40Ar/39Ar thermochronology using alkali feldspars: real thermal history or mathematical mirage of microtexture?

The Multiple Diffusion Domain (MDD) theory of Ar loss, based on the ⁴⁰Ar/³⁹Ar method, has been used to obtain thermal histories, over tens of millions of years, in the range 500 °C to near-surface temperatures, of alkali feldspars from slowly cooled geological environments and recovered in situ from sedimentary basins and geothermal systems. It assumes that Ar diffuses in Nature, over geological time, from the same domains, and by the same processes, that control its diffusion during laboratory step-heating experiments. Mathematically the model has many stringent requirements: the domains must form at T above those being modelled and must remain unchanged during geological time and during step heating, must not interact, must occur in discrete size ranges sharing one simple geometrical shape, contain no compositional gradients in ⁴⁰K, and release Ar into an infinite reservoir. We describe the nature of possible diffusion domains, and the fast pathways for diffusion between them, known to exist in alkali feldspars from direct electron microscopic observations, and explore their time–temperature evolution, based on established phase behaviour, both over geological time and during step heating. Strain-controlled microtextures, whose morphology is controlled by the minimization of coherency strain energy, do not provide fast pathways for Ar loss, and behave as crystallographically complex but single domains for Ar. Subregular dislocations may form over a range of T in some strain-controlled intergrowths, but in Nature the majority form closed loops and do not provide pathways to the crystal surface. Deuteric microtextures cut across strain-controlled microtextures, and are composed of microporous subgrain and grain mosaics with many pathways and a wide spectrum of possible domain sizes and shapes. The deuterically altered zones provide pathways for Ar diffusion out of the crystal, although some Ar is retained in fluid inclusions. All microtextures likely to define diffusion domains form below 500 °C, over a range of T and (in slowly cooled rocks) of times. The most important involve fluid–feldspar reactions, which can reset Ar, and continue to surface T. Feldspars in geothermal systems and young sedimentary basins are subject to ongoing chemical and microtextural changes. Because the microtextures are metastable during step-heating experiments in the T range over which most Ar is released, domains and pathways for rapid diffusion of Ar between domains will be significantly modified during step heating, particularly above 750 °C, when departures from a simple Arrhenius diffusion law are most marked. Pre-melting is likely to be an important cause of complexity in Ar loss behaviour. Few, if any, of the requirements of the MDD model are met, and we conclude that thermal histories cannot be obtained from alkali feldspars using this method. Received: 21 September 1998 / Accepted: 1 March 1999     

Salinity history of coastal marshes reconstructed from diatom remains

Sediment cores were collected from three Louisiana coastal marsh ponds, dated with radioisotopes, and analyzed for diatom remains to determine if long-term salinity changes were evident in the sediment record. A diatom-based salinity index formulated from a statistical comparison of available salinity data and changing diatom assemblages demonstrated that diatom remains appear to preserve salinity signals in coastal brackish and salt marsh environments. The salinity index was applied to sediment cores spanning the late 1600s to the 1990s and provided a more complete record of salinity than field data, which were temporally and spatially incomplete. The salinity reconstructions indicated that salinity has increased at two sites and decreased at a third since the early 1900s. The salinity changes are less than 1‰ per decade in all cases, and may be due to natural variability as depicted by the wide range of salinities observed between the late 1600s and 1900. Salinity regimes may be very localized (<2 km from a hydrologic source), indicating single-site studies may not be applicable to regional inferences. This study demonstrates that diatoms can be used to reconstruct past salinity in coastal marsh environments and can provide a useful tool with which to study the changing hydrology of river-influenced ecosystems.     

The Spring Phytoplankton Bloom in the Coastal Temperate Ocean: Growth Criteria and Seeding from Shallow Embayments

A method based on time-series of conductivity, temperature and depth (CTD) profiles which successfully determines favourable phytoplankton growth conditions for the spring bloom in nearshore temperate coastal waters was developed. The potential for shallow embayments to influence phytoplankton species composition in larger adjacent waters was also investigated. At temperate latitudes, such embayments should have favourable phytoplankton growth conditions earlier in the spring than open waters as bathymetry limits vertical mixing and thus increases light availability. The study area was Nanoose Bay, which is connected to the Strait of Georgia, British Columbia. Data were collected 2–3 times per week during the winter-spring of 1992 and 1993. A mooring with 5 current meters was placed at the mouth of the bay in 1992. The conservation equation for a scalar was used to estimate the balance between advective transport and biological source and sink terms. Variability in physical conditions and biological response between years was tremendous. Results indicate that seeding from the bay was not possible in 1992 but could have been in 1993. However, to conclusively determine the importance of Nanoose Bay on the spring bloom species composition in the Strait of Georgia, more extensive work is required. This revised version was published online in August 2006 with corrections to the Cover Date.     

