Six-color observations of the Eclipsing binary AR Cas

Depths and phases of both minima of AR Cas in six colors in 1959 are presented. Analysis leads to a color distribution for the secondary component which corresponds to spectral type A5–A7.     

Black Sea chemocline oscillations during the Holocene: molecular and isotopic studies of marginal sediments

We measured δ¹³C values of free and sulfur-bound lipids and framboidal pyrite-size distributions in three sediment cores from the southern margins of the Black Sea. The margin cores show a marked difference in the occurrence of biomarkers from green sulfur bacteria compared with the deep-basin cores, as a result of deepening of the chemocline resulting from enhanced mixing and/or decreased light-penetration as a consequence of high turbidity and productivity in shelf waters. Quantitation of biomarkers suggests that photic-zone anoxia along the shelf margin was generally absent during the deposition of unit I, although occurred during the deposition of Unit IIb at two sites.     

Magnetic study of serpentinized harzburgites from the Islas Orcadas Fracture Zone

Magnetic properties and bulk densities of 27 serpentinized harzburgite samples from the Islas Orcadas Fracture Zone, located in the vicinity of the Bouvet Triple Junction, have been measured and analyzed. Polished sections were examined using reflected light and scanning electron microscopy to characterize the size and geometric arrangement of opaque minerals. The relationship between the saturation magnetization (I_S) and remanent coercive force (H_R) is considered in terms of the amount of ferrimagnetic material and maghemitization. A suite of continental serpentinites from Canada is offered as contrast, to consider the role of weathering and maghemitization. Magnetite in the Islas Orcadas serpentinites is variably maghemitized, whereas continental serpentinites do not appear to contain maghemitized oxides. We verify this with optical microscopy, thermomagnetic analyses and cryogenic temperature cycling of saturation remanence. Maghemitization serves to reduce initial magnetic susceptibility, and introduce error in the use of I_S to evaluate the magnetic mode of magnetite. The presence of maghemite and the existence of a three dimensional vein network for magnetite geometry would suggest that magnetic hysteresis parameters can not reliably indicate grain size. Magnetic hysteresis ratios fall in a restricted range regardless of coercivity. The apparent grain size configured in a three dimensional vein network plus maghemitization might be responsible for this observation. Maghemitization does not affect thermal magnetic stability and enhances the geophysical importance of remanence in serpentinites. Paleomagnetic data suggest that important information about the geologic circumstances for oceanic rock serpentinization is embodied in the paleomagnetic records. This observation may be very important for generation of long wavelength aeromagnetic and possibly even satellite magnetic anomalies.     

Provenance of altered carbon phases and impact history of the Stac Fada Member,NW Scotland

The Stac Fada Member (Stoer Group) is a ~1.2 Ga melt-rich impact breccia preserved and intermittently exposed along the NW coast of Scotland. Using a combination of x-ray diffraction and micro-Raman spectroscopy, we identify potential coesite that is spatially associated with micron-sized diamonds, as well as disordered carbon phases. Comparing the graphite G-band of disordered carbon phases in the impact breccia to samples from underlying units indicates that most of the carbon in the Stoer Group was ultimately derived from the underlying Lewisian basement. Disordered carbon phases within the Stac Fada Member have been modified by mild heating within a hot ejecta blanket rather than shock pressure. We also report the first evidence for impact diamonds discovered within the Stac Fada Member. These diamonds have an average Raman shift of 1328.5 cm⁻¹ and are present within both the impact breccia and the shocked gneiss clasts that are present in sandstones directly underlying the Stac Fada Member contact, and within sandstone rafts entrapped in the unit. These findings have implications for the timing of deposition of the Stac Fada Member, which must have occurred after ballistic ejection of Lewisian basement clasts during the impact event.     

Deglaciation, basin formation and post-glacial climate change from a regional network of sediment core sites in Ohio and eastern Indiana

Many paleoclimate and landscape change studies in the American Midwest have focused on the Late Glacial and early Holocene time periods (~ 16–11 ka), but little work has addressed landscape change in this area between the Last Glacial Maximum and the Late Glacial (~ 22–16 ka). Sediment cores were collected from 29 new lake and bog sites in Ohio and Indiana to address this gap. The basal radiocarbon dates from these cores show that initial ice retreat from the maximal last-glacial ice extent occurred by 22 ka, and numerous sites that are ~ 100 km inside this limit were exposed by 18.9 ka. Post-glacial environmental changes were identified as stratigraphic or biologic changes in select cores. The strongest signal occurs between 18.5 and 14.6 ka. These Midwestern events correspond with evidence to the northeast, suggesting that initial deglaciation of the ice sheet, and ensuing environmental changes, were episodic and rapid. Significantly, these changes predate the onset of the Bølling postglacial warming (14.8 ka) as recorded by the Greenland ice cores. Thus, deglaciation and landscape change around the southern margins of the Laurentide Ice Sheet happened ~ 7 ka before postglacial changes were felt in central Greenland.     

