An investigation of upper mantle heterogeneity beneath the Archaean and Proterozoic crust of western Canada from Lithoprobe controlled-source seismic experiments

Observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle. This phenomenon is observed in wide-angle reflection data from Lithoprobe's Alberta Basement Transect [the SAREX and Deep Probe experiments of 1995] and Trans-Hudson Orogen Transect [the THoRE experiment of 1993]. SAREX and Deep Probe image the Archaean lithosphere of the Hearne and Wyoming Provinces, whereas THoRE images the Archaean and Proterozoic lithosphere of the Trans-Hudson Orogen and neighbouring areas.Finite-difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer. SAREX/Deep Probe modelling uses a 2-D visco-elastic finite-difference routine; THoRE modelling uses a pseudospectral algorithm. In both cases, the upper mantle is parameterized in terms of two media. One medium is the background matrix; the other is statistically distributed within the first as a series of elliptical bodies. Such a scheme is suitable for modelling: (1) variations in lithology (e.g., a peridotite matrix with eclogite lenses) or (2) variations in rheology (e.g., lenses of increased strain within a less strained background).The synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two Lithoprobe transects. Beneath the Trans-Hudson Orogen in Saskatchewan, the layer is best modelled to lie at depths between 80 and 150 km. Based on observations from perpendicular profiles, anisotropy of the heterogeneities is inferred. Beneath the Precambrian domains of Alberta, 400 km to the west, upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km. In both cases the heterogeneous bodies within the model have cross-sectional lengths of tens of kilometers, vertical thicknesses less than 1 km, and velocity contrasts from the background of − 0.3 to − 0.4 km/s. Based on consistency with complementary data and other results, the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle.     

Terrestrial discharge influences microbioerosion and microbioeroder community structure in coral reefs

Microbioerosion rates and microbioeroder community structure were studied in four Kenyan protected coral-reef lagoons using shell fragments of Tridacna giant clams to determine their response to the influence of terrestrial run-off. Fourteen different microbioeroder traces from seven cyanobacteria, three green algae and four fungi species were identified. The river discharge-impacted reef and ‘pristine’ reef showed similar composition but higher microbioeroder abundance and total cyanobacteria- and chlorophyte-bioeroded areas when compared with the other study reefs. Cyanobacteria dominated during the north-east monsoon (NEM) relative to the south-east monsoon (SEM) season, with algae and cyanobacteria being major microbioeroders in the river-impacted and pristine reefs. The rate of microbioerosion varied between 4.3 g CaCO3 m^?2 y^?1 (SEM) and 134.7 g CaCO₃ m^?2 y^?1 (NEM), and was highest in the river-impacted reef (127.6 g CaCO₃ m^?2 y^?1), which was almost double that in the pristine reef (69.5 g CaCO₃ m^?2 y^?1) and the mangrove-fringed reef (56.2 g CaCO₃ m^?2 y^?1). The microbioerosion rates measured in this study may not be high enough to cause concern with regard to the health and net carbonate production of Kenya’s coral reefs. Nevertheless, predicted increases in the frequency and severity of stresses related to global climate change (e.g. increased sea surface temperature, acidification), as well as interactions with local disturbances and their influence on bioerosion, may be increasingly important in the future.     

Quantifying geographic variations in associations between alcohol distribution and violence: a comparison of geographically weighted regression and spatially varying coefficient models

Past studies consistently indicate measurable local associations between alcohol distribution and the incidence of violence. These results, coupled with measurements of spatial correlation, reveal the importance of spatial analysis in the study of the interaction of alcohol and violence. While studies increasingly incorporate spatial correlation among model residuals to improve precision and reduce bias, to date, most analyses assume associations that are constant and independent of location, an assumption coming under increasing scrutiny in the quantitative geography literature. In this paper, we review and contrast two approaches for the estimation of and inference for spatially heterogeneous effects (i.e., associative factors whose impacts on the outcome of interest vary throughout geographic space). Specifically, we provide an in-depth comparison of “geographically weighted regression” models (allowing covariate effects to vary in space but only allowing relatively ad hoc inference) with “variable coefficient” models (allowing varying effects via spatial random fields and providing model-based estimation and inference, but requiring more advanced computational techniques). We compare the approaches with respect to underlying conceptual structures, computational implementation, and inferential output. We apply both approaches to violent crime, illegal drug arrest, and alcohol distribution data from Houston, Texas and compare results in light of the differing methodological structures of the two approaches.     

