Density and magnetic susceptibility of rocks from the Lockne and Tvären marine impact structures

Abstract— The Lockne and Tvären impact craters in Sweden formed in a marine environment during the Ordovician The contrast in density between the impact breccias and the surrounding target rock of these two craters is significantly lower than what has been found in craters formed in crystalline targets on land. Another marine‐target structure, the Estonian Kärdla structure, demonstrates intermediate contrast in impact breccia and target rock, which we attribute to the interpreted shallowness of the sea at the Kärdla impact site. We conclude that the main cause for these low‐density contrasts is pore and fracture filling of calcite with subordinate quartz and fluorite. Calcite is the most abundant cement, and its density differs most from that of fractured and brecciated bedrock with a low degree of cementation. Furthermore, from the studied cases, it is concluded that the target rock to impact rock contrast is generally the highest in craters formed on land in crystalline targets and the lowest in craters formed at sea, while craters formed on land in sedimentary targets are intermediate. The low density contrasts should decrease the negative gravity anomalies of marine craters.     

Contrasting regional and global climate simulations over South Asia

Climate Dynamics - Two ensembles of climate simulations, one global and one regional, are used to investigate model errors and projected climate change in seasonal mean temperature and...     

Influence from Surrounding Land on the Turbulence Measurements Above a Lake

Turbulence measurements taken at a Swedish lake are analyzed. Although the measurements took place over a relatively large lake with several km of undisturbed fetch, the turbulence structure was found to be highly influenced by the surrounding land during daytime. Variance spectra of both horizontal velocity and scalars during both unstable and stable stratification displayed a low frequency peak. The energy at lower frequencies showed a daily variation, increasing in the morning and decreasing in the afternoon. This behaviour is explained by spectral lag, where the low frequency energy due to large eddies that originate from the convective boundary layer above the surrounding land. When the air is advected over the lake the small eddies rapidly equilibrate with the new surface forcing. However, the large eddies remain for an appreciable distance and influence the turbulence in the developing lake boundary layer. The variances of the horizontal velocity and scalars are increased by these large eddies, while the turbulent fluxes are mainly unaffected. The drag coefficient, Stanton number and Dalton number used to parametrize the momentum flux, heat flux and latent heat flux respectively all compare well with current parametrizations developed for open sea conditions. The diurnal cycle of the partial pressure of methane, $p\mathrm{CH}_{4}$ , observed at this site is closely related to the diurnal cycle of the lake-air methane flux. An idealized two-dimensional model simulation of the boundary layer at a lake site indicates that the strong response of $p\mathrm{CH}_{4}$ to the surface methane flux is due to the shallow internal boundary layer that develops above the lake, allowing methane to accumulate in a relatively small volume.     

Emission metrics and sea level rise

Here we present two new metrics used for comparing climate impacts of emissions of different climate forcers: the Global Sea level rise Potential (GSP) and the Integrated Global Sea level rise Potential (IGSP). The GSP represents the Sea Level Rise (SLR) at a given time horizon due to an emission pulse of a forcer; the IGSP is similar but represents the time integrated SLR up to a given point in time. The GSP and IGSP are presented relative to the SLR caused by a comparable emission pulse of carbon dioxide. The metrics are assessed using an Upwelling-Diffusion Energy Balance Model (UDEBM). We focus primarily on the thermosteric part of SLR, denoted GSP_th. All of the examined climate forcers – even black carbon, a very Short-Lived Climate Forcer (SLCF) – have considerable influence on the thermosteric SLR on the century time scale. For a given time horizon and forcer, GSP_th lies in between the corresponding metric values obtained using Global Warming Potential (GWP) and Global Temperature change Potential (GTP), whereas IGSP_th ends up in the opposite end to GTP in the spectrum of compared metrics. GSP_th and IGSP_th are more sensitive for SLCFs than for the long-lived Greenhouse Gases (GHGs) to changes in the parameterization of the model (under the time horizons considered here). We also use a Semi-Empirical (SE) model to estimate the full SLR, and corresponding GSP_SE and IGSP_SE, as alternatives to the thermosteric approach. For SLCFs, GSP_SE is greater than GSP_th for all time horizons considered, while the opposite holds for long-lived GHGs such as SF₆.     

Modeling Fine‐Scale Geological Heterogeneity—Examples of Sand Lenses in Tills

Sand lenses at various spatial scales are recognized to add heterogeneity to glacial sediments. They have high hydraulic conductivities relative to the surrounding till matrix and may affect the advective transport of water and contaminants in clayey till settings. Sand lenses were investigated on till outcrops producing binary images of geological cross‐sections capturing the size, shape and distribution of individual features. Sand lenses occur as elongated, anisotropic geobodies that vary in size and extent. Besides, sand lenses show strong non‐stationary patterns on section images that hamper subsequent simulation. Transition probability (TP) and multiple‐point statistics (MPS) were employed to simulate sand lens heterogeneity. We used one cross‐section to parameterize the spatial correlation and a second, parallel section as a reference: it allowed testing the quality of the simulations as a function of the amount of conditioning data under realistic conditions. The performance of the simulations was evaluated on the faithful reproduction of the specific geological structure caused by sand lenses. Multiple‐point statistics offer a better reproduction of sand lens geometry. However, two‐dimensional training images acquired by outcrop mapping are of limited use to generate three‐dimensional realizations with MPS. One can use a technique that consists in splitting the 3D domain into a set of slices in various directions that are sequentially simulated and reassembled into a 3D block. The identification of flow paths through a network of elongated sand lenses and the impact on the equivalent permeability in tills are essential to perform solute transport modeling in the low‐permeability sediments.     

