William Smith's map brought alive by digital remastering

The geological mapping carried out by William Smith, which resulted in the publication of his famous map in 1815, was remarkable in many respects, not least because it relied on him being able to make consistent and accurate observations on the rock types he encountered during his fieldwork. This ability, gained from his many years studying rocks, allowed him to observe features with his own eyes (or at the very least, with the aid of a simple magnifying device) that others could not. We take a new look at William Smith's original stratigraphical sequences, and with samples collected from his classic field areas (many of which are around the city of Bath, Somerset, UK), demonstrate how spatial mineralogy mapping can be incorporated into the modern age of digital mapping.     

A numerical method for obtaining multiple linear regression parameters with physically realistic signs and magnitudes: Applications to the determination of equilibrium constants from solubility data

A common problem in experimental geochemistry is the derivation of equilibrium constants from solubility experiments. A simple method of deriving these equilibrium constants, multiple linear regression, often results in the appearance of negative values. This has been a significant obstacle to continuing research in this field. The problem occurs for the most part because of significant correlations among the “independent” concentration variables. These correlations are an inescapable result of the nature of the experiments and the physical model being fitted. Ridge regression is an appropriate modification to simple linear regression which overcomes this difficulty. Ridge regression results in a simple procedure to obtain physically plausible, yet statistically rigorous stability constants. Of course, other problems may further degrade the quality of derived equilibrium constants, e.g. uncertainty in activity coefficients and no purely statistical method can overcome these types of problems. However, ridge regression is an effective procedure to overcome the multicolinearity which is the main cause of negative equilibrium constants. We demonstrate the use of ridge regression with a general mathematical model and then illustrate its use in the determination of iron-chloro complex equilibrium constants from solubility studies of pyrite-pyrrhotite-magnetite in NaCl solutions at 250°C. Ridge regression may also be of use in other geochemical problems where one must estimate parameters with a physical interpretation and where the independent variables are significantly intercorrelated.     

Chromite-plagioclase assemblages as a new shock indicator; implications for the shock and thermal histories of ordinary chondrites

Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988°C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20-300 μm and consist of 0.2-20-μm-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al₂O₃ than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5-2 μm thick and 10-300 μm long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich “chondrules” (consisting mainly of olivine, plagioclase-normative mesostasis, and 5-15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher-energy shock events. The melt was jetted from the impact site and formed droplets due to surface tension. Crystallization of these droplets may have commenced in flight, prior to landing on the parent-body surface.Chromite-plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock-stage S1 OC of petrologic type 5 and 6 that I examined. Although these rocks contain unstrained olivine with sharp optical extinction, most possess other shock indicators such as extensive silicate darkening, numerous occurrences of metallic Cu, polycrystalline troilite, and opaque veins. It seems likely that these rocks were shocked to levels at least as high as shock-stage S3 and then annealed by heat generated during the shock event. During annealing, the olivine crystal lattices healed but other shock indicators survived. Published Ar-Ar age data for some S1 OC indicate that many shock and annealing events occurred very early in the history of the parent asteroids. The common occurrence of shocked and annealed OC is consistent with collisions being a major mechanism responsible for metamorphosing OC.     

Initial Al/Al in carbonaceous-chondrite chondrules: too little Al to melt asteroids

We report ²⁶Mg excesses correlated with Al/Mg ratios in five chondrules from the primitive CO3.0 chondrite Yamato 81020 that yield a mean initial ²⁶Al/²⁷Al ratio of only (3.8 ± 0.7) × 10⁻⁶, about half that of ordinary chondrite (OC) chondrules. Even if asteroids formed immediately after chondrule formation, this ratio and the mean Al content of CO chondrites is only capable of raising the temperature of a well-insulated CO asteroid to 940 K, which is more than 560 K too low to produce differentiation. The same ratio combined with the higher Al content of CV chondrites results in a CV asteroid temperature of 1100 K. We calculate that the mean initial ²⁶Al/²⁷Al ratio of about 7.4 × 10⁻⁶ found in LL chondrules is only able to produce small amounts of melting, too little to produce differentiation. These results cast serious doubt on the viability of ²⁶Al as the heat source responsible for asteroid differentiation. Inclusion of ⁶⁰Fe raises temperatures about 160 K, but this increment is not enough to cause differentiation, even of an LL-chondrite asteroid.     

Resonant oscillations in sunspot umbrae

The Uchida and Sakurai (1975), Thomas and Scheuer (1982), and Scheuer and Thomas (1981) theory of umbral oscillations as resonant modes of magneto-acoustic-gravity waves is re-examined. For an isothermal atmosphere it is found that the quasi-Alfvén approximation is not a good approximation to the complete linearized wave equations. The new results presented here show that 3 min umbral oscillation periods are fairly insensitive to magnetic field strength above some critical value. For a detailed model umbra (Thomas and Scheuer, 1982) the calculations presented here show that 3 min umbral oscillations do not depend to any great extent on the level of forcing of the oscillations for those magnetic field strengths which are observed in sunspot umbras. Modes outside the 3 min range appear, as the lowest mode, as the level of forcing is placed at deeper and deeper levels in the solar atmosphere.     

