The two-step shape and timing of the last deglaciation in Antarctica

The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record.     

Spatially resolved methane band photometry of Saturn: I. Absolute reflectivity and center-to-limb variations in the 6190-, 7250-, and 8900-Å bands

We present spatially resolved measurements of Saturn's absolute reflectivity in methane bands at 6190, 7250, and 8900 Å and in nearby continuum regions. Images were obtained through narrow-band interference filters with a 500 × 500-pixel charge-coupled device. Band/continuum ratios were measured to high accuracy by referencing to the ring brightness in each image. Several data processing techniques enhanced the quality of the observations. These are the use of the ring symmetry to find center position and orientation, accurate subtraction of ring light, and constrained image deconvolution. Uncertainty in the continuum absolute reflectivity is ±10%. Uncertainties in band/continuum ratios are from one to several percent. The Equatorial Zone was much brighter than any other latitude in the strong 8900 band image. Northern mid-latitudes were brighter than southern mid-latitudes. The latter observation indicates fewer high-altitude aerosols in the south, a possible result of atmospheric dynamics or seasonal sublimation of NH₃ crystals. The data are tabulated and presented in a form suitable for quantitative scattering model analyses.     

Anisotropies of in-phase,out-of-phase,and frequency-dependent susceptibilities in three loess/palaeosol profiles in the Czech Republic; methodological implications

The relationship between the anisotropy of frequency-dependent magnetic susceptibility (fdAMS) and the anisotropy of out-of-phase magnetic susceptibility (opAMS) was investigated theoretically and also empirically at three loess/palaoesol profiles in Prague and in Southern Moravia. The data treatment was made in terms of mean susceptibility, degree of AMS, and orientations of principal susceptibilities. It has shown that the fdAMS and opAMS can serve as indicators of the preferred orientations of ultrafine magnetic particles that are on transition between superparamagnetic and stable single domain states in rocks, soils and environmental materials. In loess/palaeosol sequences, the fdAMS and opAMS correlate reasonably, because they are due to magnetic particles of similar grain sizes. The fdAMS and opAMS can be both coaxial with standard AMS (i.e. anisotropy of in-phase susceptibility - ipAMS) or non-coaxial indicating slightly different orientations of viscous magnetic particles.     

Refining the ice flow chronology and subglacial dynamics across the migrating Labrador Divide of the Laurentide Ice Sheet with age constraints on deglaciation

The Laurentide Ice Sheet was characterized by a dynamic polythermal base. However, important data and knowledge gaps have led to contrasting reconstructions in areas such as the Labrador Ice Divide. In this study, detailed fieldwork was conducted at the southeastern edge of a major landform boundary to resolve the relative ice flow chronology and constrain the evolution of the subglacial dynamics, including the migration and collapse of the Labrador Ice Divide. Surficial mapping and analysis of 94 outcrop‐scale ice flow indicators were used to develop a relative ice flow chronology. ¹⁰Be exposure ages were used with optical ages to confine the timing of deglaciation within the study area. Four phases of ice flow were identified. Flow 1 was a northeasterly ice flow preserved under non‐erosive subglacial conditions associated with the development of an ice divide. Flow 2 was a northwest ice flow, which we correlate to the Ungava Bay Ice Stream and led to a westward migration of the ice divide, preserving Flow 2 features and resulting in Flow 3's eastward‐trending indicators. Flow 4 is limited to sparse fine striations within and around the regional uplands. The new optical ages and ¹⁰Be exposure ages add to the regional geochronology dataset, which further constrains the timing of ice margin retreat in the area to around 8.0 ka. Copyright © 2019 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd.     

Fluid sources for the La Guitarra epithermal deposit (Temascaltepec district, Mexico): Volatile and helium isotope analyses in fluid inclusions