The relationship between major element chemistry and tectonic environment of basic and intermediate volcanic rocks

This work reports the preliminary results of a study of the relationship between tectonic environment and major element chemistry of volcanic rocks. Using a file of 8400 analyses of rocks of varying ages and geographic distribution it has been found that a simple ternary plot of MgOFeO(total)Al₂O₃ may be used to distinguish five tectonic environments: ocean ridge and floor, ocean island, continental, orogenic, and spreading center island. Subalkaline “basaltic-andesites” (51–56% SiO₂ calculated anhydrous) are used in this study. Alkaline rocks do not generally show the simple patterns of tholeiitic or subalkaline rocks. The ocean island suite is bimodal with one mode tholeiitic and the other alkaline. A bimodal age/chemistry relationship in the continental suite is tentatively related to different conditions during and after active breaking up of continents (rifting and drifting stages).Most of the Archean rocks in our file generally do not fall in the orogenic field. It is unlikely that modern island arc models can be generally applied to Archean greenstone belts. Locally, however, there may be Archean rocks analogous in some respect to modern calc-alkali rocks.     

The flocculation and de-watering of kimberlite clay slimes

The crushing of kimberlite at certain diamond mines produces a very persistent “slime” which is exceptionally difficult to filter or de-water. The colloidally stable component has been identified as a swelling clay, saponite, which carries a strong negative charge in alkaline media.Methods of coagulating or flocculating these slimes have been investigated; the most convenient method found was the combined use of magnesium sulphate and an anionic polyacrylamide (A-130 from B.T.I. Chemicals Ltd.). Optimal results were obtained only in a narrow range of conditions but with these it was possible to build flocs of any required size.The permeability and de-watering of coagulated or flocculated kimberlite slimes has been investigated with a hydraulic apparatus and in the capillary suction time (c.s.t.) apparatus. Typically compressible filter-cakes are obtained, their character varying greatly with the flocculation conditions.     

Microfossils of Polynesian cultigens in lake sediment cores from Rano Kau, Easter Island

Previous wetland vegetation records from Easter Island showing deforestation and Polynesian agriculture are limited to cores that rely on pollen, with a single cultigen pollen type identified: Urticaceae/Moraceae, possibly Broussonetia payrifera (paper mulberry). Here we redress this by also using phytolith and starch analyses on four lake sediment cores on a ~350-m transect along the southwest edge of Rano Kau, focusing on in-washed basal clayey layers. We also use a new method, Fourier transform infrared spectroscopy, to positively identify degraded starch collected from sedimentary deposits. The cores are the first samples recovered from an area in the lake that (a) lies below the relict village of Orongo, (b) is near a section of the crater believed to be most accessible from the Pacific coast, and (c) is far from the northern crater rim and receives high solar radiation, a likely benefit for crops of tropical origin. Pollen and phytoliths are abundant in the clayey layers and sparse in overlying layers of organic lake detritus and living rhizomes. Mixing of core deposits as a result of human activity has disordered the radiocarbon sequence, precluding development of an reliable chronology. Containing microfossils of several introduced cultigens, the clayey layers represent gardened terraces that have slumped into the lake. The data indicate large-scale deforestation and a mixed-crop production system including Broussonetia papyrifera, Colocasia esculenta (taro), Dioscorea alata (greater yam), Ipomoea batatas (sweet potato), Lagenaria siceraria (bottle gourd) and Musa (banana) sp. The data show (a) the potential for using the combined analyses to provide direct evidence of Polynesian horticulture on Easter Island and (b) that the island’s wetlands potentially hold extensive horticultural records. The study highlights the concept of ‘transported landscapes,’ whereby colonising people replace indigenous forests with artificial, imported agricultural landscapes.     

