Preheated light gas gun shock experiments: hot Molybdenum and diopside-anorthite liquid revisited Paul D Asimow Daoyuan Sun T J Ahrens The possible existence of silicate liquids at the modern core-mantle boundary and in an early terrestrial deep magma ocean motivates studies of the properties of melts under such conditions. With shock wave techniques, we can compress liquid silicates to lower mantle pressures and determine their density under such conditions. Melt density is among the first things we need to know in order to model phase equilibria and dynamics of lower mantle partial melting. We have extended the techniques for pre-heated Hugoniot equation of state measurements for use on Caltech's 25 mm light gas gun at flyer velocities up to 7.5 km/s. Previous data on Mo at 1400°C and on a variety of silicate liquids were collected on a 40 mm propellant gun up to a maximum flyer velocity of 2.6 km/s. Higher impact velocities open up a range of new opportunities, including tests of previous extrapolations of low-pressure data and direct probing of the properties of molten silicates at lower mantle pressure. Our preheated liquid experiments are conducted in sealed Mo capsules and therefore we need to know the Hugoniot of Mo initially at elevated temperature, which may differ by several percent from the principal Hugoniot of Mo. Miller et al. [1] measured the Hugoniot EOS of Mo initially at 1400°C up to a particle velocity (Up) of 1.5 km/s and applied a linear fit with shock velocities slower than the principal Hugoniot in the measured range, but implying a crossover when extrapolated above 1.8 km/s (i.e., about 100 GPa pressure). Molodets [2] fit these data to a parameter-free theoretical form for the volume dependence of the Gr ¼neisen parameter that predicts a concave-downward high-temperature Hugoniot that runs below and approaches parallel with the principal Hugoniot. Our data points up to Up = 3.25 km/s (P = 300 GPa) are coincident with Molodet's theory or, in fact, any Mie-Gr ¼neisen form within error. We propose a uniform set of parameters for both cold and hot shock states of Mo. The silicate liquid composition consisting of 64 mol % anorthite and 36 mol % diopside is a simplified analogue for basalt and was chosen for study by Rigden et al. [3]. This earlier study found the expected linear Us-Up Hugoniot (with molar volume intermediate between anorthite and diopside end members) up to 25 GPa, followed by two data points that suggested a dramatic stiffening to a nearly incompressible Hugoniot. We now have three experiments at higher pressure (44, 81, and 110 GPa) that clearly show that this extrapolation was incorrect. All the data on this composition can be fit with a single linear Hugoniot. Although basaltic liquids of this composition are not expected in the lower mantle, the implication is that silicate liquids remain more compressible than solids at compressions approaching 50%. This is consistent with results from our laboratory on SiO2, MgSiO3, and Mg2SiO4 systems showing that melts in these systems become denser than coexisting solids at pressures similar to the base of the mantle. We also have one new high pressure data point on diopside liquid at 85 GPa which is consistent with extrapolation of previous data. Together, new and existing data on diopside, anorthite, and the intermediate composition are consistent with linear mixing of volumes through the entire pressure range studied. 1. Miller, G.H., T.J. Ahrens, and E.M. Stolper, The Equation of State of Molybdenum at 1400 °C. Journal of Applied Physics, 1988. 63(9): p. 4469-4475. 2. Molodets, A.M., Shock compression of preheated molybdenum. High Pressure Research, 2005. 25(3): p. 211-216. 3. Rigden, S.M., T.J. Ahrens, and E.M. Stolper, Shock compression of molten silicate - results for a model basaltic composition. Journal of Geophysical Research, 1988. 93(B1): p. 367-382.

