Ring torque on janus and the melting of Enceladus

Voyager 2 images show parts of Enceladus' surface to be very smooth, lacking craters down to the resolution limit of 4 km. This absence of craters indicates geologically recent resurfacing, probably due to internal melting. However, calculations of current heating mechanisms, including radioactive decay and tidal heating due to Enceladus' resonance with Dione, yield heating rates too small to cause melting. The orbital mean motion of Janus (1980S1) is slightly less than twice that of Enceladus and, according to theoretical calculations, is currently decreasing as Janus' orbit evolves outward due to resonant torques from Saturn's rings. If Janus were ever locked into a stable 2:1 orbital commensurability with Enceladus, the resulting angular momentum transfer could have sufficiently enhanced the eccentricity of Enceladus' orbit for the ensuing tidal heating to have melted Enceladus' interior. The existence of a Laplace-like three-body resonance including Dione, although unlikely, would have increased heating. If Janus were indeed held in resonance with Enceladus until recently (10⁷–10⁸ years B.P.) when the lock was disrupted by an unspecified event (possibly a catastrophic collision which simultaneously created the coorbital pair, or by the influence of Dione) both the recent internal activity of Enceladus and the proximity of Janus to Saturn's rings may be explained. However, the predicted rapid time scale for ring evolution due to resonant torques from Saturn's inner moons remains a major problem.     

A long-term Arctic snow depth record from Abisko, northern Sweden, 1913–2004

A newly digitized record of snow depth from the Abisko Scientific Research Station in northern Sweden covers the period 1913-present. Mean snow depths were taken from paper records of measurements made on a profile comprising 10 permanent stakes. This long-term record yields snow depths consistent with two other shorter term Abisko records: measurements made at another 10-stake profile (1974-present) and at a single stake (1956-present). The measurement interval is variable, ranging from daily to monthly, and there are no data for about half of the winter months in the period 1930-1956. To fill the gaps, we use a simple snowpack model driven by concurrent temperature and precipitation measurements at Abisko. Model snow depths are similar to observed; differences between the two records are comparable to those between profile and single stake measurements. For both model and observed snow depth records, the most statistically significant trend is in winter mean snow depths, amounting to an increase of about 2 cm or 5 % of the mean per decade over the whole measurement period, and 10% per decade since the 1930-40s, but all seasonal means of snow depth show positive trends on the longest timescales. However, the start, end, and length of the snow season do not show any statistically significant long-term trends. Finally, the relation between the Arctic Oscillation index and Abisko temperature, precipitation and snow depth is positive and highly significant, with the best correlations for winter.     

Phosphorus Cycling and Burial in Sediments of a Seasonally Hypoxic Marine Basin

Recycling of phosphorus (P) from sediments contributes to the development of bottom-water hypoxia in many coastal systems. Here, we present results of a year-long assessment of P dynamics in sediments of a seasonally hypoxic coastal marine basin (Lake Grevelingen, the Netherlands) in 2012. Sequential phosphorus extractions (SEDEX) and X-ray absorption spectroscopy (XAS) indicate that P was adsorbed to Fe-(III)-(oxyhydr)oxides when cable bacteria were active in the surface sediments in spring. With the onset of summer hypoxia, sulphide-induced dissolution of the Fe-(III)-(oxyhydr)oxides led to P release to the pore water and overlying water. The similarity in authigenic Ca-P concentrations in the sediment and suspended matter suggest that Ca-P is not formed in situ. The P burial efficiency was ≤ 32%. Hypoxia-driven sedimentary P recycling had a major impact on the water-column chemistry in the basin in 2012. Water-column monitoring data indicate up to ninefold higher surface water concentrations of phosphate in the basin in the late 1970s and a stronger hypoxia-driven seasonal P release from the sediment. The amplified release of P from the sediment in the past is attributed to the presence of a larger pool of Fe-bound P in the basin prior to the first onset of hypoxia. Given that P is not limiting, primary production in the basin has not been affected by the decadal changes in P availability and recycling over the past 40 years. The changes in P dynamics on decadal time scales were not recorded in sediment profiles of total P or organic C/total P.     

Evaluation of the CORDEX-Africa multi-RCM hindcast: systematic model errors

Monthly-mean precipitation, mean (T_AVG), maximum (T_MAX) and minimum (T_MIN) surface air temperatures, and cloudiness from the CORDEX-Africa regional climate model (RCM) hindcast experiment are evaluated for model skill and systematic biases. All RCMs simulate basic climatological features of these variables reasonably, but systematic biases also occur across these models. All RCMs show higher fidelity in simulating precipitation for the west part of Africa than for the east part, and for the tropics than for northern Sahara. Interannual variation in the wet season rainfall is better simulated for the western Sahel than for the Ethiopian Highlands. RCM skill is higher for T_AVG and T_MAX than for T_MIN, and regionally, for the subtropics than for the tropics. RCM skill in simulating cloudiness is generally lower than for precipitation or temperatures. For all variables, multi-model ensemble (ENS) generally outperforms individual models included in ENS. An overarching conclusion in this study is that some model biases vary systematically for regions, variables, and metrics, posing difficulties in defining a single representative index to measure model fidelity, especially for constructing ENS. This is an important concern in climate change impact assessment studies because most assessment models are run for specific regions/sectors with forcing data derived from model outputs. Thus, model evaluation and ENS construction must be performed separately for regions, variables, and metrics as required by specific analysis and/or assessments. Evaluations using multiple reference datasets reveal that cross-examination, quality control, and uncertainty estimates of reference data are crucial in model evaluations.     

