Tropical Glacier and Ice Core Evidence of Climate Change on Annual to Millennial Time Scales

This paper examines the potential of the stable isotopic ratios, ¹⁸O/¹⁶O ( ¹⁸O_ice)and ²H/¹H ( D_ice), preserved in mid to low latitude glaciers as a toolfor paleoclimate reconstruction. Ice cores are particularly valuable as they contain additional data, such as dust concentrations, aerosol chemistry, and accumulation rates, that can be combined with the isotopic information to assist with inferences about the regional climate conditions prevailing at the time of deposition. We use a collection of multi-proxy ice core histories to explore the ¹⁸O-climate relationship over the last 25,000 years that includes both Late Glacial Stage (LGS) and Holocene climate conditions. These results suggest that on centennial to millennial time scales atmospheric temperature is the principal control on the ¹⁸O_ice of the snowfall that sustains these high mountainice fields.Decadally averaged ¹⁸O_ice records from threeAndean and three Tibetan ice cores are composited to produce a low latitude ¹⁸O_ice history for the last millennium. Comparison ofthis ice core composite with the Northern Hemisphere proxy record (1000–2000A.D.) reconstructed by Mann et al. (1999) and measured temperatures(1856–2000) reported by Jones et al. (1999) suggests the ice cores have captured the decadal scale variability in the global temperature trends. These ice cores show a 20th century isotopic enrichment that suggests a large scale warming is underway at low latitudes. The rate of this isotopically inferred warming is amplified at higher elevations over the Tibetan Plateau while amplification in the Andes is latitude dependent with enrichment (warming) increasing equatorward. In concert with this apparent warming, in situobservations reveal that tropical glaciers are currently disappearing. A brief overview of the loss of these tropical data archives over the last 30 years is presented along with evaluation of recent changes in mean ¹⁸O_ice composition. The isotopic composition of precipitation should be viewed not only as a powerful proxy indicator of climate change, but also as an additional parameter to aid our understanding of the linkages between changes in the hydrologic cycle and global climate.     

Characteristics of Autumn Precipitation Trends in the Northeastern United States

Previous research has shown that the northeastern United States experienced a significant increase in precipitation during the twentieth century, especially during the autumn. This shift toward wetter conditions can be explained by examining trends in daily precipitation characteristics. Early season precipitation changes in the western portion of the study area were associated with increases in the frequency of precipitation‐days. Later in the season a coincident increase in precipitation‐day intensity enhanced these trends. Tropical storms had little influence on autumn precipitation increases. The study suggests that, instead, changes in regional atmospheric circulation may be responsible for changing precipitation characteristics.     

Advection and removal of Pb and stable Pb isotopes in the oceans: a general circulation model study

Natural Pb-isotope variability in the oceans encodes information about the sources of continental material to the oceans, about ocean circulation, and about Pb removal. In order to use this information, we must understand the natural cycle of Pb in the oceans, which is overprinted by large anthropogenic input. In this study we use ²¹⁰Pb, which has not been significantly anthropogenically perturbed, to investigate oceanic Pb. GEOSECS ²²⁶Ra and model-derived atmospheric fluxes of ²¹⁰Pb are used to input ²¹⁰Pb into an ocean general circulation model. Irreversible scavenging of this ²¹⁰Pb onto settling biogenic particles and at the seafloor are tuned so that the model replicates the observed pattern of ²¹⁰Pb in the oceans. The best-fit model run provides a map of the variability of residence time for Pb. The global average residence time of Pb in this model is 48 yr, but there is over an order of magnitude variation between areas of high and low productivity. This is expected to enhance provinciality of Pb isotope ratios in the oceans. Because stable Pb isotopes are expected to behave in seawater in a similar fashion to ²¹⁰Pb, the pattern of removal of ²¹⁰Pb derived by the model can be used to investigate the behavior of stable Pb isotopes. We use a simplified input of Pb consisting of five point sources representing major rivers and a disseminated dust input. Although this simplified input scheme does not allow precise reconstruction of Pb concentration and isotopes in the oceans, it allows us to answer some first-order questions about the behavior of Pb as an ocean tracer. With a total Pb input of 6.3 × 10⁷ mol/yr (Chow T. J. and Patterson C. C., “The occurrence and significance of Pb isotopes in pelagic sediments,” Geochim. Cosmochim. Acta26, 263-308, 1962), the model predicts natural seawater Pb concentrations averaging 2.2 pmol/kg. Even in the absence of anthropogenic input, the model ocean exhibits a near-surface maximum in Pb concentration. And the model suggests natural Pb concentrations in the Northern Hemisphere an order of magnitude higher than in the Southern Hemisphere. A point source of Pb is suggested to dominate the seawater Pb inventory close to the source but is reduced to typically less than 10% of the inventory by the time that Pb is advected out of the originating ocean. This length scale of advection for Pb isotope signals confirms their potential as tracers of ocean circulation. Assuming an 8% dissolution of dust, their input to the open ocean are seen to be a significant portion of Pb inventory throughout the oceans and make up >50% of the Pb inventory in the model’s Southern Ocean. Finally, a preliminary investigation of the response of Pb-isotope distributions to changes in boundary conditions between glacial and interglacial times illustrates that significant variation in the Pb isotopes are expected in some regions, even for reasonably small changes in climate conditions.     

