Subsurface tropical Pacific nitrogen isotopic composition of nitrate: Biogeochemical signals and their transport

We report measurements of the nitrogen isotopic composition of nitrate (the δ15N of NO3−) across the equatorial Pacific, for zonal transects from 165°E to 95°W and meridional transects across 95° and 110°W. The δ15N of NO3− is similar in the equatorial thermocline (≈100 m) and intermediate depth waters (≈150 to 600 m), averaging (7.1 ± 0.3)‰ and (7.1 ± 0.1)‰, respectively. These values are more than 2‰ higher than subthermocline waters of the Southern and Atlantic Oceans and are ≈1‰ higher than putative source waters in the high latitude South Pacific (Subantarctic Mode Water, SAMW). The combined constraints of nitrate concentration and δ15N of NO3−in the equatorial Pacific require (1) lateral exchange between the high‐latitude source waters and the zones of denitrification in the eastern tropical Pacific and (2) the accumulation of remineralized nutrients at depth. The zonal uniformity of the subsurface equatorial Pacificδ15N of NO3− indicates rapid transport within the equatorial zone, which works to homogenize the δ15N of NO3− across the Pacific basin. Against this backdrop of high δ15N of NO3−in the tropical Pacific, we find a discrete off‐equatorial core of lowerδ15N of NO3− (5.5 ± 0.3)‰ concentrated at 5°S and 150 to 200 m along the 110° and 95°W transects and in apparent association with the Southern Subsurface Counter Current (SSCC). We propose that the remineralized products of nitrogen fixation, at the source of the SSCC in the western south Pacific, are the origin of the low δ15N of NO3− in these waters

The Effect of Allowing Pollution Offsets with Imperfect Enforcement

Several pollution control regimes, including climate change policies, allow polluters in one sector subject to an emissions cap to offset excessive emissions in that sector with pollution abatement in another sector. The government may often find it more costly to verify offset claims than to verify compliance with emissions caps, and concerns about difficulties in enforcement may lead regulators to restrict the use of offsets. In this paper, we demonstrate that allowing offsets may increase pollution abatement and reduce illegal pollution, even if the government has a fixed enforcement budget. We explore the circumstances that may make it preferable to allow offsets when enforcement is costly

The Effect of Allowing Pollution Offsets with Imperfect Enforcement

Several pollution control regimes, including climate change policies, allow polluters in one sector subject to an emissions cap to offset excessive emissions in that sector with pollution abatement in another sector. The government may often find it more costly to verify offset claims than to verify compliance with emissions caps, and concerns about difficulties in enforcement may lead regulators to restrict the use of offsets. In this paper, we demonstrate that allowing offsets may increase pollution abatement and reduce illegal pollution, even if the government has a fixed enforcement budget. We explore the circumstances that may make it preferable to allow offsets when enforcement is costly

The Effect of Joint and Several Liability Under Superfund on Brownfields

In response to claims that the threat of environmental liability under the Superfund law deters the acquisition of potentially contaminated sites (or brownfields ) for redevelopment, the federal government has adopted programs to protect purchasers from liability. This protection may be unwarranted, however, if sellers can simply adjust property prices downward to compensate buyers for this liability. We present a model of joint and several liability under Superfund that allows us to distinguish four different reasons that this liability may discourage the purchase of brownfields. The previous literature has overlooked the effects that we identify, which all arise because a sale may increase the number of defendants in a suit to recover cleanup costs. Our analysis suggests that the brownfields problem may be more widespread than one might infer from the prior literature. Furthermore, the effects that we identify may distort not only the incentives to sell property subject to Superfund liability but also any decision of any party subject to any joint and several liability if that decision could affect the number of other defendants liable for the same harm

The Effect of Joint and Several Liability Under Superfund on Brownfields

In response to claims that the threat of environmental liability under the Superfund law deters the acquisition of potentially contaminated sites (or brownfields ) for redevelopment, the federal government has adopted programs to protect purchasers from liability. This protection may be unwarranted, however, if sellers can simply adjust property prices downward to compensate buyers for this liability. We present a model of joint and several liability under Superfund that allows us to distinguish four different reasons that this liability may discourage the purchase of brownfields. The previous literature has overlooked the effects that we identify, which all arise because a sale may increase the number of defendants in a suit to recover cleanup costs. Our analysis suggests that the brownfields problem may be more widespread than one might infer from the prior literature. Furthermore, the effects that we identify may distort not only the incentives to sell property subject to Superfund liability but also any decision of any party subject to any joint and several liability if that decision could affect the number of other defendants liable for the same harm

The conundrum of functional brain networks: small-world efficiency or fractal modularity

The human brain has been studied at multiple scales, from neurons, circuits, areas with well defined anatomical and functional boundaries, to large-scale functional networks which mediate coherent cognition. In a recent work, we addressed the problem of the hierarchical organization in the brain through network analysis. Our analysis identified functional brain modules of fractal structure that were inter-connected in a small-world topology. Here, we provide more details on the use of network science tools to elaborate on this behavior. We indicate the importance of using percolation theory to highlight the modular character of the functional brain network. These modules present a fractal, self-similar topology, identified through fractal network methods. When we lower the threshold of correlations to include weaker ties, the network as a whole assumes a small-world character. These weak ties are organized precisely as predicted by theory maximizing information transfer with minimal wiring costs.Comment: 14 pages, 8 figure

On a common circle: natural scenes and Gestalt rules

To understand how the human visual system analyzes images, it is essential to know the structure of the visual environment. In particular, natural images display consistent statistical properties that distinguish them from random luminance distributions. We have studied the geometric regularities of oriented elements (edges or line segments) present in an ensemble of visual scenes, asking how much information the presence of a segment in a particular location of the visual scene carries about the presence of a second segment at different relative positions and orientations. We observed strong long-range correlations in the distribution of oriented segments that extend over the whole visual field. We further show that a very simple geometric rule, cocircularity, predicts the arrangement of segments in natural scenes, and that different geometrical arrangements show relevant differences in their scaling properties. Our results show similarities to geometric features of previous physiological and psychophysical studies. We discuss the implications of these findings for theories of early vision.Comment: 3 figures, 2 large figures not include

A Comprehensive Study of the Alteration of Ignitable Liquids by Weathering and Microbial Degradation

The differing effects of weathering and microbial degradation are described here in a comprehensive study that involved 50 different ignitable liquids from the Ignitable Liquids Database and Reference Collection. Examples of ignitable liquid residues from each of the main classes established by the American Society of Testing and Materials are presented. Weathering was accomplished via evaporation, whereas microbial degradation was carried out on soil at room temperature for periods of up to 21 days. Major trends included the rapid degradation of long n-alkanes and monosubstituted alkyl benzenes (e.g., toluene, ethylbenzene, and propylbenzene). Surprisingly, some longer branched alkanes (e.g., trimethyloctanes) were also susceptible to microbial attack. Although all ignitable liquids examined suffered at least to some extent from microbial degradation, gasoline, petroleum distillates, and oxygenates were the most susceptible. Isoparaffinic and naphthenic–paraffinic products were the most resistant to microbial degradation