Jiddat al Harasis 422: A ureilite with an extremely high degree of shock melting

The Jiddat al Harasis (JaH) 422 ureilite was found in the Sultanate of Oman; it is classified as a ureilitic impact melt breccia. The meteorite consists of rounded polycrystalline olivine clasts (35%), pores (8%), and microcrystalline matrix (57%). Clasts and matrix have oxygen isotopic values and chemical compositions (major and trace elements) characteristic of the ureilite group. The matrix contains olivine (Fo_83–90), low‐Ca pyroxene (En_84–92Wo_0–5), augite (En_71–56Wo_20–31), graphite, diamond, Fe‐metal, sulfides, chromite, and felsic glass. Pores are partly filled by secondary Fe‐oxihydroxide and desert alteration products. Pores are surrounded by strongly reduced silicates. Clasts consist of fine‐grained aggregates of polygonal olivine. These clasts have an approximately 250 μm wide reaction rim, in which olivine composition evolves progressively from the core composition (Fo_79–81) to the matrix composition (Fo_84–87). Veins crossing the clasts comprise pyroxene, Fe‐oxihydroxide, C‐phases, and chromite. Clasts contain Ca‐, Al‐, and Cr‐rich glass along olivine grain boundaries (<1 μm wide). We suggest that a significant portion of JaH 422, including olivine and all the pyroxenes, was molten as a result of an impact. In comparison with other impact‐melted ureilites, JaH 422 shows the highest melt portion. Based on textural and compositional considerations, clasts and matrix probably originated from the same protolith, with the clasts representing relict olivine that survived, but was recrystallized in the impact melt. During the melt stage, the high availability of FeO and elevated temperatures controlled oxygen fugacity at values high enough to stabilize olivine with Fo_～83–87 and chromite. Along pores, high Mg# compositions of silicates indicate that in a late stage or after melt crystallization FeO became less available and fO₂ conditions were controlled by C?CO + CO₂.     

Ground water dependence of endangered ecosystems: Nebraska's eastern saline wetlands

Many endangered or threatened ecosystems depend on ground water for their survival. Nebraska's saline wetlands, home to a number of endangered species, are ecosystems whose development, sustenance, and survival depend on saline ground water discharge at the surface. This study demonstrates that the saline conditions present within the eastern Nebraska saline wetlands result from the upwelling of saline ground water from within the underlying Dakota Aquifer and deeper underlying formations of Pennsylvanian age. Over thousands to tens of thousands of years, saline ground water has migrated over regional scale flowpaths from recharge zones in the west to the present-day discharge zones along the saline streams of Rock, Little Salt, and Salt Creeks in Lancaster and Saunders counties. An endangered endemic species of tiger beetle living within the wetlands has evolved under a unique set of hydrologic conditions, is intolerant to recent anthropogenic changes in hydrology and salinity, and is therefore on the brink of extinction. As a result, the fragility of such systems demands an even greater understanding of the interrelationships among geology, hydrology, water chemistry, and biology than in less imperiled systems where adaptation is more likely. Results further indicate that when dealing with ground water discharge-dependent ecosystems, and particularly those dependent on dissolved constituents as well as the water, wetland management must be expanded outside of the immediate surface location of the visible ecosystem to include areas where recharge and lateral water movement might play a vital role in wetland hydrologic and chemical mixing dynamics.     

A method for producing digital probabilistic seismic landslide hazard maps

The 1994 Northridge, California, earthquake is the first earthquake for which we have all of the data sets needed to conduct a rigorous regional analysis of seismic slope instability. These data sets include: (1) a comprehensive inventory of triggered landslides, (2) about 200 strong-motion records of the mainshock, (3) 1:24 000-scale geologic mapping of the region, (4) extensive data on engineering properties of geologic units, and (5) high-resolution digital elevation models of the topography. All of these data sets have been digitized and rasterized at 10 m grid spacing using ARC/INFO GIS software on a UNIX computer. Combining these data sets in a dynamic model based on Newmark's permanent-deformation (sliding-block) analysis yields estimates of coseismic landslide displacement in each grid cell from the Northridge earthquake. The modeled displacements are then compared with the digital inventory of landslides triggered by the Northridge earthquake to construct a probability curve relating predicted displacement to probability of failure. This probability function can be applied to predict and map the spatial variability in failure probability in any ground-shaking conditions of interest. We anticipate that this mapping procedure will be used to construct seismic landslide hazard maps that will assist in emergency preparedness planning and in making rational decisions regarding development and construction in areas susceptible to seismic slope failure.     

Wisconsinan and Holocene Climate History from an Ice Core at Taylor Dome, Western Ross Embayment, Antarctica

Geochemical data and geophysical measurements from a 554-m ice-core from Taylor Dome, East Antarctica, provide the basis for climate reconstruction in the western Ross Embayment through the entire Wisconsinan and Holocene. In comparison with ice cores from central East and West Antarctica, Taylor Dome shows greater variance of temperature, snow accumulation, and aerosol concentrations, reflecting significant variability in atmospheric circulation and air mass moisture content. Extreme aridity during the last glacial maximum at Taylor Dome reflects both colder temperatures and a shift in atmospheric circulation patterns associated with the advance of the Ross Sea ice sheet and accounts for regional alpine glacier retreats and high lake levels in the Dry Valleys. Inferred relationships between spatial accumulation gradients and ice sheet configuration indicate that advance of the Ross Sea ice sheet began in late marine isotope stage 5 or early stage 4. Precise dating of the Taylor Dome core achieved by trace-gas correlation with central Greenland ice cores shows that abrupt deglacial warming at Taylor Dome was near-synchronous with the ∼14.6 ka warming in central Greenland and lags the general warming trend in other Antarctic ice cores by at least 3000 years. Deglacial warming was following by a warm interval and transient cooling between 14.6 and 11.7 ka, synchronous with the Bølling/Allerød warming and Younger Dryas cooling events in central Greenland, and out of phase with the Antarctic Cold Reversal recorded in the Byrd (West Antarctica) ice core. Rapid climate changes during marine isotope stages 4 and 3 at Taylor Dome are similar in character to, and may be in phase with, the Northern Hemisphere stadial–interstadial (Dansgaard–Oeschger) events. Results from Taylor Dome illustrate the importance of obtaining ice cores from multiple Antarctic sites, to provide wide spatial coverage of past climate and ice dynamics.