When Molluscs Took Over the World: Ecological Change, Environmental Stress, and the End-Permian Mass Extinction The severe biotic crises during the Permian-Triassic interval had a profound impact on the composition and structure of marine ecosystems, by forcibly disassembling Paleozoic community structure and giving rise to ecological dominance by the molluscan Modern Evolutionary Fauna. Previous compilations of global biodiversity suggested that the transition to the Modern fauna was abrupt and occurred during the catastrophic end-Permian mass extinction, suggesting that the severity and selectivity of the biotic crisis was largely responsible for the taxonomic shift. However, the ecological shift, in terms of relative abundance within marine fossil assemblages, has not been well constrained and the relative importance of the end-Permian extinction and another extinction 8 Myr earlier at the end of the Middle Permian remain unclear. Quantitative counts of relative abundance from silicified Early and Middle Permian fossil assemblages from North America and Thailand confirm that members of the Paleozoic fauna, such as brachiopods, were strongly numerically dominant, consistent with inferences from global diversity compilations. However, there was a dramatic ecological shift recorded in Late Permian assemblages from south China and Greece, which were instead dominated by molluscs of the Modern fauna, including abundant burrowing bivalves that were typically rare in the older assemblages. Although these changes were coincident with the previously-recognized large extinction at the end of the Middle Permian, the existence of elevated extinction rates during that interval is not supported by a new analysis that incorporates standardization techniques to correct for unequal sampling of the fossil record. Apparent extinction at the end of the Middle Permian results more from the shifting locus of fossil sampling due to cessation of marine deposition in regions such as western North America than from a global biotic crisis. Although the ecological change did not occur during a pronounced extinction event, it was coincident with the onset of anoxic conditions in deep marine basins. This relationship suggests that variable environmental stress in proximity to anoxic deep waters during the Late Permian may have favored ecological dominance by the Modern fauna. The abrupt end-Permian mass extinction, which resulted from upwelling of those anoxic waters into shallow shelf environments, may therefore represent the climax of a protracted ecological crisis beginning around the Middle-Late Permian boundary.

Special Brown Bag Talk on WEDNESDAY
in E&MS, Room A340 at 11:30 A.M.

More than 80% of the Earth's biosphere is cold (<5°C) with much of it existing as ice. Cold icy environments are now known to support a surprising diversity of active microbial life. The Taylor Glacier in the McMurdo Dry Valleys, Antarctica is host to one such icy microbial hotspot. The episodic release of subglacial brine from the Taylor Glacier is known as Blood Falls. The name derives from a visible accumulation of iron-oxides at the point where outflow meets the atmosphere at the snout of the glacier. This subglacial water from the Taylor Glacier provides a sample of what is believed to be cryoconcentrated Pliocene age seawater that became trapped in the upper Taylor Valley and was eventually covered by the glacier as it advanced. Biogeochemical measurements, culture-based techniques, and molecular analysis (based on 16S rRNA gene sequences) were used to characterize microbes and chemistry associated with the subglacial outflow. The majority of the microbial assemblage detected in Blood Falls shares high 16S rRNA gene sequence homology with other reported phylotypes from marine systems, including cultured relatives that are known to metabolize iron and sulfur compounds. These findings are consistent with the high iron and sulfate concentrations in the outflow, which are likely due to the interactions of the subglacial brine with underlying iron-rich bedrock. Together these results indicate that glaciated systems can retain a biological imprint of their preglacial heritage. An emerging theme is that subglacial systems on Earth host metabolically active and potentially autonomous ecosystems that can cycle iron and sulfur compounds for energy in permanent cold and darkness.

Special Brown Bag Talk on WEDNESDAY
in E&MS, Room A340 at 11:30 A.M.
Abstract: The subglacial system provides a unique model to study microbial systems in relative geological isolation. The long time-scale of entrapment relative to average lifetimes of a microbial cell provides an opportunity to explore questions ranging form the linkages between metabolic pathways, thermodynamics and substrate availability; to the potential for insight about fundamental rates of evolution and constraints on biodiversity. Using new techniques that couple molecular biology and isotopic geochemistry may provide insight into carbon cycling and microbial metabolism occurring in cold, dark, isolated, low biomass systems. This talk will discuss current and planned experiments with stable isotope probing (SIP), compound specific isotopic measurements of nucleic acids and natural isotopic abundance measurements made on important geochemical substrates. The goal is to combine these data to elucidate in situ subglacial processes and the geochemical legacy that subglacial microbial activity imparts on the proglacial system.