Petrogenesis of the magmatic complex at Mount Ascutney,Vermont, USA

The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had ⁸⁷Sr/⁸⁶Sr=0.7039, ¹⁴³Nd/¹⁴⁴Nd=0.512678 ( _Nd=+3.8) and ¹⁸O=6.1, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in ¹⁸O, to about 7.8, is the result of fractional crystallization. Mafic rocks show a large range in initial ¹⁴³Nd/¹⁴⁴Nd ratio, from about 0.51267 to 0.51236 ( _Nd= +3.7 to –2.5), which is correlated with elevated ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement.The Precambrian gneisses of the Chester dome and overlying lower Paleozoic schists have essentially identical Nd isotope systematics which suggest a crustal formation age of about 1.6. b.y. The parental sediments for the schists were apparently derived from a protolith similar to the gneissic basement without appreciable Sm/Nd fractionation.     

High-resolution 40Ar/39Ar chronology of multiple intrusion igneous complexes

The Mount Brome complex of the Monteregian province of southern Quebec, Canada, consits of several major intrusions ranging compositionally from gabbro to syenite. The relative ages of these intrusives have been investigated with high-resolution ⁴⁰Ar/³⁹Ar analyses, including a specially designed irradiation configuration to cancel the effects of fluence gradients. Small yet distinct apparent age differences are observed. While a number of analytical and geological factors could be proposed to explain the small variations, evaluation of these suggests the age differences reflect those in emplacement times. The gabbro and nepheline diorite were emplaced within a short span 123.1 Ma ago. Generally more evolved lithologies (biotite monzodiorite, pulaskite, nordmarkite) appear to have been emplaced within a restricted interval 1.4±0.3 Ma later. Whole-rock Rb-Sr systematics do not give acceptable isochrons because of significant scatter interpreted to reflect initial ⁸⁷Sr/⁸⁶Sr heterogeneities resulting from crustal contamination. Considering the variations in initial ratio, the Rb-Sr data are consistent with the ⁴⁰Ar/³⁹Ar age.     

A comparison of model calculations and measurements of acetone in the troposphere and stratosphere

We present 1-D eddy diffusion model calculations of the distributions of propane and acetone in the atmosphere for continental conditions. The magnitude of the surface seasonal variation in propane mixing ratios that we obtain is in general agreement with measurements at the surface and in the free troposphere. A comparison of the absolute values of the model with propane measurements suggests that a larger surface flux than we have used may be more appropriate for continental conditions. The acetone model results for summer conditions that we obtain are also in reasonable accord with measurements. However, we find serious disagreement between the model winter profiles of acetone and the measurements at the tropopause and in the lower stratosphere. The measured values are lower than the model values at 45° N by a factor of 7–30. In addition, it is also surprising that, given the relatively long lifetime of acetone, free tropospheric values of acetone more representative of surface values have not been measured. The results simulating the decay of elevated levels of propane in the upper troposphere caused by rapid convective transport of boundary layer air indicate that propane will be primarily dispersed by transport rather than destroyed photochemically. Thus, the impact on acetone and PAN is minimal.     

The behaviour of tungsten in granitic melt-vapour systems

An experimental study has been carried out to determine the effect of solution composition on the partitioning behaviour of tungsten in granitic melt-vapour systems at 800° C and 1 kbar. With chloride and phosphate solutions, tungsten partitions strongly into the aqueous phase, whereas with fluoride, carbonate and borate solutions, and water alone, tungsten partitions in favour of the melt. With chloride solutions, the fluid/melt partition coefficients (K _D) for W show a marked positive correlation with chloride concentration, and suggest that at low chloride concentrations W-Cl complexes with low Cl∶W ratios (such as associated equivalents of (WO₃)₂C1⁻) may be present. In contrast, at higher chloride concentrations complexes with high Cl∶W ratios (such as WOCl₄, WCl₆ and associated ionic equivalents) may predominate. With phosphate solutions, K _D shows little variation with phosphate concentration, and phosphorus heteropolytungstates (such as H₃[PW₁₂O₄₀]) may be present. There is no evidence to suggest that fluoride, carbonate or borate complexes of tungsten are important under the experimental conditions: the data for these compositions can be interpreted assuming that isopolytungstates (such as H₆[H₂W₁₂O₄₀]) are present. Within high temperature hydrothermal solutions tungsten may be transported principally as isopolytungstates and heteropolytungstates in addition to chloride complexes, and this may, in part, account for the common association of apatite and arsenopyrite with scheelite and wolframite in tungsten deposits.     

Temperature-dependent Fe2+–Mn2+ order–disorder behaviour in amphiboles

The partitioning of Fe and Mn between the large M(4) site and the octahedral sites, M(1,2,3) in the amphibole structure has been investigated in two natural manganogrunerites of compositions Ca_0.1Mn_1.9 Mg_1.25Fe²⁺ _3.56Fe³⁺ _0.38Si_7.81O₂₂(OH)₂ and Ca_0.24Mn_1.57 Mg_2.27 Fe²⁺ _2.76Fe³⁺ _0.32Si_7.84O₂₂(OH)₂. The long-range cation distribution in the two samples has been elucidated by in situ neutron powder diffraction revealing that Mn is preferentially ordered onto M(4) ? M(2) >M(1) >M(3) in both samples. Partitioning of Mn from M(4) into the octahedral sites begins at 350 °C, with site exchange energies of ?16.6 kJ mol^?1 and ?14.9 kJ mol^?1, in samples containing 1.90 and 1.57 Mn apfu, respectively. Mössbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mössbauer data agree well with the diffraction results, confirming that high-temperature cation distributions are retained during cooling. The fine structure in the hydroxyl-stretching region of the IR absorption spectra has been used to discuss qualitatively the site occupancies of the coordinating M(1)M(3)M(1) triplet, linked by O(3). On the basis of such modelling, we conclude that a degree of local clustering is present in both samples.     

A Novel Surrogate-Assisted Multi-objective Optimization Method for Well Control Parameters Based on Tri-Training

Natural Resources Research - Multi-objective well control optimization, which considers simultaneous optimization of more than one objective, has become an essential tool for efficient intelligent...