The forward problem of electromagnetic induction: accurate finite-difference approximations for two-dimensional discrete boundaries with arbitrary geometry

We describe finite-difference approximations to the equations of 2-D electromagnetic induction that permit discrete boundaries to have arbitrary geometrical relationships to the nodes. This allows finite-difference modelling with the flexibility normally ascribed to finite-element modelling. Accuracy is demonstrated by comparison with finite-element computations. We also show that related approximations lead to substantially improved accuracy in regions of steep, but not discontinuous, conductivity gradient.     

Differential unroofing within the central metasedimentary Belt of the Grenville Orogen: constraints from 40Ar/39Ar thermochronology

An ⁴⁰Ar/³⁹Ar thermochronological investigation of upper greenschist to granulite facies gneiss, amphibolite and marble was conducted in the Central Metasedimentary Belt (CMB), Ontario, to constrain its cooling history. Incremental ⁴⁰Ar/³⁹Ar release spectra indicate that substantial differential unroofing occurred in the CMB between 1000 and 600 Ma. A consistent pattern of significantly older hornblende and phlogopite ⁴⁰Ar/³Ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation. Variations in hornblende ⁴⁰Ar/³⁹Ar age plateaus exceeding 200 Ma occur over distances less than 50 km with major age discontinuities occurring across the Robertson Lake shear zone and the Sharbot Lake mylonite zone which separate the Sharbot Lake terrane from the Elzevir and Frontenac terranes. Extensional displacements of up to 14 km are inferred between the Frontenac and Elzevir terranes of the CMB. No evidence for significant post argon-closure vertical displacement is indicated in the vicinity of the Perth Road mylonite within the Frontenac terrane. Variations of nearly 100 Ma in phlogopite ⁴⁰Ar/³⁹Ar plateau ages occur in undeformed marble on either side of the Bancroft Shear Zone. Phlogopites from sheared and mylonitized marble within the shear zone yield ⁴⁰Ar/³⁹Ar diffusional loss profiles, but have older geologically meaningless ages thought to reflect incorporation of excess argon. By 900 Ma, southeast directed extension was occurring throughout the CMB, possibly initiated along previous zones of compressional shearing. An easterly migration of active zones of extension is inferred, possibly related to an earlier, overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone. The duration of extensional shearing is not well constrained, but must have ceased before 600 Ma as required by the deposition of overlying undeformed Cambrian and/or Ordovician sedimentary rocks.Contribution No. 481 from the Mineralogical Laboratory, University of Michigan     

Climate model simulation of anthropogenic influence on greenhouse-induced climate change (early agriculture to modern): the role of ocean feedbacks