Abstract: One of the most marked environmental changes in Earth history, the rise of atmospheric oxygen, stems from a major biological innovation. The evolution of oxygenic photosynthesis conferred the ability to use water as a photosynthetic substrate (earlier photosynthesis was anoxygenic and required reduced iron, sulfur, or hydrogen). Primary productivity was no longer limited by source of electrons. Molecular oxygen became widely available for use in anabolic and catabolic metabolisms. This innovation profoundly altered biogeochemical cycles, led to the buildup of oxidants in the atmosphere and oceans, and ultimately paved the way for modern surface environments bathed in free oxygen. In this seminar I will present research from a 2.52 billion-year-old carbonate platform preserved along the western margin of the Kaapvaal Craton in South Africa that provides constraints on our understanding of life just prior to the rise of atmospheric oxygen. I will focus on two aspects of Late Archean geobiology: 1) operation of the carbon cycle as it relates to the generation of environmental oxidants, and 2) deposition and significance of banded iron formation.

Special Brown Bag Talk on WEDNESDAY
in E&MS, Room A340 at 11:30 A.M.

Abstract: Over the past 50 years a substantial body of knowledge about Precambrian life has accumulated. Building on this foundation, I will present several approaches where recent advances in molecular biology and biochemistry can be combined with geological data to reveal more about the early history of life than traditional micropaleontology. Examples include the interpretation of lipid biomarkers as signals of environmental oxygen, and the evolution of steroid and hopanoid biosynthesis. These methods offer a new perspective on the deep history of metabolism, particularly the evolution of geologically-important biosynthetic pathways.

Microbial Geochemistry at the Edge of the Biosphere: Geocatalysis, growth, and diagenesis Interdisciplinary studies of the Earth's crust and upper mantle have greatly expanded the range of conditions known to support life and thereby the range of systems impacted by biogeochemical processes. Of these systems, studies of subsurface environments, only indirectly linked to photosynthetic processes at the Earth's surface, offer tremendous potential for elucidating the intricate relationships between abiotic, mineral-catalyzed chemistry, microbial geochemistry, and the remineralization or diagenesis of organic matter. Subsurface biogeoscience has benefited from the genomics revolution in microbiology, which has improved our understanding of microbial diversity in natural systems, but left many other topics such as the physiological status or metabolic rates of microbial communities under-explored. My research focuses upon two characteristics of subsurface ecosystems; the combined effects of high temperature and high pressures upon the microbial activities, and the role of microbe-mineral interactions in structures known as biofilms. I will show that iterative experimental, theoretical, and observational studies are necessary to understand the controlling factors on microbial ecosystems in deep-seated environments and their impacts upon geochemistry. Furthermore, these systems provide opportunities to delineate linkages between biotic and abiotic processes and may be useful in identifying and evaluating biomarkers for the assessment of both ancient environments and putative extraterrestrial habitats.

Special Brown Bag Talk on WEDNESDAY
in E&MS, Room A340 at 11:30 A.M.

Peering at the subsurface biosphere through a diamond window: Improving our ability to observe and quantify microbial activities in rock-hosted environments Investigations of the subsurface biosphere have pushed the depth limits of microbial ecosystems to greater than 800 meters below the seafloor in marine sediments and 3-4 km into the continental lithosphere. In subsurface environments, the mode-of-growth for microorganisms is attached to minerals in structures known as biofilms. In many rock-hosted, subsurface environments, organisms are confronted with multiple stressors including not only high pressures, but elevated temperatures and low energy fluxes. Subsurface microorganisms live at a precarious boundary between geologically-supported growth and cell death and remineralization. A limitation to our study of deep ecosystems has been an inability to distinguish and quantify microbial activities under conditions found in their native habitats. I will describe a research plan aimed at improving our ability to observe and characterize biogeochemical processes under conditions relevant to the deep subsurface environment. This plan relies upon the establishment of a one-of-a-kind research facility optimized for conducting high pressure experimental microbiology, borrowing from tools developed for materials science and hydrothermal geochemistry applications. A future goal is to make the systems modular, allowing for its use in both laboratory and field-based studies. An analytical suite necessary to characterize biogeochemical transformations at microbe-mineral interfaces can be employed to follow the products of high pressure experiments, but will also be amenable to a range of studies in near-surface environments. The data obtained from these experiments will be important aid in deciphering both the extent and the biogeochemical consequences of a deep subsurface biosphe