The La Guitarra deposit (Temascaltepec district, South-Central Mexico), belongs to the low/intermediate sulfidation epithermal type, has a polymetallic character although it is currently being mined for Ag and Au. The mineralization shows a polyphasic character and formed through several stages and sub-stages (named I, IIA, IIB, IIC, IID, and III). The previous structural, mineralogical, fluid inclusion and stable isotope studies were used to constrain the selection of samples for volatile and helium isotope analyses portrayed in this study. The N₂/Ar overall range obtained from analytical runs on fluid inclusion volatiles, by means of Quadrupole Mass Spectrometry (QMS), is 0 to 2526, and it ranges 0 to 2526 for stage I, 0 to 1264 for stage IIA, 0 to 1369 for stage IIB, 11 to 2401 for stage IIC, 19 to 324 for stage IID, and 0 to 2526 for stage III. These values, combined with the CO₂/CH₄ ratios, and N₂-He-Ar and N₂-CH₄-Ar relationships, suggest the occurrence of fluids from magmatic, crustal, and shallow meteoric sources in the forming epithermal vein deposit. The helium isotope analyses, obtained by means of Noble Gas Mass Spectrometry, display R/Ra average values between 0.5 and 2, pointing to the occurrence of mantle-derived helium that was relatively diluted or “contaminated” by crustal helium. These volatile analyses, when correlated with the stable isotope data from previous works and He isotope data, show the same distribution of data concerning sources for mineralizing fluids, especially those corresponding to magmatic and crustal sources. Thus, the overall geochemical data from mineralizing fluids are revealed as intrinsically consistent when compared to each other.The three main sources for mineralizing fluids (magmatic, and both deep and shallow meteoric fluids) are accountable at any scale, from stages of mineralization down to specific mineral associations. The volatile and helium isotope data obtained in this paper suggest that the precious metal-bearing mineral associations formed after hydrothermal pulses of predominantly oxidized magmatic fluids, and thus it is likely that precious metals were carried by fluids with such origin. Minerals from base-metal sulfide associations record both crustal and magmatic sources for mineralizing fluids, thus suggesting that base metals could be derived from deep leaching of crustal rocks. At the La Guitarra epithermal deposit there is no evidence for an evolution of mineralizing fluids towards any dominant source. Rather than that, volatile analyses in fluid inclusions suggest that this deposit formed as a pulsing hydrothermal system where each pulse or set of pulses accounts for different compositions of mineralizing fluids.The positive correlation between the relative content of magmatic fluids (high N₂/Ar ratios) and H₂S suggests that the necessary sulfur to carry mostly gold as bisulfide complexes came essentially from magmatic sources. Chlorine necessary to carry silver and base metals was found to be abundant in inclusion fluids and although there is no evidence about its source, it is plausible that it may come from magmatic sources as well.     

Ultra-Deep Optical Spectroscopy with PMAS. Using the Nod-and-Shuffle Technique

PMAS, the Potsdam Multi-Aperture Spectrophotometer, is a new integral field spectrograph in the optical, which is optimized for good transmission and high image quality from 350 nm to 1 μm. We present our plan to implement a CCD charge-shuffle mode to allow for beam switching with a very high degree of sky subtraction accuracy for faint object 3-D spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detection of new hydrocarbons in Uranus' atmosphere by infrared spectroscopy

Ethane (C₂H₆), methylacetylene (CH₃C₂H or C₃H₄) and diacetylene (C₄H₂) have been discovered in Spitzer 10-20 μm spectra of Uranus, with 0.1-mbar volume mixing ratios of (1.0±0.1)×¹⁰⁻⁸, (2.5±0.3)×¹⁰⁻¹⁰, and (1.6±0.2)×¹⁰⁻¹⁰, respectively. These hydrocarbons complement previously detected methane (CH₄) and acetylene (C₂H₂). Carbon dioxide (CO₂) was also detected at the 7-σ level with a 0.1-mbar volume mixing ratio of (4±0.5)×¹⁰⁻¹¹. Although the reactions producing hydrocarbons in the atmospheres of giant planets start from radicals, the methyl radical (CH₃) was not found in the spectra, implying much lower abundances than in the atmospheres of Saturn or Neptune where it has been detected. This finding underlines the fact that Uranus' atmosphere occupies a special position among the giant planets, and our results shed light on the chemical reactions happening in the absence of a substantial internal energy source.     

Error analyses of resonant orbits for geodesy

An error analysis of resonant orbits for geodesy indicates that attempts to use resonance to recover high order geopotential coefficients may be seriously hampered by errors in the geopotential. This effect, plus the very high correlations (up to .99) of the resonant coefficients with each other and the orbital period in single satellite solutions, makesindividual resonant orbits of limited value for geodesy. Multiple-satellite, single-plane solutions are only a slight improvement over the single satellite case. Accurate determination of high order coefficients from low altitude resonant satellites requires multiple orbit planes and small drift-periods to reduce correlations and effects of errors of non-resonant geopotential terms. Also, the effects of gravity model errors on low-altitude resonant satellites make the use of tracking arcs exceeding two to three weeks of doubtful validity. Because high-altitude resonant orbits are less affected by non-resonant terms in the geopotential, much longer tracking arcs can be used for them.