Climatic and topographic limits to the abundance of bog pools

On patterned peatlands, open water pools develop within a matrix of terrestrial vegetation (‘ridges’). Regional patterns in the distribution of ridge–pool complexes suggest that the relative cover of these two surface types is controlled in part by climate wetness, but landscape topography must also be an important controlling factor. In this paper, a functional model that relates relative cover of ridges and pools to climate and surface gradient was developed and tested. The model was formulated in terms of a water budget, based on the differential effects of ridges and pools on losses by evapotranspiration and subsurface flow. It predicts a positive relationship between surface gradient and ridge proportion, with a linear effect related to water supply and ridge hydraulic conductivity, modified at high ridge proportion by differences in evapotranspiration between ridges and pools. The limit to patterned peatland distribution occurs where the surface is completely covered by ridges. The model may be sensitive or insensitive to climate differences between localities, depending on whether hydraulic characteristics of ridge peat co‐vary with water supply. To distinguish between these alternative hypotheses, surface gradient and ridge proportion were surveyed along 20 transects in each of three localities in Scotland that differ threefold in net precipitation to pools. The results of the field survey served to reject the climate‐sensitive hypothesis, but were consistent with the climate‐insensitive hypothesis. Analysis of the residuals suggested that variation within localities was related more to topographic control of water supply than to ridge hydraulic conductivity or developmental stage. Hence, within this maritime climate region, the distribution of ridge–pool complexes and the relative abundance of pools are controlled mainly by topographic variables. Field surveys across both maritime and continental regions are required to confirm a subtle climatic effect that allows pools to occur on higher gradients in drier climates than in wetter climates. Further development and testing of the functional model will provide a stronger basis for assessing potential feedback between climate change, peatland surface structure and methane emission from pools. Copyright © 2006 John Wiley & Sons, Ltd.     

Borehole image analysis of the Nankai Accretionary Wedge, ODP Leg 196: Structural and stress studies

Electrical images recorded with Resistivity-At-Bit (RAB) from two sites drilled during Ocean Drilling Program (ODP) Leg 196 were analyzed to study the effects of subduction at the Nankai margin. For the first time in the history of scientific deep-sea drilling in ODP, in situ complete borehole images of the décollement zone were obtained. Analyses of all drilling-induced fracture data indicated that the maximum horizontal compressive stress (S_Hmax) axes have an azimuth of 303°, and analyses of breakout data from RAB images indicated an azimuth of 310°. These azimuths approximate the convergence direction of the Philippine Sea plate towards the Eurasian plate. The frontal thrust at Site 808 was encountered at about 389 mbsf. Density, porosity, resistivity, and gamma ray data change across the frontal thrust. The décollement zone at the deformation front was identified between 937 and 965 mbsf. The base of the décollement is sharply defined as the maximum extent of conductive fracturing and is marked by abrupt changes in physical properties [Mikada, H., Becker, K., Moore, J.C., Klaus, A., Austin, G.L., Bangs, N.L., Bourlange, S., Broilliard, J., Brückmann, W., Corn, E.R., Davis, E.E., Flemings, P.B., Goldberg, D.B., Gulick, S.S., Hansen, M.B., Hayward, N., Hills, D.J., Hunze, S., Ienaga, M., Ishiguro, H., Kinoshita, M., Macdonald, R.D., McNeill, L., Obana, S., Hong, O.S., Peacock, S., Pettigrew, T.L., Saito, S., Sawa, T., Thaiprasert, N., Tobin, H.J., Tsurumi, H., 2002. Proc. ODP, Initial Rep., 196, College Station, TX, (Ocean Drilling Program)]. The upper boundary of the décollement is marked by several sets of conductive fractures and by high variability in physical properties. The décollement zone is characterized by intense brittle fracturing. These fractures are considered to be the consequence of cyclic stresses and high fluid pressures in this zone. We analyzed fracture dips and their orientations at both sites and found that they are all consistent with a unique stress field model surrounding the two sites.     

The remineralization of organic carbon on the North Carolina continental slope

The sources and fates of metabolizable organic carbon were examined at three sites on the North Carolina slope positioned offshore of Cape Fear, Cape Lookout and Cape Hatteras. The¹³C/¹²C compositions (δ¹³C) of the solid phase organic matter, and the dissolved inorganic carbon (ΣCO₂) produced during its oxidation, suggested that the labile fraction was predominantly marine in origin. The ΣCO₂ concentration gradient across the sediment-water interface, and by inference the ΣCO₂ flux and production rate, increased northward from Cape Fear to Cape Hatteras. Methane distributions and ΣCO₂ δ¹³C values suggest that the rate of anaerobic diagenesis increased northward as well. The differences in sedimentary biogeochemistry are most likely driven by an along-slope gradient of reactive organic carbon flux to the seabed. This trend in reactive organic carbon flux correlates well with macrofaunal densities previously observed at the three sites. Proximity to the shelf and the transport of particulate material by surface boundary currents may control the deposition of metabolizable material on the Carolina slope.Evidence for methanogenesis was found only on the Cape Hatteras slope. The methane, which was produced at a depth of approximately 1 m in the seabed, was consumed nearly quantitatively in the biologically mixed layer as it diffused upward. Irrigation of the sediments by infauna may have provided the necessary oxidant for the consumption of the methane.     