The role of biodegradation and photo-oxidation in the transformation of terrigenous organic matter

Microbial and photochemical decomposition are two major processes regulating organic matter (OM) transformation in the global carbon cycle. However, photo-oxidation is not as well understood as biodegradation in terms of its impact on OM alteration in terrigenous environments. We examined microbial and photochemical transformation of OM and lignin derived phenols in two plant litters (corn leaves and pine needles). Plant litter was incubated in the laboratory over 3 months and compositional changes to OM were measured using nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry. We also examined the susceptibility of soil organic matter (SOM) to ultraviolet (UV) radiation. Solid-state ¹³C NMR spectra showed that O-alkyl type structures (mainly from carbohydrates) decreased during biodegradation and the loss of small carbohydrates and aliphatic molecules was observed by solution-state ¹H NMR spectra of water extractable OM from biodegraded litters. Photochemical products were detected in the aliphatic regions of NaOH extracts from both litter samples by solution-state ¹H NMR. Photo-oxidation also increased the solubility of SOM, which was attributed to the enhanced oxidation of lignin derived phenols and photochemical degradation of macromolecular SOM species (as observed by diffusion edited ¹H NMR). Overall, our data collectively suggests that while biodegradation predominates in litter decomposition, photo-oxidation alters litter OM chemistry and plays a role in destabilizing SOM in soils exposed to UV radiation.     

Re-examination of the formation ages of the Apollo 16 regolith breccias

The lunar regolith is exposed to irradiation from the solar wind and to bombardment by asteroids, comets and inter-planetary dust. Fragments of projectiles in the lunar regolith can potentially provide a direct measure of the sources of exogenous material being delivered to the Moon. Constraining the temporal flux of their delivery helps to address key questions about the bombardment history of the inner Solar System.Here, we use a revised antiquity calibration (after Eugster et al., 2001) that utilises the ratio of trapped ⁴⁰Ar/³⁶Ar (‘parentless’ ⁴⁰Ar derived from radioactive decay of ⁴⁰K, against solar wind derived ³⁶Ar) to semi-quantitatively calculate the timing of the assembly of the Apollo 16 regolith breccias. We use the trapped ⁴⁰Ar/³⁶Ar ratios reported by McKay et al. (1986). Our model indicates that the Apollo 16 ancient regolith breccia population was formed between ∼3.8 and 3.4 Ga, consistent with regoliths developed and assembled after the Imbrium basin-forming event at ∼3.85 Ga, and during a time of declining basin-forming impacts. The material contained within the ancient samples potentially provides evidence of impactors delivered to the Moon in the Late-Imbrian epoch. We also find that a young regolith population was assembled, probably by local impacts in the Apollo 16 area, in the Eratosthenian period between ∼2.5 and 2.2 Ga, providing insights to the sources of post-basin bombardment. The ‘soil-like’ regolith breccia population, and the majority of local Apollo 16 soils, were likely closed in the last 2 Ga and, therefore, potentially provide an archive of projectile types in the Eratosthenian and Copernican periods.     

Production of n-alkyl lipids in living plants and implications for the geologic past

Leaf waxes (i.e., n-alkyl lipids or n-alkanes) are land-plant biomarkers widely used to reconstruct changes in climate and the carbon isotopic composition of the atmosphere. There is little information available, however, on how the production of leaf waxes by different kinds of plants might influence the abundance and isotopic composition of n-alkanes in sedimentary archives. This lack of information increases uncertainty in interpreting n-alkyl lipid abundance and δ¹³C signals in ancient settings. We provide here n-alkyl abundance distributions and carbon isotope fractionation data for deciduous and evergreen angiosperm and gymnosperm leaves from 46 tree species, representing 24 families. n-Alkane abundances are significantly higher in angiosperms than gymnosperms; many of the gymnosperm species investigated did not produce any n-alkanes. On average, deciduous angiosperms produce 200 times more n-alkanes than deciduous gymnosperms. Although differences between angiosperms and gymnosperms dominate the variance in n-alkane abundance, leaf life-span is also important, with higher n-alkane abundances in longer-lived leaves. n-Alkanol abundances covary with n-alkanes, but n-alkanoic acids have similar abundances across all plant groups. Isotopic fractionation between leaf tissue and individual alkanes (ε_lipid) varies by as much as 10‰ among different chain lengths. Overall, ε_lipid values are slightly lower (−4.5‰) for angiosperm than for gymnosperm (−2.5‰) n-alkanes. Angiosperms commonly express slightly higher Δ_leaf (photosynthetic discrimination) relative to gymnosperms under similar growth conditions. As a result, angiosperm n-alkanes are expected to be generally 3-5‰ more depleted in ¹³C relative to gymnosperm alkanes for the same locality. Differences in n-alkane production indicate the biomarker record will largely (but not exclusively) reflect angiosperms if both groups were present, and also that evergreen plants will likely be overrepresented compared with deciduous ones. We apply our modern lipid abundance patterns and ε_lipid results to constrain the magnitude of the carbon isotope excursion (CIE) at the onset of the Paleocene-Eocene Thermal Maximum (55.8 Ma). When Bighorn Basin (WY) sediment n-alkanes are interpreted in context of floral changes and modern n-alkane production estimates for angiosperms and gymnosperms, the CIE is greater in magnitude (−5.6‰) by ∼1‰ compared to previous estimates that do not take into account n-alkane production.