V43C-1431 1340h
Oxygen Isotope Variability of the Tagish Lake Meteorite
Triple oxygen isotope data for anhydrous phases and bulk samples provide insight into the complex history of the Tagish Lake carbonaceous chondrite meteorite. Oxygen was extracted from hand-picked separates of anhydrous phases by laser fluorination and from powdered bulk samples by conventional methods, both of which used BrF5. The oxygen isotope compositions of pyroxene and olivine concentrates ($>$95% purity) and individual chondrules describe a line (d17O = 0.974 * d18O - 3.11; R$^2$ = 0.9959) for the Tagish Lake meteorite that is very similar to the Murchison anhydrous mineral mixing line. Three possible sources of pyroxene identified in Tagish Lake include: pyroxene chondrules, disaggregated pyroxene fragments, and olivine-(pyroxene) chondrules. The oxygen isotope values of pyroxene separates (d18O = -6.2 to -4.5 permil; d17O = -9.3 to -7.3 permil) are significantly lower than those obtained for olivine (d18O = 3.1 to 5.1 permil; d17O = -0.2 to 2.2 permil). These data are interpreted to indicate that pyroxene acquired its oxygen isotopic composition under different conditions than olivine in the early solar nebula. Olivine-(pyroxene) chondrule oxygen isotope compositions (d18O = 0.8 to 4.7 permil; d17O = -3.5 to 0.8 permil) typically plot between those of olivine and pyroxene, suggestive of a two-component mixture. The triple oxygen isotope values for nine "pristine" and "degraded" bulk samples of the Tagish Lake meteorite are much higher than those obtained for pyroxene, olivine and chondrules separates. "Pristine" samples were collected immediately after the meteorite fall, whereas "degraded" material was obtained approximately 4 months later, from the ice-(water) cover on Tagish Lake. The "pristine" samples (d18O = 16.9 to 17.5 permil; d17O = 8.5 to 9.0 permil) plot close to, but below the Terrestrial Fractionation Line (TFL). "Degraded" samples also plot just below the TFL, but are more enriched in 18O and 17O (d18O = 21.2 to 23.5 permil; d17O = 10.3 to 12.2 permil) than "pristine" samples. All bulk samples follow a trend that is close to the CM meteorite mixing line, as do results for two bulk samples reported earlier by Clayton et al. (2001). The variation in oxygen isotope compositions of the bulk samples is consistent with mixing between anhydrous phases and a much more abundant, 18O and 17O-rich clay matrix, whose composition is expected to lie on or slightly above the TFL.
V43C-1432 1340h
New Results from High Pressure Phase Equilibria Experiments on the Richardton H-Chondrite
If the accretion of the Earth was accomplished by chondritic building blocks then melting and crystallization of chondritic material can be used to simulate the accreting and segregating bulk Earth. Because we may not have a representative "bulk Earth chondrite," silicate high P-T phase equilibria studies of a variety of chondrite compositions are needed to model liquids and crystallizing phases in a primordial segregating mantle. Crystallizing phases in a liquid magma ocean, whether persistent or intermittent, could significantly change the composition (and oxidation state) of that liquid by crystal flotation or settling. Also, crystals settling to the core-mantle boundary could provide a source for light elements in the core, or constraints on the abundance of some light elements. The Richardton H-chondrite was chosen as starting material for experiments because of its resemblance to the bulk Earth and higher FeO content than in a similar previous study of the Allende CV chondrite (Agee et al., 1995). Runs were conducted in a Walker-type multi-anvil apparatus and an end loaded piston cylinder. Pressures between 3 and 23 GPa and temperatures between 800 and $2300\deg$C were investigated, with a focus on higher temperatures bracketing the liquidus. Run durations were up to 50 minutes. Compositional identification of silicate run products was accomplished using either the Cameca SX-50 electron microprobe at the University of Arizona and the JEOL 8600 at Arizona State University. Mineral phases have been tentatively identified by stoichiometry, but Raman spectroscopy (ASU) is underway for more definitive mineral identification. The liquidus (olivine-out) at pressures less than 14 GPa occurs at temperatures less than about $1750\deg$C. Appropriate experiments for the full range of the liquidus at the garnet-out line have not yet been conducted. However, the probable garnet-olivine cotectic appears at about 13 GPa. Mg-Fe perovskite may be the liquidus phase above 21 GPa, with a perovskite-magnesiowustite cotectic occurring above 23 GPa. The lower liquidus below 14 GPa, relative to Allende and peridotite, is consistent with the higher FeO content. If Mg-Fe perovskite is the liquidus phase at higher pressures, an early crystallizing layer of perovskite could provide a source of Si and O for the core at the expense of other light elements, in addition to providing a potential sink for water in the lower mantle.
