Negative δO values in Allan Hills 84001 carbonate: Possible evidence for water precipitation on mars     

G. Holland  J.M. Saxton  G. Turner 《Geochimica et cosmochimica acta》2005,69(5):1359-1370

The Martian meteorite ALH84001 contains ∼1% by weight of carbonate formed by secondary processes on the Martian surface or in the shallow subsurface. The major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere. This study concentrates upon carbonate regions ∼200 μm across which possess previously unobserved magnesium rich inner cores, interpreted here as rosette fragments, surrounded by a later stage cement containing rare Ca-rich carbonates (up to Ca₈₁Mg₀₇Fe₀₄Mn₀₇) intimately associated with feldspar. High spatial resolution ion probe analyses of Ca-rich carbonate surrounding rosette fragments have δ¹⁸O_V-SMOW values as low as −10‰. These values are not compatible with deposition from a global Martian atmosphere invoked to explain ALH84001 rosettes. The range of δ¹⁸O values are also incompatible with a fluid that has equilibrated with the Martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition. At Martian atmospheric temperatures, the small CO₂(gas)-CO₂(ice) fractionation makes meteoric CO₂ an unlikely source for −10‰ carbonates. In contrast, closed system Rayleigh fractionation of H₂O can generate δ¹⁸O_H2O −30‰, as observed at high latitudes on Earth. We suggest that atmospheric transport and precipitation of H₂O in a similar fashion to that on Earth provides a source of suitably ¹⁸O depleted water for generation of carbonate with δ¹⁸O_V-SMOW = −10‰.  相似文献   

Stratigraphy of Pleistocene glaciations in the St Elias Mountains,southwest Yukon,Canada          下载免费PDF全文

Derek G. Turner  Brent C. Ward  Duane G. Froese  Michel Lamothe  Jeffrey D. Bond  Nancy H. Bigelow 《Boreas: An International Journal of Quaternary Research》2016,45(3):521-536

At least five Middle to Late Pleistocene advances of the northern Cordilleran Ice Sheet are preserved at Silver Creek, on the northeastern edge of the St Elias Mountains in southwest Yukon, Canada. Silver Creek is located 100 km up‐ice of the Marine Isotope Stage (MIS) 2 McConnell glacial limit of the St Elias lobe. This site contains ~3 km of nearly continuous lateral exposure of glacial and non‐glacial sediments, including multiple tills separated by thick gravel, loess and tilted lake beds. Infrared‐stimulated luminescence (IRSL) and AMS radiocarbon dating constrain the glacial deposits to MIS 2, 4, either MIS 6 or mid‐MIS 7, and two older Middle Pleistocene advances. This chronology and the tilt of the lake beds suggest Pleistocene uplift rates of up to 1.9 mm a^?1 along the Denali Fault since MIS 7. The non‐glacial sediment consists of sand, gravel, loess and organic beds from MIS 7, MIS 3 and the early Holocene. The MIS 3 deposits date to between 30–36 ¹⁴C ka BP, making Silver Creek one of the few well‐constrained MIS 3‐aged sites in Yukon. This confirms that ice receded close to modern limits in MIS 3. Pollen and macrofossil analyses show that a meadow‐tundra to steppe‐tundra mosaic with abundant herbs and forbs and few shrubs or trees, dominated the environment at this time. The stratigraphy at Silver Creek provides a palaeoclimatic record since at least MIS 8 and comprises the oldest direct record of Pleistocene glaciation in southwest Yukon.  相似文献   

Hominins without fellow travellers? First appearances and inferred dispersals of Afro-Eurasian large-mammals in the Plio-Pleistocene     

H.J. O'Regan  A. Turner  L.C. Bishop  S. Elton  A.L. Lamb 《Quaternary Science Reviews》2011,30(11-12):1343-1352

Discoveries of fossil Homo outside Africa predating 1.0 Ma have generated much discussion about hominin dispersal routes. However, tool-using bipeds were only one element of the inter-continental mammalian dispersals that occurred during the climatic changes of the Pliocene and Pleistocene. This paper will place hominin movements in the context of those of the wider mammalian fauna, which includes carnivores, bovids and non-human primates. The distribution of these different taxa suggests that species moved individually when the environmental conditions were right for them, rather than in multi-species waves of dispersal, and allows evaluation of the contextual evidence for the newly emerging ‘Out of Asia’ paradigm as well as the established ‘Out of Africa’ model.  相似文献   

Distribution and Characteristics of Dinosaur Eggs in China     

HAO Ziguo  FEI Hongcai  HAO Qingqing  LIU Lian  Susan Turner 《《地质学报》英文版》2016,90(1):370-375

正China is one of the most prolific countries for dinosaur material in the world,from skeletons to footprints and eggs.At present,China has documented more than 160kinds of dinosaur,35 genera and 39 ichnospecies of dinosaur footprints,and 17 genera and 44 oospecies of dinosaur eggs.The dinosaur eggs in China are mainly distributed in Xinjiang,Inner Mongolia,Ningxia,Gansu,  相似文献   

Influence of the Mississippi River on <Emphasis Type="Italic">Pseudo-nitzschia</Emphasis> spp. Abundance and Toxicity in Louisiana Coastal Waters     

Sibel Bargu  Melissa M. Baustian  Nancy N. Rabalais  Ross Del Rio  Benjamin Von Korff  R. Eugene Turner 《Estuaries and Coasts》2016,39(5):1345-1356

