Gary P. Landis

Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica

A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice. Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patterned-ground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feedback associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondary-ice lenses contrast markedly with glacial ice by lying on a δD versus δ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs. The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains >25% striated clasts in the >16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarse-grained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ± 0.08 Ma and 0.18 ± 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. Although Granite drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates. Nevertheless, these rates are orders of magnitude lower than those computed on theoretical grounds. During eruptions of the nearby McMurdo Group volcanic centers, ash-fall debris collects at the surface of Granite drift, either in open thermal-contraction cracks or in deep troughs that lie above contraction cracks; these deposits subsequently lower passively as the underlying glacier ice sublimes. The fact that some regions of Granite drift have escaped modification by patterned ground for at least 8.1 Ma indicates long-term geomorphic stability of individual polygons. Once established, polygon toughs likely persist for as long as 10 5 –10 6 yr. Our model of patterned-ground formation, which applies to the hyperarid, cold-desert, polar climate of Antarctica, may also apply to similar-sized polygons on Mars that occur over buried ice in Utopia Planitia.

STABLE HYDROGEN ISOTOPE ANALYSIS OF BAT HAIR AS EVIDENCE FOR SEASONAL MOLT AND LONG-DISTANCE MIGRATION

Abstract Although hoary bats (Lasiurus cinereus) are presumed to be migratory and capable of long-distance dispersal, traditional marking techniques have failed to provide direct evidence of migratory movements by individuals. We measured the stable hydrogen isotope ratios of bat hair (δDh) and determined how these values relate to stable hydrogen isotope ratios of precipitation (δDp). Our results indicate that the major assumptions of stable isotope migration studies hold true for hoary bats and that the methodology provides a viable means of determining their migratory movements. We present evidence that a single annual molt occurs in L. cinereus prior to migration and that there is a strong relationship between δDh and δDp during the molt period. This presumably reflects the incorporation of local δDp into newly grown hair. Furthermore, we present evidence that individual hoary bats are capable of traveling distances in excess of 2,000 km and that hair is grown at a wide range of latitudes and elevations. Stable hydrogen isotope analysis offers a promising new tool for the study of bat migration.

Genesis of sediment-hosted disseminated-gold deposits by fluid mixing and sulfidization: Chemical-reaction-path modeling of ore-depositional processes documented in the Jerritt Canyon district, Nevada

Integrated geologic, geochemical, fluid-inclusion, and stableisotope studies of the gold deposits in the Jerritt Canyon district, Nevada, provide evidence that gold deposition was a consequence of both fluid mixing and sulfidization of host-rock iron. Chemical-reaction-path models of these ore-depositional processes confirm that the combination of fluid mixing, including simultaneous cooling, dilution, and oxidation of the ore fluid, and wall-rock reaction, with sulfidization of reactive iron in the host rock, explains the disseminated nature and small size of the gold and the alteration zonation, mineralogy, and geochemistry observed at Jerritt Canyon and at many other sediment-hosted disseminated gold deposits.

Rare-element–enriched, S-type ash-flow tuffs containing phenocrysts of muscovite, andalusite, and sillimanite, southeastern Peru

Ash-flow tuffs of Neogene age exposed over 2,500 km 2 in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high 87 Sr/ 86 Sr i (0.7258 and 0.7226) and δ 18 O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.” Mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a HF/ a H 2 O. The chemical, isotopic, and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock.

Gas bubbles in fossil amber as possible indicators of the major gas composition of ancient air

Gases trapped in Miocene to Upper Cretaceous amber were released by gently crushing the amber under vacuum and were analyzed by quadrupole mass spectrometry. After discounting the possibility that the major gases N2, O2, and CO2 underwent appreciable diffusion and diagenetic exchange with their surroundings or reaction with the amber, it has been concluded that in primary bubbles (gas released during initial breakage) these gases represent mainly original ancient air modified by the aerobic respiration of microorganisms. Values of N2/(CO2 + O2) for each time period give consistent results despite varying O2/CO2 ratios that presumably were due to varying degrees of respiration. This allows calculation of original oxygen concentrations, which, on the basis of these preliminary results, appear to have changed from greater than 30 percent O2 during one part of the Late Cretaceous (between 75 and 95 million years ago) to 21 percent during the Eocene-Oligocene and for present-day samples, with possibly lower values during the Oligocene-Early Miocene. Variable O2 levels over time in general confirm theoretical isotope-mass balance calculations and suggest that the atmosphere has evolved over Phanerozoic time.