Fossilization and degradation of intact polar lipids in deep subsurface sediments: A theoretical approach

Intact polar membrane lipids (IPLs) are frequently used as markers for living microbial cells in sedimentary environments. The assumption with these studies is that IPLs are rapidly degraded upon cell lysis and therefore IPLs present in sediments are derived from in situ microbial production. We used a theoretical approach to assess whether IPLs in surface sediments can potentially represent fossilized IPLs derived from the upper part of the water column and whether IPLs can be preserved during sediment burial. Previous studies which examined the degradation kinetics of IPLs show that phospholipids, i.e. ester-linked lipids with a phosphor-containing head group, degrade more rapidly than glycosidic ether lipids, i.e. ether-linked lipids with a glycosidically bound sugar moiety. Based on these studies, we calculate that only a minor fraction of phospholipids but a major fraction of glycosidic ether lipids biosynthesized in the upper part of the water column can potentially reach deep-sea surface sediments. Using a simple model and power law kinetic degradation parameters reported in the literature, we also evaluated the degradation of IPLs during sediment burial. Our model predicts a log-log relationship between IPL concentrations and depth, consistent with what has been observed in studies of IPLs in subsurface sediments. Although our results do not exclude production of IPLs in subsurface sediment, they do suggest that IPLs present in the deep biosphere may contain a substantial fossil component potentially masking in situ IPL production.     

Contact metamorphism of impure dolomitic limestone in the Boulder Aureole,Montana

Progressive metamorphism of impure dolomitic limestone in the 1.5 to 2.5 km wide contact aureole surrounding the northernmost portion of the boulder batholith has resulted in a consistent sequence of uniformly distributed zones of low-variance mineral parageneses separated by abrupt and distinctive isograds. In silica-undersaturated, aluminous marbles, the following mineral assemblages occur, in order of increasing grade: calcite-dolomite-calcic amphibole-chlorite, calcite-dolomite-calcic amphibole-chlorite-spinel, calcite-dolomite-calcic amphibole-chlorite-olivine-spinel, calcite-dolomite-chlorite-olivine-spinel, calcite-dolomite-olivine-spinel. The spatial distribution of parageneses and the occurrence of low-variance parageneses indicate buffering of the pore fluid composition by the local mineral assemblages. The observed sequence of mineral reactions and the spacing of isograds is in good agreement with experimental and calculated equilibria in terms of P-T-X _{CO 2}and temperatures of equilibration inferred from calcite-dolomite geothermometry, which range from 435 to 607 °C across the aureole.Microprobe analyses of coexisting minerals indicate attainment of exchange equilibrium. Calcic amphibole and chlorite coexisting with calcite and dolomite become progressively more aluminous with increasing grade; calcic amphibole changes rapidly from Al-poor tremolite to pargasite, while Al^IV in Cte increases from 2.0 to 2.3 atoms per 8 tetrahedral sites. Observed low-variance assemblages fix the activities of calcic amphibole and chlorite end-member components as a function of P and T, and hence the systematic compositional variation in these phases is not an independent variable, but is controlled by the local mineral assemblage.     

Characteristics of asbestiform and non-asbestiform calcic amphiboles

In terms of morphology there are four major types of calcic amphibole; massive, prismatic, finely acicular and asbestos. Representatives of each of these types have been examined by optical microscopy, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Massive specimens (nephrite) consist of randomly oriented clusters of fine, roughly lath-shaped, sub-microscopic crystals; within each cluster the lath lengths (z) are approximately aligned but neighbouring laths are rotated with respect to one another. Finely acicular specimens (“byssolites”) have well-formed crystals bounded mainly by {110} (100) and (010) faces and characteristically have striations parallel to their lengths. Asbestiform varieties range from finer (flexible) to coarser (more brittle) specimens and many specimens contain a mixture of fine and coarse fibrils. The fibrils in a bundle are aligned parallel to z but are in a range of azimuthal orientations. It is inferred that they are formed by multiple independent nucleation and growth parallel to z rather than through parting or cleavage on {110} planes. (100) defect or twin planes, or on (010) planar defects.

The {110} cleavage in amphiboles is well reported but (100) features are rarely mentioned in the literature. Our observations reveal the importance of (100) as a cleavage or parting as well as the tendency in nephrites, byssolites and asbestos towards a lath-like (parallel to z) morphology with flattening on (100). In the latter varieties therefore, the y-direction is that of second fastest crystal growth, after z.

When subjected to moderate grinding, the comminution of asbestos fibres proceeds more by separation of fibrils and less by fracturing to shorter lengths as compared with prismatic and byssolite specimens. Prolonged grinding does, however, shorten lengths of even the least brittle asbestos.

Transmission electron microscopy revealed extensive sub-grain boundaries and dislocation networks (suggesting a deformation history) in all prismatic and nephrite specimens. Fine multiple (100) twinning was observed in asbestos but not in other varieties. Although chain-width defects [on (010)], with visibility enhanced by beam damage, were most abundant in nephrites and fibrous tremolites, there appears to be no completely consistent relationship between such features and morphological type.

Electron probe analyses showed that specimens that contain more than a very small amount of aluminium do not have asbestiform habit. Asbestos specimens also have lower contents of Mn, Na and K and have formulae closer to the ideal Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂. Small departures from this in asbestos involve Na in the A site compensated by Na for Ca rather than Al for Si whereas the reverse is true in byssolites. Chemical substitutions in prismatic specimens are much less constrained.

The characteristics of the four morphological sub-groups correlate reasonably well with what is known of their geological environments.