The presence of domoic acid (DA) toxin from multiple species of Pseudo-nitzschia is a concern in the highly productive food webs of the northern Gulf of Mexico. We documented the Pseudo-nitzschia presence, abundance, blooms, and toxicity over three years along a transect ～100 km west of the Mississippi River Delta on the continental shelf. Pseudo-nitzschia were present throughout the year and occurred in high abundances (>10⁴ cells l^?1) in the early spring months during high Mississippi River (MSR) flow (～20,000 m³ s^?1) but were most abundant (>10⁶ cells l^?1) when MSR discharge was relatively lower among the spring months. A high particulate toxin production (maximum reaching 13 μg DA l^?1) was associated with the high cell abundances and exceeded, by an order of magnitude, prior reports of particulate DA concentrations in Louisiana coastal waters. Differences in Pseudo-nitzschia peak times and its toxicity were correlated mainly with the timing and magnitude of MSR discharge and changes in associated parameters such as nutrient stoichiometry and salinity. A negative relationship between high MSR discharge and Pseudo-nitzschia and particulate DA concentrations was documented. These riverine dynamics have the potential to influence DA contamination in pelagic and benthic food webs in the coastal waters of the northern Gulf of Mexico.  相似文献   

Geophysical surveys of the sediments of Loch Ness,Scotland: implications for the deglaciation of the Moray Firth Ice Stream,British–Irish Ice Sheet     

Andrew J. Turner  John Woodward  Stuart A. Dunning  Adrian J. Shine  Chris R. Stokes  Colm Ó Cofaigh 《第四纪科学杂志》2012,27(2):221-232

We present results from three geophysical campaigns using high‐resolution sub‐bottom profiling to image sediments deposited in Loch Ness, Scotland. Sonar profiles show distinct packages of sediment, providing insight into the loch's deglacial history. A recessional moraine complex in the north of the loch indicates initial punctuated retreat. Subsequent retreat was rapid before stabilisation at Foyers Rise formed a large stillstand moraine. Here, the calving margin produced significant volumes of laminated sediments in a proglacial fjord‐like environment. Subsequent to this, ice retreated rapidly to the southern end of the loch, where it again deposited a sequence of proglacial laminated sediments. Sediment sequences were then disturbed by the deposition of a thick gravel layer and a large turbidite deposit as a result of a jökulhlaup from the Spean/Roy ice‐dammed lake. These sediments are overlain by a Holocene sheet drape. Data indicate: (i) a former tributary of the Moray Firth Ice Stream migrated back into Loch Ness as a major outlet glacier with a calving margin in a fjord‐like setting; (ii) there was significant sediment supply to the terminus of this outlet glacier in Loch Ness; and (iii) that jökulhlaups are important for sediment supply into proglacial fjord/lake environments and may compose >20% of proglacial sedimentary sequences. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Tide gauge records,water level rise,and subsidence in the Northern Gulf of Mexico     

R. Eugene Turner 《Estuaries and Coasts》1991,14(2):139-147

Long-term water level changes in the northern Gulf of Mexico were examined using tide gauge records for this century. Strong coherence exists between the annual mean water changes at Galveston, Texas, and (1) the relatively geologically-stable west coast of Florida, (2) global mean sea level, and 93) the subsiding Louisiana coast. Water levels at the Galveston gauge, one of the longest records (81 yr), have risen steadily, but not accelerated over the long-term. The apparent acceleration of water rise in the recent two decades is within the historical pattern, and is probably driven by regional or global, but not local climatic factors. Because eustatic sea level has risen steadily this century, the analysis supports the conclusion that regional geologic subsidence has not varied significantly over the tide gauge record (1909–1988). Variations in the estimates of subsidence in the surface layers are generally consistent with the generally accepted understanding of the geology of deltaic processes on this coast.  相似文献   

The role of stratification in the deoxygenation of Mobile Bay and adjacent shelf bottom waters     

R. E. Turner  W. W. Schroeder  Wm. J. Wiseman 《Estuaries and Coasts》1987,10(1):13-19

Oxygen depletion in the shallow bottom waters of Mobile Bay, Alabama, and in adjacent nearshore and continental shelf waters, is shown to be directly related to the intensity of water column stratification. Low winds speeds are coincidental with the onset of water column stratification and the occurrence of hypoxic events. Hourly, daily, and seasonal changes in the relationship between percent oxygen saturation or oxygen concentration in the bottom waters and surface-bottom density differences indicate that the oxidized materials are recently formed, and not relic or overwintering carbon sources. The influence of density structure (water column stratification) in other oxygen-depleted coastal water masses is compared to Mobile Bay.  相似文献   

Ultimate removal mechanisms of elements from the ocean—a comment     

M. Whitfield  D.R. Turner 《Geochimica et cosmochimica acta》1982,46(10):1989-1992

Li (1981) has proposed a surface complexation model to rationalise the partitioning of the elements between the oceans and pelagic clays. In this comment we compare the resulting scattered correlations with the coherent and statistically significant correlations obtained from a simple electrostatic model.  相似文献   

An experimental investigation of volatile exsolution in evolving magma chambers     

J. Stewart Turner  Herbert E. Huppert  R.Stephen J. Sparks 《Journal of Volcanology and Geothermal Research》1983,16(3-4)

Previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas. Turbulent transfer of heat between a layer of dense, hot and volatile-rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer. Small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing, followed by further exsolution of gas and explosive eruption. These processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of CO₂. We have investigated both the initial overturning produced by gas release in the lower layer, and the subsequent evolution of gas due to intimate mixing of the two layers. The latter experiments, in which the reactants remained isolated in the two layers until overturning occurred, demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat. This shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other. The magmas can coexist in the same stratified chamber, until their bulk densities become equal and they mix together. The processes illustrated in these experiments could occur in H₂O-bearing magmas such as in the calcalkaline association and in CO₂-bearing mafic magmas such as in silica undersaturated suites.  相似文献