Diagenesis of Cretaceous sandstones of the Kootenay Formation at Elk Valley (southeastern British Columbia) and Mt Allan (southwestern Alberta)

Vitrinite reflectance data for two sections of the Kootenay Formation are different, but increase with depth. Illite crystallinities (XRD) are irregular with depth but detailed S.E.M. examination shows that crystal size and morphology increase with depth.At Elk Valley (low reflectance) S.E.M. examination shows kaolinite + dolomite as the authigenic assemblage. At Mt Allan (45 km north, higher reflectance) kaolinite disappears at 265 m and chlorite + calcite appears implying the equilibrium 5 CaMg(CO₃)₂ + Al₂Si₂O₅(OH)₄ + SiO₂ + H₂OagMg₅Al₂Si₃O₁₀(OH)₈ + 5CaCO₃ + 5CO₂. (1)Equilibrium (1) involves CO₂ and H₂O which form an immiscible fluid at diagenetic conditions. Thermochemical data, activity coefficients for the fluid mixture (Margules solution model), and fugacity coefficients allow the position of intersection of the miscibility surface and equilibria similar to (1) to be calculated. Maximum temperatures (180–250°C) in water-rich fluids are in accord with geothermal gradients of 33°C/km and temperatures from vitrinite reflectance (220°C). It is suggested that produced CO₂ may be responsible for the development of secondary porosity during deep burial.     

Pressure-temperature conditions in the Wadi Kid metamorphic complex: Implications for the pan-african event in SE Sinai

Geothermobarometry using continuous equilibria in pelitic rocks of the central and northeastern Wadi Kid areas, Sinai, yields pressure estimates of 320 MPa (3.2 kbar) and temperature estimates of 565° C and >620° C for staurolite-andalusite zone assemblages and anatectic rocks, respectively. This corresponds to geothermal gradients of 47° C/km and 50° C/km. TheseP-T conditions are shown to be compatible with burial under a volcanic cover. The principal heat source for high-grade metamorphism and anatexis is thought to be rising magmatic diapirs. Uplift was relatively slow and the final stages of heating and early stages of cooling probably occurred isobarically. The data imply a high heat flow regime for the Pan-African event in Sinai.     

The age,origin, and volcanological significance of the Y-5 ash layer in the Mediterranean

The Y-5 ash is the most widespread layer in deep-sea sediments from the eastern Mediterranean. This ash layer was previously correlated with the Citara-Serrara tuff on Ischia Island and dated at approximately 25,000 yr B.P. New data on the glass chemistry of the Y-5 ash and pyroclastic deposits from the Neopolitan volcanic province suggest that the layer is correlative with the large-volume Campanian ignimbrite and not with the deposit from Ischia Island. The volume of the Y-5 ash is approximately 65 km³ which is comparable in magnitude to the volume of the Campanian ignimbrite. An interpolated age of approximately 38,000 yr B.P. is estimated based on sedimentation rates derived from δ¹⁸O stratigraphy. There is a discrepancy between this estimate and previously reported radiocarbon ages which range from 24,000 to 35,000 yr B.P. We propose that the “Campanian tuff ash layer” should be adopted as the full stratigraphic name for the Y-5 ash. The deep-sea ash layer is divisible into two units in proximal localities, probably correlating with two major phases of the eruption: plinian and ignimbrite.     

Environmental Correlates of Growth and Stable Isotopes in Intertidal Species Along an Estuarine Fjord

Production and resource use by intertidal taxa were studied in the estuarine fjord of Puget Sound, Washington, USA. Along nearly 100 km, salinity is similar, and intertidal habitat and immersion time were kept consistent, thus allowing a focus on other environmental variables such as temperature and resource availability that could influence growth. Primary producers (ulvoid and fucoid macroalgae) and suspension feeders (oysters and barnacles) were transplanted on three beaches in each of three regions and assessed for individual-level growth, carbon and nitrogen ratios, and stable isotopes. In most transplants, δ¹³C and C/N showed no regional variation but δ¹⁵N was enriched up-estuary. Among environmental variables, chlorophyll a, total suspended solids, and particulate organic matter had small and/or inconsistent regional variation, but temperature was higher up-estuary. For the most intensively studied species (Pacific oyster, Crassostrea gigas) transplanted four times over 2 years, seasonal and regional variation in growth were best predicted by temperature rather than resource availability. Growth rates continued to increase into Totten Inlet, a shallow finger inlet at the head of Puget Sound. As indicators of environmental conditions, the growth and tissue chemistry of intertidal study taxa affirm that sources and amounts of resources show no strong gradients along this estuarine fjord, and they also support temperature as a key factor for performance, with species-specific responses. Higher temperatures may also have community-level impacts, given prior evidence linking beach temperatures to reduced intertidal diversity and biomass into Puget Sound.