Late-Glacial and Holocene Record of Lake Levels of Mathews Pond and Whitehead Lake,Northern Maine,USA

Paleohydrology studies at Mathews Pond and Whitehead Lake in northern Maine revealed synchronous changes in lake levels from about 12,000 ¹⁴C yrs BP to the present. We analyzed gross sediment structure, organic and carbonate content, mineral grain size, and macrofossils of six cores from each of the two lakes, and obtained 72 radiocarbon dates. Interpretation of this paleo-environmental data suggests that the late-glacial and Younger Dryas climate was dry, and lake levels were low. Early Holocene lake levels were considerably higher but declined for an interval from about 8000 to 7200 ¹⁴C yrs BP. Sediment of both lakes contains evidence of a dry period at ∼7400 ¹⁴C yrs BP (8200 cal yr). Lake levels of both sites declined abruptly about 4800 ¹⁴C yrs BP and remained low until 3000 ¹⁴C yrs BP. Modern lake levels were achieved only within the past 600 years. The west-to-east, time-transgressive nature of lake-level changes from several sites across northeastern North America suggests periodic changes in atmospheric circulation patterns as a driving force behind observed moisture balance changes. Electronic supplementary material to this article is available at and accessible for authorized users.     

Solubility of Fe-ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O)

The solubility of Fe-ettringite (Ca₆[Fe(OH)₆]₂(SO₄)₃ · 26H₂O) was measured in a series of precipitation and dissolution experiments at 20 °C and at pH-values between 11.0 and 14.0 using synthesised material. A time-series study showed that equilibrium was reached within 180 days of ageing. After equilibrating, the solid phases were analysed by XRD and TGA while the aqueous solutions were analysed by ICP-OES (calcium, sulphur) and ICP-MS (iron). Fe-ettringite was found to be stable up to pH 13.0. At higher pH-values Fe-monosulphate (Ca₄[Fe(OH)₆]₂(SO₄) · 6H₂O) and Fe-monocarbonate (Ca₄[Fe(OH)₆]₂(CO₃) · 6H₂O) are formed. The solubilities of these hydrates at 25 °C are:      

Determination of the Atmospheric Boundary Layer Height from Radiosonde and Lidar Backscatter

The height of the atmospheric boundary layer is derived with the help of two different measuring systems and methods. From radiosoundings the boundary layer height is determined by the parcel method and by temperature and humidity gradients. From lidar backscatter measurements a combination of the averaging variance method and the high-resolution gradient method is used to determine boundary layer heights. In this paper lidar-derived boundary layer heights on a 10 min basis are presented. Datasets from four experiments – two over land and two over the sea – are used to compare boundary layer heights from both methods. Only the daytime boundary layer is investigated because the height of the nighttime stable boundary layer is below the range of the lidar. In many situations the boundary layer heights from both systems coincide within ±200 m. This corresponds to the standard deviation of lidar-derived 10-min values within a 1-h interval and is due to the time and space variability of the boundary layer height. Deviations appear for certain situations and depend on which radiosonde method is applied. The parcel method fails over land surfaces in the afternoon when the boundary layer stabilizes and over the ocean when the boundary layer is slightly stable. An automatic radiosonde gradient method sometimes fails when multiple layers are present, e.g. a residual layer above the growing convective boundary layer. The lidar method has the advantage of continuous tracing and thus avoids confusion with elevated layers. On the other hand, it mostly fails in situations with boundary layer clouds     

A finite element method for localized erosion in porous media with applications to backward piping in levees

