Carbon dioxide on the satellites of Saturn: Results from the Cassini VIMS investigation and revisions to the VIMS wavelength scale

Several of the icy satellites of Saturn show the spectroscopic signature of the asymmetric stretching mode of C-O in carbon dioxide (CO₂) at or near the nominal solid-phase laboratory wavelength of 4.2675 μm (2343.3 cm⁻¹), discovered with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft. We report here on an analysis of the variation in wavelength and width of the CO₂ absorption band in the spectra of Phoebe, Iapetus, Hyperion, and Dione. Comparisons are made to laboratory spectra of pure CO₂, CO₂ clathrates, ternary mixtures of CO₂ with other volatiles, implanted and adsorbed CO₂ in non-volatile materials, and ab initio theoretical calculations of CO₂ * nH₂O. At the wavelength resolution of VIMS, the CO₂ on Phoebe is indistinguishable from pure CO₂ ice (each molecule’s nearby neighbors are also CO₂) or type II clathrate of CO₂ in H₂O. In contrast, the CO₂ band on Iapetus, Hyperion, and Dione is shifted to shorter wavelengths (typically ∼4.255 μm (∼2350.2 cm⁻¹)) and broadened. These wavelengths are characteristic of complexes of CO₂ with different near-neighbor molecules that are encountered in other volatile mixtures such as with H₂O and CH₃OH, and non-volatile host materials like silicates, some clays, and zeolites. We suggest that Phoebe’s CO₂ is native to the body as part of the initial inventory of condensates and now exposed on the surface, while CO₂ on the other three satellites results at least in part from particle or UV irradiation of native H₂O plus a source of C, implantation or accretion from external sources, or redistribution of native CO₂ from the interior.The analysis presented here depends on an accurate VIMS wavelength scale. In preparation for this work, the baseline wavelength calibration for the Cassini VIMS was found to be distorted around 4.3 μm, apparently as a consequence of telluric CO₂ gas absorption in the pre-launch calibration. The effect can be reproduced by convolving a sequence of model detector response profiles with a deep atmospheric CO₂ absorption profile, producing distorted detector profile shapes and shifted central positions. In a laboratory blackbody spectrum used for radiance calibration, close examination of the CO₂ absorption profile shows a similar deviation from that expected from a model. These modeled effects appear to be sufficient to explain the distortion in the existing wavelength calibration now in use. A modification to the wavelength calibration for 13 adjacent bands is provided. The affected channels span about 0.2 μm centered on 4.28 μm. The maximum wavelength change is about 10 nm toward longer wavelength. This adjustment has implications for interpretation of some of the spectral features observed in the affected wavelength interval, such as from CO₂, as discussed in this paper.     

Statistical Feature Recognition for Multidimensional Solar Imagery

A maximum a posteriori (MAP) technique is developed to identify solar features in cotemporal and cospatial images of line-of-sight magnetic flux, continuum intensity, and equivalent width observed with the NASA/National Solar Observatory (NSO) Spectromagnetograph (SPM). The technique facilitates human understanding of patterns in large data sets and enables systematic studies of feature characteristics for comparison with models and observations of long-term solar activity and variability. The method uses Bayes’ rule to compute the posterior probability of any feature segmentation of a trio of observed images from per-pixel, class-conditional probabilities derived from independently-segmented training images. Simulated annealing is used to find the most likely segmentation. New algorithms for computing class-conditional probabilities from three-dimensional Gaussian mixture models and interpolated histogram densities are described and compared. A new extension to the spatial smoothing in the Bayesian prior model is introduced, which can incorporate a spatial dependence such as center-to-limb variation. How the spatial scale of training segmentations affects the results is discussed, and a new method for statistical separation of quiet Sun and quiet network is presented.     

International collaboration on marine bioinvasions--the ICES response

The International Council for the Exploration of the Sea (ICES) noted the risks associated with uncontrolled species introductions and transfers more than 40 years ago and launched two working groups to address the issue, i.e. the ICES Working Group on Introductions and Transfers of Marine Organisms (WGITMO) to deal with the movement of non-indigenous species for e.g. aquaculture purposes and the ICES/IOC/IMO Working Group on Ballast and Other Ship Vectors which focuses on species movements with ships. Both groups are actively working until today and the key achievements of the groups are outlined.     

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C₁–C₄, CO₂]). Molecular ratios of LMWHC (C₁/[C₂ + C₃] > 1000) and stable isotopic compositions of methane (δ¹³C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C₁/C₂₊ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C₁–C₄, CO₂]) relative to the other gas types and the virtual lack of ¹⁴C–CH₄ indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C₁–C₄, CO₂)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of ¹⁴C–CH₄ (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.     