V43C-1433 1340h
Sulfur Isotopic Compositions of Sulfides From the Lower Huronian Supergroup, Ontario, Canada
Mass-independent isotopic fractionation (MIF) of sulfur found in sedimentary rocks older than 2470 Ma implies that the atmospheric oxygen level was lower than 10$^{-5}$ PAL (present atmospheric level) in the Archean atmosphere [e.g., 1, 2]. Sulfides from the Rooihoogte and Timeball Hill Formations, Transvaal Supergroup, South Africa, (2316 Ma [3]) show only a small degree of MIF [4], which suggests that the atmospheric oxygen level reached 10$^{-5}$ PAL by 2316 Ma [4]. The Huronian Supergroup in E. Canada recording three Paleoproterozoic glaciatial events between 2450 and 2220 Ma is correlated with the Rooihootge and Timeball Hill Formations, and may record the evidence of the great oxygenation event. The sulfur isotopic compositions of sulfides in the Matinenda to Gordon Lake Formations of the Huronian Supergroup exhibit D$^{33}$S, deviation from a mass-dependent fractionation line, of less than 0.5 permil [e.g., 1]. This suggests that the atmospheric oxygen reached a level high enough to vanish the MIF signature before the Matinenda deposition. Here we report sulfur isotopic compositions of sulfides from the Livingstone Creek, Thessalon, and Matinenda Formations of the Lower Huronian Supergroup in the Elliot lake area. The Livingstone Creek and Thessalon Formations underlie the uraniferous Matinenda Formation. In-situ ion microprobe measurements showed that most of sulfides have a small degree of sulfur MIF, as seen in other formations, except for clastic sulfide blocks in the polymictic conglomerates of the Livingstone Creek Formation, the lowermost Huronian Supergroup. The sulfides in the Livingstone sulfide blocks show a clear evidence of MIF (D$^{33}$S = -1.7 to +3.6 permil) with d$^{34}$S of _|4 to +2 permil. The range of MIF from the sulfide blocks is an order of magnitude larger than that for other sulfides from the Huronian Supergroup. This indicates that the atmospheric oxygen may have started to increase after the formation of the sulfide blocks and before the deposition of the Livingstone Creek Formation. [1] Farquhar J. et al. (2000) Science, 289, 756. [2] Pavlov and Kasting (2002) Astrobiology, 2, 27. [3] Hannah J. L. et al. (2004) EPSL, 225, 43. [4] Bekker A. et al. (2004) Nature, 427, 117.
V43C-1434 1340h
Organic Haze Aerosols on the Early Earth: A Laboratory Investigation
An organic haze layer in the upper atmosphere of Titan plays a crucial role in the atmospheric composition and climate of that moon. Such a haze layer may also have existed on the early Earth, providing a UV shield for greenhouse gases needed to warm the planet enough for life to arise and evolve. Despite the implications of such a haze layer, little is known about the organic material produced under early Earth conditions when both CO2 and CH4 may have been abundant in the atmosphere. We have developed a method by which we can analyze the aerosol products in real time, using an Aerosol Mass Spectrometer (AMS) to study the chemical composition and size of particles as a function of trace gas composition. We use a deuterium lamp with a spectral range from 115 - 400 nm to produce particles from gaseous mixtures of CH4/CO2/N2, thus simulating the low ultraviolet wavelengths available to the early Earth's atmosphere. Ongoing studies explore the chemical and physical properties of these aerosols as function of C/O ratio and other experimental parameters such as total pressure. The properties of the haze aerosols formed using the UV lamp are also compared to those from previous studies using an electrical discharge source.