This paper presents a computational method able to effectively model both the simultaneous processes typically observed in backward erosion piping, ie, the pipe tip propagation and the conduit cross-section enlargement. The numerical method is based on the novel formulation of a problem of localized erosion along a line propagating in a multidimensional porous medium. In this line, a conduit with evolving transverse size is embedded, which conveys a multiphase flow. The two systems, porous medium and pipe, are bridged by exchange terms of multiphase fluid mass and by a shared fluid pressure field. On the contrary, different fields are considered to describe flows, which are assumed as Darcian in the porous medium and turbulent in the conduit. These two flows drive pipe propagation and enlargement, respectively, as modeled by means of proper erosion kinetic laws. The corresponding numerical formulation is based on the combination between one- and multidimensional finite elements, to model the erosion conduit and the porous medium, respectively. Several simulations are proposed to demonstrate the ability of the proposed approach in reproducing available experimental data of real-scale tests on levees. Our results point out the crucial role played by the combined influence of pipe propagation and enlargement, as well as of three-dimensional (3D) effects. We also assess the mesh independence of the proposed numerical solution, particularly as concerns the calculated pipe propagation history.     

Seismological constraints on the 2018 Mt. Etna (Italy) flank eruption and implications for the flank dynamics of the volcano

3D earthquake locations, focal mechanisms and stress tensor distribution in a 16‐month interval covering the 2018 Mt. Etna flank eruption, enabled us to investigate the relationship between magma intrusion and structural response of the volcano and shed light on the dynamic processes affecting the instability of Mt. Etna. The magma intrusion likely caused tension in the flanks of the volcano, leading to significant ground deformation and redistribution of stress on the neighbouring faults at the edge of Mt. Etna's unstable sector, encouraging the ESE sliding of the eastern flank of the volcano. Accordingly, FPSs of the post‐eruptive events show strike slip faulting mechanisms, under a stress regime characterized by a maximum compressive σ₁, NE‐SW oriented. In this perspective, any flank eruption could temporarily enhance the sliding process of both the southern and eastern flanks of the volcano.     

Flow dynamics at the continental scale: Streamflow correlation and hydrological similarity

Streamflow variability in space and time critically affects anthropic water uses and ecosystem services. Unfortunately, spatiotemporal patterns of flow regimes are often unknown, as discharge measurements are usually recorded at a limited number of hydrometric stations unevenly distributed along river networks. Advances in understanding the physical processes that control the spatial patterns of river flows are therefore necessary to predict water availability at ungauged locations or to extrapolate pointwise streamflow observations. This work explores the use of the spatial correlation of river flows as a metric to quantify the similarity between hydrological responses of two catchments. Following a stochastic framework, 340,000 cross‐correlations between pairs of daily streamflows time series are predicted at a seasonal timescale across the contiguous United States using 413 catchments of the MOPEX dataset. Model predictions of streamflow correlation obtained in absence of run‐off information are successfully used to identify catchment outlets sharing similar discharge dynamics and flow regimes across a broad range of geomorphoclimatic conditions, without relying on calibration. The selection of reference streamgauges based on predicted streamflow correlation generally outperforms the selection based on spatial proximity, especially as the density of available gauged sections decreases. Interestingly, correlated outlets share a broad spectrum of hydrological signatures (mean discharge, flow variability, and recession properties), suggesting that catchments forced by analogous frequency and intensity of effective rainfall events might exhibit common geomorphoecological traits leading to similar hydrological responses. The proposed framework provides a physical basis to assist the regionalization of flow dynamics and to interpret the spatial variability of flow regimes along stream networks.     

Subglacial and ice‐marginal landforms in south‐central Ontario: implications for ice‐sheet reconfiguration during deglaciation