A climatological study of the effect of sea-surface temperature on North Atlantic hurricane intensification

The climatic influence of sea-surface temperature (SST) on intensification is examined for North Atlantic hurricanes by averaging hourly intensity increases from best-track data over the period 1986–2013 in 4° by 4° latitude–longitude grid cells. Independent monthly SST data over the same period are averaged in the same cells. After removing cells with cold water or fast moving hurricanes, the SST effect on intensification, at the climate scale, is quantified by regressing intensification onto SST while controlling for average intensity. The regression is performed using a generalized linear model from a gamma family and a logarithmic link function. The model shows a statistically significant relationship, with higher intensification values associated with higher SST values. On average, mean intensification increases by 16% [(9,?20)% uncertainty interval] for every 1 °C increase in mean SST. A clustered region where the model underpredicts intensification is noted over the southeastern Caribbean Sea, perhaps related to the fresh water plume from the Orinoco River.     

Climate-driven changes in water level: a decadal scale multi-proxy study recording the 8.2-ka event and ecosystem responses in Lake Sarup (Denmark)

A two-stage change in lake level during the 8.2-ka event was identified in Lake Sarup, Denmark (55°N), using a multiproxy approach on precise radiocarbon wiggle-matched annually laminated sediments deposited 8740–8060 cal. yr BP. Changes in δ¹³C and δ¹⁸O indicated closed lake hydrology driven by precipitation. The isotopic, sedimentary and plant macrofossil records suggested that the lake level started to decrease around 8400 cal. yr BP, the decrease accelerating during 8350–8260 before an abrupt increase during 8260–8210. This pattern shows that the climate anomaly started ~150 years before the onset of the 8.2-ka cooling event registered in Greenland ice cores, but was synchronous with hydrologic change in the North American Lake Agassiz drainage. The lake level decrease was accompanied by a higher accumulation rate of inorganic matter and lower accumulation rates of cladoceran subfossils and algal pigments, possibly due to increased turbidity and reduced nutrient input during this drier period. Pigment analysis also showed added importance of diatoms and cryptophytes during this climate anomaly, while cyanobacteria became more important when the water level rose. Moreover, Nymphaeaceae trichosclereids were abundant during the period of algal enrichment. Cladoceran taxa associated with floating leaved plants or benthic habitats responded in a complex way to changes in water level, but the cladoceran assemblages generally reflected deep lake conditions throughout the period. The lake did not return to its pre-8.2-ka event status during the period of analysis, but remained more productive for centuries after the climatic anomaly as judged from the pigment accumulation and assemblage composition. The change to more eutrophic conditions may have been triggered by erosion of marginal deposits. Together, these data confirm the chronology of hydrologic changes and suggest, for the first time, that lake levels exhibited both a decline and an increase in rapid succession in response to the 8.2-ka event in southern Scandinavia.     

Use of the weighted Kappa coefficient in classification error assessment of thematic maps

Abstract

The weighted Kappa coefficient is applied to the comparison of thematic maps. Weighted Kappa is a useful measure of accuracy when the map classes are ordered, or when the relative seriousness of the different possible errors may vary. The calculation and interpretation of weighted Kappa are demonstrated by two examples from forest surveys. First, the accuracy of thematic site quality maps classified according to an ordinal scale is assessed. Error matrices are derived from map overlays, and two different sets of agreement weights are used for the calculation. Weighted Kappa ranges from 0.34 to 0.55, but it does not differ significantly between two separate areas. Secondly, weighted Kappa is calculated for a tree species cover classified according to a nominal scale. Weights reflecting the economic loss for the forest owner due to erroneous data are used for the computation. The value of weighted Kappa is 0.56.     

Large prandtl number finite-amplitude thermal convection with maxwell viscoelasticity

Abstract

Nonlinear interactions of deviatoric stress components and the velocity field occur in all dynamic flows where convected elasticity is accounted for. By incorporating a linear Maxwellian constitutive relation (Oldroyd ‘B’ type) into a finite-amplitude convection model we quantify the magnitude of some of the effects of these nonlinear interactions. For viscoelastic flows the relevant nondimensional parameter is the ratio of viscoelastic constitutive relaxation time constant, λ₁, to the basic flow process time. The Rayleigh number, Ra, and the nondimensional ratio of λ₁ to thermal conduction time, τ_c, are part of the parameter space investigated. However, shorter basic flow time scales than that for thermal equilibration are of interest since most viscoelastic fluids have relatively small values of λ₁ The ratio of λ₁ to buoyant time [bcirc], or λ₁/[bcirc], is, therefore, a pertinent parameter. Using both lithospheric and aesthenospheric values for λ₁, the ratio appropriate to mantle convection is roughly bounded by O(1)[bcirc]>λ₁/[bcirc]>O(10^?6). Employing these bounds and computing low Rayleigh number time-dependent convective flows in a two-dimensional box, it is demonstrated that viscoelasticity has a negligible influence on quasi-steady heat transport even for λ₁/[bcirc]～O(1) For any time-dependent behavior with time scales as short, or shorter than, the buoyant time, [bcirc], viscoelasticity might be important to the local exchange of mechanical energy. The recoverable strain energy in the descending portion of the lithosphere is comparable to the local viscous dissipation. The magnitude of this recoverable component of shear is proportional to λ₁/[bcirc].