Downstream changes in antimony and arsenic speciation in sediments at a mesothermal gold deposit in British Columbia,Canada

This study investigates Sb speciation in sediments along the drainage of the Upper Peter adit at the Bralorne Au mine in southern British Columbia, Canada, and compares the behavior of Sb with that of As. The Upper Peter mineralization consists of native Au in quartz-carbonate veins with 1 wt.% sulfides dominated by pyrite and arsenopyrite although stibnite, the primary Sb-bearing sulfide mineral, can be locally significant. Dissolved Sb concentrations can reach up to 349 μg L⁻¹ in the mine pool. Sediments were collected for detailed geochemical and mineralogical characterization at locations along the 350-m flow path, which includes a 100-m shallow channel within the adit, a sediment settling pond about 45 m beyond the adit portal and an open wetland another 120 m farther downstream. From the mine pool to the wetland outlet, dissolved Sb in the drainage drops from 199 μg L⁻¹ to below the detection limit due to the combined effect of dilution and removal from solution. Speciation analyses using X-ray absorption near-edge structure (XANES) spectroscopy indicate that Sb(III)–S accounts for around 70% of total Sb in the sediments in the main pool at the far end of the adit. At a short distance (24 m) downstream of the main adit pool, however, Sb(III)–O and Sb(V)–O species represent ?50% of total Sb in the bulk sediments, indicating significant oxidation of the primary sulfides inside the adit. Although Sb appears largely oxidized in the bulk samples collected near the portal, Sb(III)–S species are nevertheless present in the <53-μm fraction, suggesting a higher oxidation rate for stibnite in the coarser grains, possibly due to galvanic interaction with pyrite. Secondary Sb species released from the sulfide oxidation are most likely sorbed/co-precipitated with Fe-, Mn-, and Al-oxyhydroxides along the flow channel in the adit and in the sediment settling pond, with the Fe phase being the dominant sink for Sb.     

Undersampling power-law size distributions: effect on the assessment of extreme natural hazards

The effect of undersampling on estimating the size of extreme natural hazards from historical data is examined. Tests using synthetic catalogs indicate that the tail of an empirical size distribution sampled from a pure Pareto probability distribution can range from having one-to-several unusually large events to appearing depleted, relative to the parent distribution. Both of these effects are artifacts caused by limited catalog length. It is more difficult to diagnose the artificially depleted empirical distributions, since one expects that a pure Pareto distribution is physically limited in some way. Using maximum-likelihood methods and the method of moments, we estimate the power-law exponent and the corner size parameter of tapered Pareto distributions for several natural hazard examples: tsunamis, floods, and earthquakes. Each of these examples has varying catalog lengths and measurement thresholds, relative to the largest event sizes. In many cases where there are only several orders of magnitude between the measurement threshold and the largest events, joint two-parameter estimation techniques are necessary to account for estimation dependence between the power-law scaling exponent and the corner size parameter. Results indicate that whereas the corner size parameter of a tapered Pareto distribution can be estimated, its upper confidence bound cannot be determined and the estimate itself is often unstable with time. Correspondingly, one cannot statistically reject a pure Pareto null hypothesis using natural hazard catalog data. Although physical limits to the hazard source size and attenuation mechanisms from source to site constrain the maximum hazard size, historical data alone often cannot reliably determine the corner size parameter. Probabilistic assessments incorporating theoretical constraints on source size and propagation effects are preferred over deterministic assessments of extreme natural hazards based on historical data.     

Time–temperature evolution of microtextures and contained fluids in a plutonic alkali feldspar during heating