Origin of late dolomite cement by CO2-saturated deep basin brines: Evidence from the Ozark region, central United States

Studies of fluid inclusions in regionally extensive late dolomite cement (LDC) throughout the Ozark region show that CO2 effervescence was widespread during dolomite precipitation. On the basis of quantitative analyses of inclusion fluids, reaction-path modeling shows that LDC with trace amounts of sulfides can be deposited by effervescence of a CO2-saturated basin brine as it migrates to shallower levels and lower confining pressures. This precipitation mechanism best explains occurrences of LDC in the Ozark region and may account for LDC found in sedimentary basins worldwide.

Harding Iceland spar: A new δ18O−δ13C carbonate standard for hydrothermal minerals

Abstract An isotopically homogeneous calcite, Harding Iceland Spar (HIS), having 6180 = +11.78 ± 0.07‰ (= +22.15‰ for CO2) and δ13C = −4.80 ± 0.02‰, has been prepared in quantities suitable for use as a working standard in mass spectrometric analysis. The isotopic values are well-suited for analysis of minerals of hydrothermal origin.

Analysis of gases in fossil amber

Fossilized tree resin, commonly known as amber, has long served as a robust source of paleobiologic information via the preservation of embalmed invertebrates. Another more radical idea is that gas bubbles enclosed in fossil amber may represent ancient air trapped at the time the original resin was exuded from its host tree. Alternatively, gas trapped in ancient amber may instead represent contamination by modern air. Here, we analyze gaseous inclusions from ambers sampled worldwide spanning the Cretaceous, Eocene, Eocene–Oligocene, Eocene–Miocene, U. Oligocene–Miocene, and modern. After careful consideration of a number of possible explanations, we suggest the gas inclusions may represent, at least in part, ancient air.

Localization of Tabular, Sediment-Hosted Uranium-Vanadium Deposits of Henry Structural Basin, Utah: ABSTRACT

Tabular uranium-vanadium mineralization characteristic of the Colorado Plateau occurs in fluvial sandstones of the Salt Wash Member of the Morrison Formation (Jurassic) within the Henry structural basin, south-central Utah. The ore consists of a mineralized interval (MI) of two closely spaced uranium and vanadium-rich zones separated by one barren of uranium but enriched in vanadium. No known stratigraphic feature controls the position of this MI which occurs at successively higher stratigraphic levels toward the interior of the basin. The dominant clay mineral throughout the MI is an unusual vanadium-rich di,trioctahedral chlorite. Laterally continuous with and below the MI, mixed-layer chlorite/smectite and illite/smectite (greater than 75% expandable layers) predominat . Above the MI, kaolinite in sandstone beds and illite/smectite plus kaolinite in bentonitic beds are the dominant authigenic clay minerals. The MI and its unmineralized lateral extensions are bounded, both above and below, by zones rich in authigenic dolomite cement. Petrographic evidence places the dolomite as pre-ore to contemporaneous with ore, and the chlorite contemporaneous with ore. Geochemical and mineralogical data, ^dgr18O to ^dgrD values of clay minerals and ^dgr18O to ^dgr13C values of dolomite indicate the presence of an interface between two isotopically and chemically distinct fluids. The lower fluid was typical of closed-basin evaporated brines with a high Mg/Ca ratio and high SO42- content. The upper fluid was meteo ic water. Elemental zoning patterns and isotopic data suggest that the upper (meteoric) fluid carried the uranium and vanadium to the solution interface, but that ore grade mineralization occurred only where the brine-meteoric water interface intersected horizons with anomalous concentrations of organic matter (dominantly detrital plant debris). End_of_Article - Last_Page 613------------