We present further steps in our analysis of the early anthropogenic hypothesis (Ruddiman, Clim Change 61:261–293, 2003) that increased levels of greenhouse gases in the current interglacial, compared to lower levels in previous interglacials, were initiated by early agricultural activities, and that these increases caused a warming of climate long before the industrial era (～1750). These steps include updating observations of greenhouse gas and climate trends from earlier interglacials, reviewing recent estimates of greenhouse gas emissions from early agriculture, and describing a simulation by a climate model with a dynamic ocean forced by the low levels of greenhouse gases typical of previous interglacials in order to gauge the magnitude of the climate change for an inferred (natural) low greenhouse gas level relative to a high present day level. We conduct two time slice (equilibrium) simulations using present day orbital forcing and two levels of greenhouse gas forcing: the estimated low (natural) levels of previous interglacials, and the high levels of the present (control). By comparing the former to the latter, we estimate how much colder the climate would be without the combined greenhouse gas forcing of the early agriculture era (inferred from differences between this interglacial and previous interglacials) and the industrial era (the period since ～1750). With the low greenhouse gas levels, the global average surface temperature is 2.7 K lower than present day—ranging from ～2 K lower in the tropics to 4–8 K lower in polar regions. These changes are large, and larger than those reported in a pre-industrial (～1750) simulation with this model, because the imposed low greenhouse gas levels (CH₄ = 450 ppb, CO₂ = 240 ppm) are lower than both pre-industrial (CH₄ = 760 ppb, CO₂ = 280 ppm) and modern control (CH₄ = 1,714 ppb, CO₂ = 355 ppm) values. The area of year-round snowcover is larger, as found in our previous simulations and some other modeling studies, indicating that a state of incipient glaciation would exist given the current configuration of earth’s orbit (reduced insolation in northern hemisphere summer) and the imposed low levels of greenhouse gases. We include comparisons of these snowcover maps with known locations of earlier glacial inception and with locations of twentieth century glaciers and ice caps. In two earlier studies, we used climate models consisting of atmosphere, land surface, and a shallow mixed-layer ocean (Ruddiman et al., Quat Sci Rev 25:1–10, 2005; Vavrus et al., Quat Sci Rev 27:1410–1425, 2008). Here, we replaced the mixed-layer ocean with a complete dynamic ocean. While the simulated climate of the atmosphere and the surface with this improved model configuration is similar to our earlier results (Vavrus et al., Quat Sci Rev 27:1410–1425, 2008), the added information from the full dynamical ocean is of particular interest. The global and vertically-averaged ocean temperature is 1.25 K lower, the area of sea ice is larger, and there is less upwelling in the Southern Ocean. From these results, we infer that natural ocean feedbacks could have amplified the greenhouse gas changes initiated by early agriculture and possibly account for an additional increment of CO₂ increase beyond that attributed directly to early agricultural, as proposed by Ruddiman (Rev Geophys 45:RG4001, 2007). However, a full test of the early anthropogenic hypothesis will require additional observations and simulations with models that include ocean and land carbon cycles and other refinements elaborated herein.     

Morphology of the Wadati-Benioff zone,andesitic volcanism,and active fracture zones in Central America

Summary The morphology of the Wadati-Benioff zone in the region of Central America, based on the distribution of 1377 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Cocos plate. Four deep seismically active fracture zones, genetically connected with the process of subduction, and three fracture zones manifesting the possible boundary between the Americas and Caribbean plates were identified in the continental wedge.     

Phytoplankton biomass and primary production responses to physico-chemical forcing across the northeastern New Zealand continental shelf

Phytoplankton biomass and primary production were monitored in the Hauraki Gulf and on the northeastern continental shelf, New Zealand - using ship surveys, moored instruments and satellite observations (1998-2001) - capturing variability across a range of space and time scales. A depth-integrated primary production model (DIM) was used to predict integrated productivity from surface parameters, enabling regional-specific estimates from satellite data. The shelf site was dominated by pico-phytoplankton, with low chlorophyll-a (<1 mg m⁻³) and annual production (136 g C m⁻² yr⁻¹). In contrast, the gulf contained a micro/nano-phytoplankton-dominated community, with relatively high chlorophyll-a (>1 mg m⁻³) and annual production (178 g C m⁻² yr⁻¹). Biomass and productivity responded to physico-chemical factors; a combination of light, critical mixing depths and/or nutrient limitation—particularly new nitrate-N. Relatively low biomass and production was observed during 1999. This coincided with inter-annual variability in the timing and extent of upwelling- and downwelling-favourable along-shelf wind-stress, influencing the fluxes of new nitrate-N to the shelf and gulf. Relationships with the Southern Oscillation Index are also discussed. Our multi-scaled sampling highlighted details associated with stratification and de-stratification events, and deep sub-surface chlorophyll-a not visible to satellite sensors. This study demonstrates the importance of multi-scaled sampling in gaining estimates of regional production and its responses to physico-chemical forcing.     

Shark interactions in pelagic longline fisheries

Substantial ecological, economic and social problems result from shark interactions in pelagic longline fisheries. Improved understanding of industry attitudes and practices towards shark interactions assists with managing these problems. Information on fisher knowledge and new strategies for shark avoidance may benefit sharks and fishers. A study of 12 pelagic longline fisheries from eight countries shows that incentives to avoid sharks vary along a continuum, based on whether sharks represent an economic disadvantage or advantage. Shark avoidance practices are limited, including avoiding certain areas, moving when shark interaction rates are high, using fish instead of squid for bait and deeper setting. Some conventionally employed fishing gear and methods used to target non-shark species contribute to shark avoidance. Shark repellents hold promise; more research and development is needed. Development of specifically designed equipment to discard sharks could improve shark post release survival prospects, reduce gear loss and improve crew safety. With expanding exploitation of sharks for fins and meat, improved data collection, monitoring and precautionary shark management measures are needed to ensure that shark fishing mortality levels are sustainable.     