V43C-1435 1340h
Evolution of Redox State of Shallow and Deep Seawater
Redox state of seawater and atmosphere of early Earth is still controversial. Generally speaking, many previous works indicated that oxygen was free even in the shallow seawater before 2.7 Ga, and gradually increased because of emergence of oxygen-producing lives since then. However, it is still poorly known the detailed secular change of redox state of shallow and deeper part of the seawater, respectively. It is well known that carbonates are deposited equilibrated with ambient seawater in microbial or abiotic environment. Therefore, the distribution, composition, and mineralogy of the carbonate rocks and minerals give constraints on physical and chemical properties of paleoseawater. This work presents in-situ analyses of major, trace and rare earth elements of well-preserved carbonate minerals in shallow and deep-sea deposits with EPMA and LA-ICP-MS. We focus on carbonates with original depositional textures because of elimination of post-depositional alteration. The shallow marine deposits include sedimentary carbonates in 3.0 Ga Pongola, 2.7 Ga Tumbiana, 2.5 Ga Wittenoom and Campbellrand, 2.4 Ga Mooidraai, 2.3 Ga Kazput, 2.2 Ga Duck Creek, 1.9 Ga Slave, 1.0 Nepal, 0.58 Ga Altai and modern Solomon Islands, and amygdaloidal carbonates within hot-sport basalts in 3.5 Ga North Pole, 2.7 Ga Belingwe, Mount Roe and Maddina, 2.5 Ga Hamersley, and modern OIB. The deep-sea carbonates include amygdaloidal carbonates within mid-oceanic and mature rift-type basalts in 3.5 Ga North Pole, 2.7 Ga Belingwe, 2.5 Ga Hamersley, 1.9 Ga Glengarry and modern MORB. Deep-sea carbonates have LREE-enriched pattern with faint Ce and Eu anomalies between 3.5 and 1.9 Ga, but modern equivalents have significant negative Ce anomaly. In contrast, negative Ce anomalies in shallow carbonates were frequently deviated from those in deep-sea carbonate with the equivalent ages. The negative Ce anomalies increase since 2.78 Ga Mount Roe Basalt, but they decreased until 2.72 Ga, again. They significantly increased after 2.6 Ga, but vanished just after 2.4 Ga global glaciation, again. They gradually increased between 2.2 and 1.0 Ga, but vanished just after 0.8 Ga global glaciation, again. They suddenly increased since 0.6 Ga. The evidence implies complicated secular change of redox state even in shallow water as well as in spite of presence of free-oxygen in atmosphere, paleoseawater was anoxic until Proterozoic in deep-sea environment.
V43C-1436 1340h
Neoproterozoic Land Colonisation, Rising Oxygen, Global Cooling and the Cambrian Explosion
The Neoproterozoic (1000-542 Ma BP) was a time of severe glaciations and a major transition from microscopic to macroscopic life forms. We develop the hypothesis that a rise in atmospheric oxygen in the Neoproterozoic was driven by the biological colonization of the land surface. If early forms of photosynthetic land life selectively weathered continental rock in order to extract nutrients, this would have led to an increase in the flux of biologically available phosphorus to the ocean. We show that recent models for coupled biogeochemical cycles, despite differences in the feedback mechanisms represented, predict this would lead to a rise in atmospheric oxygen concentration, consistent with biological and geochemical evidence. Increased weathering of silicate rocks would also have caused a decline in atmospheric carbon dioxide, which could have been a causal factor in the Neoproterozoic glaciations. A rise in oxygen may have provided a necessary condition for the evolution of animals with hard skeletons seen in the Cambrian explosion. Furthermore, an increase in phosphorus supply to the ocean may have driven an increase in the phosphorus content of marine primary producers. This would have represented an increase in food quality for grazing animals, which have a high phosphorus requirement, and may thus have removed a further limitation on their evolutionary radiation.
V43C-1437 1340h
Experimental and Temporal Observations on the Occurrence and Abundance of Pyrogenic PAH Relative to Atmospheric Oxygen Levels
The Phanerozoic record of atmospheric O2 is based on a global mass balance of several dynamic geochemical cycles, with error margins reflecting such complexity. The potential for accurately determining the record of atmospheric O2 may significantly improve with the proposed method, which relies on a direct relationship between atmospheric oxygen, fuels, and fire products. The interaction between combustion and atmospheric-oxygen level during biomass burning events is investigated by comparing the occurrence and abundance of pyrogenic polycyclic aromatic hydrocarbons (PAH) from experimentally-produced, modern, Triassic, and Carboniferous chars. The combustion process consumes O2 directly from the atmosphere and thermally alters organic matter to produce chars. PAH that develop through combustion, and are preserved within chars, are expected to be more abundant at times of elevated atmospheric O2, which facilitates biomass burning efficiency. To investigate the hypothesized relationship in the geologic record, PAH have been extracted from chars of three periods - modern, Triassic, and Carboniferous - relating to 21%, 15%, and 35% atmospheric O2, respectively (Berner and Canfield, 1989). Surface samples of modern chars that developed in controlled burns set by National Park Service personnel in a mixed conifer-deciduous forest were collected at Zion National Park, Utah. Triassic fusain from Petrified Forest National Park, AZ, and Carboniferous fusain from Joggins Fossil Cliffs, Nova Scotia, were collected from floodplain- and coastal plain-deposited mudstone and sandstone. Target PAH shown to be uniquely pyrogenic include: phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, and benzo(ghi)perylene. The abundance of PAH from modern chars at Zion National Park, Triassic fusain from Petrified Forest National Park, and Carboniferous fusain from Joggins Fossil Cliffs seem to reflect differences in atmospheric oxygen levels. These temporal trends are compared to experimentally-derived pyrogenic PAH produced in a combustion apparatus with different levels of oxygen.