Regional‐scale, high‐resolution terrain data permit the study of landforms across south‐central Ontario, where the bed of the former Laurentide Ice Sheet is well exposed and passes downflow from irregular topography on Precambrian Shield highlands to flat‐lying Palaeozoic carbonate bedrock, and thick (50 to >200 m) unconsolidated sediment substrates. Rock drumlins and megagrooves are eroded into bedrock and mega‐scale glacial lineations (MSGL) occur on patchy streamlined till residuals in the Algonquin Highlands. Downflow, MSGL pass into juxtaposed rock and drift drumlins on Palaeozoic bedrock and predominantly till‐cored drumlins in areas of thick drift. The Lake Simcoe Moraines, now traceable for more than 80 km across the Peterborough drumlin field (PDF), form a distinct morphological boundary: downflow of the moraine system, drumlins are larger, broader and show no indication of subsequent reworking by the ice, whereas upflow of the moraines, a higher degree of complexity in bedform pattern and morphology is distinguished. Discrete radial and/or cross‐cutting flowset terminate at subtle till‐cored moraine ridges downflow of local topographic lows, indicating multiple phases of late‐stage ice flow with strong local topographic steering. More regional‐scale flow switching is evident as NW‐orientated bedforms modify drumlins south of the Oak Ridges Moraine, and radial flowset emanate from areas within the St. Lawrence and Ottawa River valleys. Most of the drumlins in the PDF formed during an early, regional drumlinization phase of NE–SW flow that followed the deposition of a thick regional till sheet. These were subsequently modified by local‐scale, topographically controlled flows that terminate at till‐cored moraines, providing evidence that the superimposed bedforms record dynamic ice (re)advances throughout the deglaciation of south‐central Ontario. The patterns and relationships of glacial landform distribution and characteristics in south‐central Ontario hold significance for many modern and palaeo‐ice sheets, where similar downflow changes in bed topography and substrate lithology are observed.     

Cold subduction zone in northern Calabria (Italy) revealed by lawsonite–clinopyroxene blueschists

Lawsonite blueschists are important markers of cold subduction zones, subjected to intense fluid circulation. This is because lawsonite preservation in exhumed blueschists and eclogites is typically linked to cold exhumation paths, accompanied by hydration. In the Catena Costiera (Calabria, southern Italy), lawsonite–clinopyroxene blueschists of the Diamante–Terranova Unit, affected by ductile shearing and retrogression, are exposed. Blueschists contain zoned clinopyroxene crystals, showing core–rim compositional variation from diopside to omphacite and hosting primary inclusions of lawsonite and titanite. Thermodynamic modelling of phase equilibria in the NCKFMASHTO system revealed peak metamorphic conditions of 2.0–2.1 GPa and 475–490°C for the Alpine subduction in Calabria. The subsequent post-peak metamorphic evolution mainly proceeded along a decompression and cooling path up to ~1.1 GPa and ~380°C. The final exhumation stages are recorded in the sheared blueschists where a mylonitic to ultramylonitic foliation developed at ~0.7 GPa and 290–315°C. Therefore, the P–T evolution of the Diamante–Terranova blueschists mostly occurred in the stability field of lawsonite, sustained by H₂O-saturated conditions during the exhumation path. The results of this study indicate that the blueschists underwent peak metamorphic conditions higher than previously thought, reaching a maximum depth of ~70 km under a very cold geothermal gradient (~6.6°C/km), during the Eocene subduction of the Ligurian Tethys oceanic crust in Calabria.     

An approach for providing quasi‐convexity to yield functions and a generalized implicit integration scheme for isotropic constitutive models based on 2 unknowns

Yield and plastic potential surfaces are often affected by problems related to convexity. One such problem is encountered when the yield surface that bounds the elastic domain is itself convex; however, convexity is lost when the surface expands to pass through stress points outside the current elastic domain. In this paper, a technique is proposed, which effectively corrects this problem by providing linear homothetic expansion with respect to the centre of the yield surface. A very compact implicit integration scheme is also presented, which is of general applicability for isotropic constitutive models, provided that their yield and plastic potential functions are based on a separate mathematical definition of the meridional and deviatoric sections and that stress invariants are adopted as mechanical quantities. The elastic predictor‐plastic corrector algorithm is based on the solution of a system of 2 equations in 2 unknowns only. This further reduces to a single equation and unknown in the case of yield and plastic potential surfaces with a linear meridional section. The effectiveness of the proposed convexification technique and the efficiency and stability of the integration scheme are investigated by running numerical analyses of a notoriously demanding boundary value problem.