Assessing market and non-market costs of freshwater flooding due to climate change in the community of Fredericton, Eastern Canada

There is a general consensus that climate change will increase the frequency and severity of freshwater flooding in many parts of the world. Communities prone to such flooding have struggled to understand the manner in which this will affect them (in both economic and social terms) and the appropriate way to adapt. In this study, we conduct a case-study investigation into the costs of freshwater flooding due to climate change along the Saint John River in Fredericton, NB, Canada. We develop a four-step framework that combines extreme event analysis, downscaled general circulation models, hydrologic analysis, and the contingent valuation method. Using this framework, together with primary data on a 2005 flooding event, we estimate market and non-market annual average flood damage under a number of climate and population scenarios. We find that non-market costs can represent up to 50% of total household costs of flooding events, and 23–42% of the total costs of flooding due to climate change, depending on the different climate and population scenarios considered. Incorporating such costs into flood adaptation planning may substantially increase support for active adaptation activities, especially in ‘worst case’ climate scenarios.     

A simultaneous interpretation,by wind speed and direction,of ambient air negative ion density in Athens,Greece

A recently developed data presentation technique, the rectangular wind frequency isopleth diagram, has been applied to examine the effect of wind speed and direction, acting simultaneously, on ambient air negative ion densities. The large negative ion concentration decreases in the majority of directions and wind speed intervals, while that of the small ions increases, as the wind speed increases. The local maxima and minima, occurring in some sectors, are properly discussed.     

Hurricane effects on surface gulf stream currents

A 3-dimensional model methodology for predicting the upper ocean currents under the combined influence of Gulf Stream and hurricane is described. Predicted currents are highly dependent on the turbulence closure scheme and nonlinear interactions. New field data will be required to determine the appropriate formulation for turbulent momentum transfer. However, the model points out the importance of nonlinear terms in the equations of motions for preserving the jet-like structure of the Gulf Stream and for properly accounting for interaction between the Gulf Stream and a hurricane. A potentially worst case condition is obtained for the so-called resonant hurricane.     

Fluorescence spectral analysis of resinite macerals from coals of the Hanna Formation, Wyoming, U.S.A.

Flourescence spectral analysis indicates that resinite macerals from Tertiary Hanna Formation coals (Hanna Coal Field, southcentral Wyoming, U.S.A.) can be separated into five distinct groups. The first resinite group fluoresces a a medium green (in blue light); its average spectral maximum occurs at or below 440 mm with a red/green quotient of 0.22. The second resinite group fluoresces yellow-green with an average spectral maximum of 500 nm and a red/green quotient of 0.53. The third resinite group displays a yellow fluorescence having an average spectral maximum of 580 nm and a red/green quotient of 0.86. The fourth resinite group fluorescence orange-brown having an average spectral maximum of 610 nm and a red/green quotient of 1.20. These four groups mostly occur as primary globular resinites exhibiting scratches and fractures, indicating that they are brittle, solid substances. Primary cell-filling and secondary fracture-filling resinites also occur in these four groups. The fifth group only occurs as a secondary void-filling material and lacks evidence of br of brittle properties. It fluoresces a reddish-brown, has a spectral maximum at 690 nm, and a red/green quotient of 1.54. The fifth group has properties resembling exsudatinite. The five resinite groups can be separated on the basis of their nine spectral properties alone, without qualitative petrographic interpretation.The relative quantities of the five resinite groups vary among Hanna Formation coals. The origins of these five resinite groups are probably related to their botanical properties and pre- and post-depossitional conditions. Overall, Hanna Formation resinites have petrographic characteristics similar to other North American resinites; however, only four resinite groups have been distinguished in in certain coals from Utah and New Mexico (U.S.A.), and western Canada.