V43C-1438 1340h
The Pb Isotope Records of Archean and Proterozoic Shales Limit Early O-Rich Atmosphere
The relative behavior of Th and U under Earth's surface conditions is well known today, with these elements separated due to the oxidation of U (4+) to U (6+). Thorium does not undergo such a transition, and remains in its 4+ state. U and Th have very limited solubility in their 4+ states, but U (6+) is quite soluble. These differences, and their resulting radiogenic Pb isotope signatures, can be used to provide clues about conditions that existed during crustal weathering and sedimentary transport and diagenesis. Early and Middle Archean fine-grained sediments from Greenland (Rosing and Frei 2004) and southern Africa (Krogstad et al. 2004) have Pb isotopic records (Pb6/4 versus Pb8/4) that show that Th and U behaved in a well-correlated manner. These sample suites have integrated long-term Th/U near the Archean upper crust average. Variations in Pb6/4 versus Pb8/4 of Early Proterozoic shales (2.5 to 2.2 Ga) from North America (Krogstad and Walker 1996, McLennan et al. 2000) also behave in such a manner. This suggests that until 2.2 Ga separation of U from Th due to oxidation of U (4+) was not important. By contrast, Phanerozoic shales show fanning patterns in their Pb6/4 versus Pb8/4, indicating that separation of U from Th occurred in weathering of their provenance, as well during their transport and/or diagenesis. This Pb isotopic record confirms conclusions from other studies that the atmosphere and hydrosphere in Paleoproterozoic and earlier Earth were not oxygen-rich. This conclusion differs from recent suggestions from the Pb isotopic record that oxidizing conditions existed by 3.7 Ga.
V43C-1439 1340h
Evolution of High Temperature Early Atmosphere Under the Interaction of H$_{2}$O-CO$_{2}$ Super-critical Fluid With Minerals
The evolution of atmosphere-lithosphere system of the early Earth is controlled by mutual interaction of high temperature atmosphere with rocks and minerals. It is assumed that the total pressure of the early atmosphere and the surface temperature above initial magma ocean are 26MPa (H$_{2}$O 20MPa, CO$_{2}$ 6MPa) and $130-330\deg$C, respectively. This composition, temperature and pressure are very close to an azeotropic critical point of the H$_{2}$O-CO$_{2}$ system. Cooling of the hot H$_{2}$O-CO$_{2}$ atmosphere brings the first precipitation of liquid phase at above $300\deg$C. During the early period, hot rain of the Earth should be a supercritical acid rain. Cooling rate of the hot atmosphere is regulated by energy transportation capacity among the surface, atmosphere and radiation of the early Earth. In this study, we discuss evolution of the early atmosphere-lithosphere system based on the results of the alteration experiments of minerals simulated early crust with the H$_{2}$O-CO$_{2}$ fluid and the cooling rate estimation of the high temperature atmosphere. The H$_{2}$O-CO$_{2}$ fluid easily reacts with silicate minerals at around critical point of the fluid to produce carbonate and hydrous minerals. Consumption of CO$_{2}$ increases up to approximately 80% at around $250\deg$C for olivine starting material. This means that most of Mg and Fe in the olivine starting material react with CO$_{2}$. The Formation of carbonate minerals reduces the CO$_{2}$ composition of fluid in the capsule to approximately one fifth. The fixation of CO$_{2}$ by carbonate formation should be very effective to reduce CO$_{2}$ pressure from the early atmosphere in cooling through $250\deg$C. The first sediment of the primitive ocean should contain dolomite and hydrous silicate. The CO$_{2}$ and H$_{2}$O fixed in the first sediment should take an important role in the evolution of the early crust. Composition of the early atmosphere, or partial pressures of CO$_{2}$ and H$_{2}$O and temperature gradient of the atmosphere are essential factors controlling cooling history of the high temperature early atmosphere. We discuss evolution of the early atmosphere including effects of precipitation of super-critical H$_{2}$O-CO$_{2}$ fluid and atmosphere-rock interaction.