Hydrogeological and hydrochemical characterization of two karstic discharge areas in San Luis Potosí, Mexico

Two karstic discharge areas, Río Verde (RV) and Ciénega de Cabezas (CC), located in a distance of 80 km to each other are chosen to determine the influence of local variations in geology and climatic condition on water chemistry and to examine if the groundwater, supplying the discharge areas, undergoes the same evolution and has a common source. Both study areas are situated on the carbonate platform Valles-San Luis Potosí and comprise a similar geological setting, but despite of their spatial vicinity the climate is semiarid in RV and humid in CC presenting an important factor on the amount of discharge and the concentrations of ions in the discharge. The investigation encompasses discharge, hydrochemical and physico-chemical parameter evaluations as well as the determination of saturation indices, hydrochemical modelling and water type characterization of surface water samples to derive knowledge of the groundwater systems. Scatterplots and saturation indices were used to proof the influence of lithological variability. Both study areas represent normal alkaline water, marked by high concentrations of calcium and magnesium with varying concentrations of bicarbonate and sulphate. In RV, the water interacts with dolomite rocks and gypsum layers, whereas in CC the dolomite content is depleted and the influence of limestone rocks increases. The climatic impact on the groundwater in RV is noticeable by the increase in ionic concentrations due to higher evaporation. In CC the higher amount of precipitation dilutes the groundwater and causes decreasing ionic concentrations.     

An assessment of climate change impacts and adaptation for the Torres Strait Islands, Australia

Adaptive practices are taking place in a range of sectors and regions in Australia in response to existing climate impacts, and in anticipation of future unavoidable impacts. For a rich economy such as Australia’s, the majority of human systems have considerable adaptive capacity. However, the impacts on human systems at the intra-nation level are not homogenous due to their differing levels of exposure, sensitivity and capacity to adapt to climate change. Despite past resilience to changing climates, many Indigenous communities located in remote areas are currently identified as highly vulnerable to climate impacts due to their high level of exposure and sensitivity, but low capacity to adapt. In particular, communities located on low-lying islands have particular vulnerability to sea level rise and increasingly intense storm surges caused by more extreme weather. Several Torres Strait Island community leaders have been increasingly concerned about these issues, and the ongoing risks to these communities’ health and well-being posed by direct and indirect climate impacts. A government agency is beginning to develop short-term and long-term adaptation plans for the region. This work, however, is being developed without adequate scientific assessment of likely ‘climate changed futures.’ This is because the role that anthropogenic climate change has played, or will play, on extreme weather events for this region is not currently clear. This paper draws together regional climate data to enable a more accurate assessment of the islands’ exposure to climate impacts. Understanding the level of exposure and uncertainty around specific impacts is vital to gauge the nature of these islands’ vulnerability, in so doing, to inform decisions about how best to develop anticipatory adaptation strategies over various time horizons, and to address islanders’ concerns about the likely resilience and viability of their communities in the longer term.     

Ontogenetic patterns of concentration indicate lagoon nurseries are essential to common grunts stocks in a Puerto Rican bay

Estimates of abundance and size of three commercially exploited grunt species indicate ontogenetic changes in habitat utilization concentrate their juveniles within the lagoon of the Bay of La Parguera, Puerto Rico. Eleven biotopes, defined by four benthic structures (reef, mangrove, vegetation beds and unconsolidated sediments) and three geographic zones (inner lagoon, outer lagoon and bank shelf) were sampled randomly by visual surveys. French, bluestriped and white grunt (Haemulon flavolineatum, Haemulon sciurus and Haemulon plumeri) were common in the bay and appeared to exhibit similar life history patterns of cross-shelf migration and habitat selection. Recently settled grunts were dispersed over vegetated and unconsolidated soft-bottom sediments of the bay. The juvenile stage occurred in highest densities in shallow lagoon biotopes among the submerged prop-roots of mangrove stands and on inshore reefs. Length data indicates that grunts migrate offshore to adult habitat via increasingly deep reefs. Indices of biotope nursery function based on standing stock estimates of juveniles identified three biotopes, all within the inner lagoon as essential habitat for juveniles of 5–10 cm length interval. This concentration of juveniles within biotopes of the lagoon could represent a bottleneck to recruitment for grunt stocks. Evidence that quantity and quality of lagoon nurseries may limit recruitment indicates that these areas represent a key component of a marine protected area designed to restore fisheries within the bay.     

The linkage between fault throw and footwall scarp erosion patterns: an example from the Bremstein Fault Complex,offshore Mid‐Norway

Studies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault‐related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity‐driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid‐Norway. This 22‐km‐long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first‐order control on footwall erosion patterns is the overall fault‐throw distribution.     

Learning about climate change and implications for near-term policy

Climate change is an issue of risk management. The most important causes for concern are not the median projections of future climate change, but the low-probability, high-consequence impacts. Because the policy question is one of sequential decision making under uncertainty, we need not decide today what to do in the future. We need only to decide what to do today, and future decisions can be revised as we learn more. In this study, we use a stochastic version of the DICE-99 model (Nordhaus WD, Boyer J (2000) Warming the world: economic models of global warming. MIT Press, Cambridge, MA, USA) to explore the effect of different rates of learning on the appropriate level of near-term policy. We show that the effect of learning depends strongly on whether one chooses efficiency (balancing costs and benefits) or cost-effectiveness (stabilizing at a given temperature change target) as the criterion for policy design. Then, we model endogenous learning by calculating posterior distributions of climate sensitivity from Bayesian updating, based on temperature changes that would be observed for a given true climate sensitivity and assumptions about errors, prior distributions, and the presence of additional uncertainties. We show that reducing uncertainty in climate uncertainty takes longer when there is also uncertainty in the rate of heat uptake by the ocean, unless additional observations are used, such as sea level rise.