Day, Thursday
in The Natural Science Annex, Room 101, 4:00PM

Mineral weathering provides nutrients to vegetation, neutralizes acid deposition, and controls soil and stream water composition - functions that are vital to the growth and maintenance of terrestrial and aquatic ecosystems. In the northeastern USA, the pool of exchangeable calcium, a plant-essential nutrient dominantly supplied by mineral weathering, has been declining for several decades. To investigate the interaction between vegetation and mineral weathering in a region affected by base cation loss, I examined the chemical and mineralogical composition of soils at the Hubbard Brook Experimental Forest, NH. During the first part of this talk, I will discuss the influence of vegetation and landscape position (e.g., slope, elevation) on mineral weathering rates, and the relative contributions of different calcium-bearing minerals (e.g., plagioclase, apatite) as nutrient sources. The second part of the talk will examine how trees access nutrients via their symbiotic fungi. The roots of certain trees, such as pine (Pinus), spruce (Picea), fir (Abies) and birch (Betula), have a symbiotic association with ectomycorrhizal fungi which extend hyphae (threadlike filaments) into the soil beyond the reach of tree roots and provide increased access to nutrients. These hyphae exude organic acids that weather minerals such as biotite, apatite, and feldspar. To determine the dissolution of calcium-bearing minerals by fungi, I buried mesh bags, which contained either apatite or wollastonite, in HBEF soils. Phospholipid fatty acids (PLFA) that are specific to fungi and bacteria were used to estimate fungal and bacterial response to the dissolution of these minerals. The results of this research improve our understanding of the role that trees and fungi play in mineral weathering, the importance of apatite as a nutrient source, and the response of fungi to certain calcium-bearing minerals.

Special Talk on FRIDAY
in E&MS, Room A340 at 2:00 P.M.

Calcium is an essential plant nutrient whose concentration has been decreasing in soils in the northeastern USA for several decades. During this talk, I will discuss recent discoveries regarding calcium cycling in terrestrial ecosystems, and introduce future research directed at better quantifying fluxes and pools of plant-available nutrients. I will also discuss on-going research which examines the response of ecosystem components to a watershed-scale calcium addition. Wollastonite (CaSiO3, 1.2 metric tonnes per hectare) was added to an experimental watershed at the Hubbard Brook Experimental Forest (NH) to increase the base saturation of the soil in order to determine the influence of exchangeable calcium on the structure and function of a forest. The wollastonite has a distinct chemical and isotopic composition which allows its dissolved constituents to be traced through the ecosystem.

TITLE: Quantitative macrostratigraphy: implications for evolution SUMMARY: A new approach to quantifying the temporal and spatial architecture of the rock record reveals important short- and long-term dynamics. Numerous quantitative similarities between the sedimentary record and the macroevolutionary history of marine life suggest that geologic processes have consistently controlled biodiversity, rates of biotic turnover, and faunal composition for the last 542 million years.

TITLE: Quantitative macrostratigraphy: implications for climate and tectonics SUMMARY: The temporal and spatial architecture of the rock record both chronicles and influences climate and tectonics. Quantifying the rock record will therefore provide important new data for testing and generating numerous hypotheses in earth systems science. Preliminary results suggest that such data may provide new constraints on the evolution of the carbon cycle and long-term rates of global tectonics. The range of questions that can be addressed by macrostratigraphy will be discussed.

Special Brown Bag Talk on WEDNESDAY
in E&MS, Room A340 at 11:30 A.M.