction (assumed to be representative of sinking particles). POC fluxes calculated from a steady-state Th scavenging model range from 7 to 91 mmol C m⁻² d⁻¹ during late January–early February, with the greatest flux observed at station Minke late in the cruise. Fluxes estimated with a non-steady-state Th model are 85 mmol C m⁻² d⁻¹ at Minke (1/13–2/1/97) and 50 mmol C m⁻² d⁻¹ at Orca (1/19–2/1/97). The decline in POC inventories (0–100 m) is most rapid in the southern Ross Sea during the austral summer cruise (Smith et al., 2000. The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3119–3140. Gardner et al., 2000. Seasonal patterns of water column particulate organic carbon and fluxes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3423–3449), and the ²³⁴Th-derived POC fluxes indicate that the sinking flux of POC is 30–50% of the POC decrease, depending on whether steady-state or non-steady-state Th fluxes are used. Rate constants for particle POC aggregation and disaggregation rates are calculated at station Orca by coupling particulate ²³⁴Th data with ²²⁸Th data on the same samples. Late in the early spring cruise, as well as during the summer cruise, POC aggregation rates are highest in near-surface waters and decrease with depth. POC disaggregation rates during the same time generally increase to a maximum and are low at depth (>200 m). Subsurface aggregation rates increase to high values late in the summer, while disaggregation rates decrease. This trend helps explain higher values of POC in the >70 m fraction relative to the 1–70 m fraction late in the summer cruise. Increases in disaggregation rate below 100 m transfer POC from the large to small size fraction and may attenuate the flux of POC sinking out of the euphotic zone.     

Microstructurally controlled trace element (Zr,U–Pb) concentrations in metamorphic rutile: An example from the amphibolites of the Bergen Arcs

As a common constituent of metamorphic assemblages, rutile provides constraints on the timing and conditions of rock transformation at high resolution. However, very little is known about the links between trace element mobility and rutile microstructures that result from synmetamorphic deformation. To address this issue, here we combine in situ LA-ICP-MS and sensitive high-resolution ion microprobe trace element data with electron back-scatter diffraction microstructural analyses to investigate the links between rutile lattice distortions and Zr and U–Pb systematics. Furthermore, we apply this integrated approach to constrain further the temperature and timing of amphibolite facies metamorphism and deformation in the Bergen Arcs of southwestern Norway. In outcrop, the formation of porphyroblastic rutile in dynamically hydrated leucocratic domains of otherwise rutile-poor statically hydrated amphibolite provides key contextual information on both the ambient conditions of hydration and deformation and the composition of the reactive fluid. Rutile in amphibolite recorded ambient metamorphic temperatures of ~590–730°C during static hydration of the granulitic precursor. By contrast, rutile from leucocratic domains in the directly adjacent shear zone indicates that deformation was accompanied by a localized increase in temperature. These higher temperatures are recorded in strain-free rutile (~600–860°C) and by Zr concentration measurements on low-angle boundaries and shear bands (620–820°C). In addition, we also observe slight depletions of Zr and U along rutile low-angle boundaries relative to strain-free areas in deformed grains from the shear zone. This indicates that crystal–plastic deformation facilitated the compositional re-equilibration of rutile upon cooling to slightly below the peak temperature of deformation. Cessation of deformation at mid-crustal conditions near ~600°C is recorded by late stage growth of small (<150 µm) rutile in the high-strain zones. U–Pb age data obtained from the strain-free and distorted rutile grains cluster in distinct populations of 437.4 ± 2.7 Ma and c. 405–410 Ma, respectively. These different ages are interpreted to reflect the difference in closure for thermally induced Pb diffusion between undeformed and deformed rutile during post-deformation exhumation and cooling. Thus, our results provide a reconstruction of the thermochronological history of the amphibolite facies rocks of the Lindås Nappe and highlight the importance of integration of microstructural data during application of thermometers and geochronometers.