Krieger’s conceptualization and measurement of discrimination and internalized oppression in studies of adverse health outcomes

Embodiment is a central concept in Krieger’s ecosocial theory, and is said to be of relevance to the understanding of the relationship between social conditions and a variety of adverse health outcomes. The most detailed empirical investigation of this in Krieger’s work is to be found in her studies of the relationship between racial discrimination and high blood pressure. Of especial relevance here is the idea of internalized oppression which is said to explain the observed association between self-reports of no racial discrimination and increased levels of blood pressure among working-class African Americans. Here we critically examine the empirical evidence pertaining to internalized oppression. Specifically, we focus on the measurement of the construct and the quality of the empirical evidence that has been presented in support of the hypothesis that there is an association between internalized oppression and adverse health outcomes. We argue that the validity of the concept has yet to be established and that the available data linking it to poor health outcomes are open to alternative explanations, notably measurement error and misclassification.

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Energetic Constraints on Precipitation Under Climate Change

Energetic constraints on precipitation are useful for understanding the response of the hydrological cycle to ongoing climate change, its response to possible geoengineering schemes, and the limits on precipitation in very warm climates of the past. Much recent progress has been made in quantifying the different forcings and feedbacks on precipitation and in understanding how the transient responses of precipitation and temperature might differ qualitatively. Here, we introduce the basic ideas and review recent progress. We also examine the extent to which energetic constraints on precipitation may be viewed as radiative constraints and the extent to which they are confirmed by available observations. Challenges remain, including the need to better demonstrate the link between energetics and precipitation in observations and to better understand energetic constraints on precipitation at sub-global length scales.     

Pollen, phytoliths, arthropods and high-resolution 14C sampling from Rano Kau, Easter Island: evidence for late Quaternary environments, ant (Formicidae) distributions and human activity

A 20-m sediment core from Rano Kau, Easter Island provides plant microfossil, arthropod fossil and high-resolution ¹⁴C sampling evidence for late Quaternary environments, ant (Formicidae) distributions and human activity. The record commences prior to or during the LGM, providing the two oldest dates for Rano Kau thus far, 20,340 ± 160 and 34,260 ± 440 BP. The vegetation at the time was mostly Arecaceae (palm)-dominated grassland-woodland, suggesting cooler/drier conditions than present. Near the start of the Holocene c. 12,500 cal BP, climate alleviation is indicated by Asteraceae shrubland increasing at the expense of grassland. There is an early to mid-Holocene sediment hiatus. The record recommences c. 3,500 cal BP, with in-washed clayey soils bearing charcoal and phytoliths of Polynesian-introduced Musa (banana) and mixed with wetland detritus, reflecting slumping as a result of forest clearance and gardening. Dates of material of other plants from the clay/detritus layers containing the Musa phytoliths are older than expected, within the range 3,680–2,750 cal BP, well before settlement of Eastern Polynesia. This could reflect horticultural material settling in an older part of the sediment column or age increase by reservoir effects. A long section of overlying unconsolidated detritus provides a progressively younger upward sequence from 2,840–2,870 to 930–810 cal BP, but dates are variable after 1,290–1,180 cal BP, where charcoal, disturbance-related pollen and older than expected ant exoskeletons demonstrate the inclusion of pre-aged material from the floating vegetation mat or upslope erosion in the sediment column. Arecaceae pollen declines sharply after 930–810 cal BP. Arthropod analysis reveals two new weevil species for Easter Island, and two ant taxa, Tetramorium bicarinatum and Pheidole sp., the latter of which is also new for the island. Three of the four age determinations on ant remains suggest that ants were present prior to c. 2,500 cal BP, and probably associated with the period following the resumption of sediment accumulation c. 3,500 cal BP. Unless the apparent ant ¹⁴C ages have been increased by reservoir effects, these results question the long-accepted assumption that all Eastern Polynesia’s ant species have been introduced.