In situ U–Pb dating of micro-baddeleyite by secondary ion mass spectrometry

We introduce a technique for U–Pb dating of baddeleyite using secondary ion mass spectrometry (SIMS) in situ analysis of ng-mass crystals that cannot be efficiently extracted by conventional mineral separation techniques. Average ²⁰⁷Pb/²⁰⁶Pb ages for Precambrian baddeleyite crystals are within < 0.3% of the respective isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages. ²⁰⁶Pb/²³⁸U ratios are corrected for instrumental fractionation calibrated through linear regression in a Pb/U relative sensitivity vs. UO₂⁺/U⁺ calibration plot. Calibration is performed on separated baddeleyite crystals (～ 100–200 μm in maximum dimension) mounted in random crystallographic orientation. ²⁰⁶Pb/²³⁸U ages for baddeleyite from Duluth gabbro (FC4b) and Kovdor are accurate within 1–2% when averaging 15–30 individual spot analyses and relative sensitivities calibrated on Phalaborwa baddeleyite. The relative difference of ²⁰⁶Pb/²³⁸U between large crystals and micro-baddeleyite from FC4b is within ～ 1%. Comparison between silicate glass and baddeleyite, as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in Pb/U sensitivity in baddeleyite due to variable crystal orientations. We successfully utilized oxygen flooding and a UO₂⁺/U⁺-based calibration to significantly reduce orientation dependent bias.     

In situ U–Pb SIMS (IN-SIMS) micro-baddeleyite dating of mafic rocks: Method with examples

An in situ U–Pb SIMS (IN-SIMS) method to date micro-baddeleyite crystals as small as 3 μm is presented with results from three samples that span a variety of ages and geologic settings. The method complements ID-TIMS geochronology by extending the range of dateable crystals to sizes smaller than can be recovered by physical separation. X-ray mapping and BSE imaging are used to locate target grains in thin section, followed by SIMS analysis on a CAMECA ims 1270, using the field aperture in the transfer column to screen out ions from host phases. Internal age precisions for the method are anticipated to range from 0.1% for Precambrian rocks to 3–7% for Phanerozoic rocks. Results establish a 2689 ± 5 Ma age for mafic dikes in the Wyoming craton, USA, a 1540 ± 30 Ma age for a subaerial lava flow from the Thelon Basin of northern Canada, and a 457 ± 34 Ma age for mafic dikes in the platform sequence of southeastern Siberia. The method is ideal for relatively non-destructive dating of small samples such as extraterrestrial rocks and precious terrestrial samples.     

Pediment response to drainage basin evolution in south-central Arizona

The Sonoran Desert portion of the Basin and Range physiographic province contains a number of streams that now flow across once-closed basins. We explore here the research questions of if and how granitic rock pediments respond to the transition from rimming endorheic basins to bordering through-flowing streams. Granitic rock pediments of the northern Usery and eastern McDowell Mountains once graded to the closed Miocene–Pliocene Pemberton basin that occupied the present-day location of the confluence of the Salt and Verde Rivers. The process of lake overflow, which integrated these rivers, first aggraded fill terraces that, in turn, caused aggradation of a mantle of transported grus on bedrock pediments. Subsequent episodic incision of the Salt and Verde rivers lowered the base level; this led to the development of erosional features such as rolling topography of a degrading pediment mantle; exposure of the former piedmont angle and its associated zones of enhanced bedrock decay and regolith carbonate; and exposure of spheroidally weathered bedrock and emerging tors, some of which experienced 20^th century erosion. The granitic pediments of the former Pemberton Basin, which now transport grus to the Salt and Verde rivers, have actively adjusted to aggradational and degradational events associated with drainage integration and do not appear to be inherited from an ancient wet climatic interval.     

Atlantic watermass and circulation response to persistent freshwater forcing in two coupled general circulation models

The sensitivity of the Atlantic circulation and watermasses to biases in the convergence of moisture into the basin is examined in this study using two different general circulation models. For a persistent positive moisture flux into the tropical Atlantic, the average salinity and temperature in the basin is reduced, mainly below mid-depths and in high latitudes. A transient reduction in the Atlantic overturning strength occurs in this case, with a recovery timescale of 1–2 centuries. In contrast, a similar amount of freshwater directed into the Subpolar North Atlantic results in a persistent reduction in overturning and an increase in basin heat and salt content. In the unperturbed pre-industrial simulations, the Atlantic is unambiguously warmer and saltier than historical observations below mid-depths and in the Nordic Seas. The models’ tropical freshwater flux sensitivities project strongly onto the spatial pattern of this bias, suggesting a common atmospheric deficiency. The integrated Atlantic plus Arctic surface freshwater flux in these models is between ?0.5 and ?0.6 Sv, compared with an observational estimate of ?0.28 Sv. Our results suggest that shortcomings in the models’ ability to reproduce realistic bulk watermass properties are due to an overestimation of the inter-basin moisture export from the tropical Atlantic.