Microtextural changes brought about by heating alkali feldspar crystals from the Shap granite, northern England, at atmospheric pressure, have been studied using transmission and scanning electron microscopy. A typical unheated phenocryst from Shap is composed of about 70 vol% of tweed orthoclase with strain-controlled coherent or semicoherent micro- and crypto-perthitic albite lamellae, with maximum lamellar thicknesses <1 μm. Semicoherent lamellae are encircled by nanotunnel loops in two orientations and cut by pull-apart cracks. The average bulk composition of this microtexture is Ab_27.6Or_71.8An_0.6. The remaining 30 vol% is deuterically coarsened, microporous patch and vein perthite composed of incoherent subgrains of oligoclase, albite and irregular microcline. The largest subgrains are ~3 μm in diameter. Heating times in the laboratory were 12 to 6,792 h and T from 300°C into the melting interval at 1,100°C. Most samples were annealed at constant T but two were heated to simulate an ⁴⁰Ar/³⁹Ar step-heating schedule. Homogenisation of strain-controlled lamellae by Na↔K inter-diffusion was rapid, so that in all run products at >700°C, and after >48 h at 700°C, all such regions were essentially compositionally homogeneous, as indicated by X-ray analyses at fine scale in the transmission electron microscope. Changes in lamellar thickness with time at different T point to an activation energy of ~350 kJmol⁻¹. A lamella which homogenised after 6,800 h at 600°C, therefore, would have required only 0.6 s to do so in the melting interval at 1,100°C. Subgrains in patch perthite homogenised more slowly than coherent lamellae and chemical gradients in patches persisted for >5,000 h at 700°C. Homogenisation T is in agreement with experimentally determined solvi for coherent ordered intergrowths, when a 50–100°C increase in T for An₁ is applied. Homogenisation of lamellae appears to proceed in an unexpected manner: two smooth interfaces, microstructurally sharp, advance from the original interfaces toward the mid-line of each twinned, semicoherent lamella. In places, the homogenisation interfaces have shapes reflecting the local arrangements of nanotunnels or pull-aparts. Analyses confirm that the change in alkali composition is also relatively sharp at these interfaces. Si–Al disordering is far slower than alkali homogenisation so that tweed texture in orthoclase, tartan twinning in irregular microcline, and Albite twins in albite lamellae and patches persisted in all our experiments, including 5,478 h at 700°C, 148 h at 1,000°C and 5 h at 1,100°C, even though the ensemble in each case was chemically homogeneous. Nanotunnels and pull-aparts were modified after only 50 min at 500°C following the simulated ⁴⁰Ar/³⁹Ar step-heating schedule. New features called ‘slots’ developed away from albite lamellae, often with planar traces linking slots to the closest lamella. Slot arrays were often aligned along ghost-like regions of diffraction contrast which may mark the original edges of lamellae. We suggest that the slot arrays result from healing of pull-aparts containing fluid. At 700°C and above, the dominant defects were subspherical ‘bubbles’, which evolved from slots or from regions of deuteric coarsening. The small degree of partial melting observed after 5 h at 1,100°C was often in the vicinity of bubbles. Larger micropores, which formed at subgrain boundaries in patch perthite during deuteric coarsening, retain their shape up to the melting point, as do the subgrain boundaries themselves. It is clear that modification of defects providing potential fast pathways for diffusion in granitic alkali feldspars begins below 500°C and that defect character progressively changes up to, and beyond, the onset of melting.     

Ten guiding principles for the delineation of priority habitat for endangered small cetaceans

The adoption of endangered species laws in various nations has intensified efforts to better understand, and protect, at-risk species or populations, and their habitats. In many countries, delineating a portion of a species' habitat as particularly worthy of protection has become a mantra of these laws. Unfortunately, the laws themselves often provide scientists and managers with few, if any, guidelines for how to define such habitat. Conservationists and scientists may view protecting part of the habitat of an endangered species as an ineffectual compromise, while managers may be under pressure to allow a range of human activities within the species' habitat. In the case of small cetaceans, establishing boundaries for such areas can also be complicated by their mobility, the fluid nature of their environment, and the often ephemeral nature of their habitat features. The convergence of multiple human impacts in coastal waters around the world is impacting many small cetaceans (and other species) that rely on these areas for feeding, reproducing, and resting. The ten guiding principles presented here provide a means to characterize the habitat needs of small, at-risk cetaceans, and serve as a basis for the delineation of ‘priority habitat’ boundaries. This conceptual approach should facilitate a constructive discourse between scientists and managers engaged in efforts to recover endangered species. The degree to which the recovery of an at-risk species can be reconciled with sustainable economic activity will depend in part on how well these principles are incorporated into the delineation of priority habitat.     

Sweeping scientific data under a polar bear skin rug: The IUCN and the proposed listing of polar bears under CITES Appendix I

In 2008 the US Government listed the polar bear as “threatened” under the US Endangered Species Act, largely due to scientific analyses of climate change data and polar bear distribution conducted by the US Geological Survey (USGS), which projected a 2/3 reduction in the polar bear population within the next 40 years. In 2009 the US Government announced that it would submit a proposal to the 15th Conference of the Parties to the Convention on International Trade in Endangered Species of Wild Fauna and Flora to uplist polar bears from CITES Appendix II to Appendix I. A report produced by the IUCN/Traffic stated that this was not warranted. However, the IUCN/Traffic report did not consider the USGS data noted above. Ultimately the proposal for CITES uplisting failed. In this paper we examine the differences in the IUCN/Traffic report and the best available science, and conclude that listing polar bears under CITES Appendix I was scientifically warranted.