Role of the bedrock topography in the Quaternary filling of a giant estuarine basin: the Lower St. Lawrence Estuary,Eastern Canada

The geometry of estuarine and/or incised‐valley basins and their protected character compared with open sea basins are favourable for the preservation of sedimentary successions. The Lower St. Lawrence Estuary Basin (LSLEB, eastern Canada) is characterized by a thick (>400 m in certain areas) Quaternary succession. High‐ and very high‐resolution seismic reflection data, multibeam bathymetry coverage completed by core and chronostratigraphic data as well as a 3‐D seismic stratigraphic model are used to document the geometrical relationships between the bedrock and the Quaternary units of the LSLEB. The bedrock geometry of LSLEB is characterized by two large troughs that are interpreted as resulting mainly from repeated (?) periods of glacial overdeepening of a pre‐Quaternary drainage system. However, other mechanisms with complex feedback effects such as differential glacio‐isostatic uplift, erosion, sedimentary supply, and subsidence may have contributed to the formation of bedrock troughs. The two large bedrock troughs are mostly filled by ～200 m thick Wisconsinan (Marine Isotopic Stages 2–4) and possibly older sediments. Overlying units recorded the retreat of the Laurentian Ice Sheet during the Late Wisconsinan (Marine Isotopic Stage 2) and estuarine conditions during the Holocene. The strong correlation existing between the bedrock topography and the thickness of the Quaternary succession is indicative of the effectiveness of the LSLEB as a sediment trap.     

Fe-Ti deposits in Rogaland anorthosites (South Norway): geochemical characteristics and problems of interpretation

The Rogaland anorthosite province (S. Norway) contains numerous Fe-Ti oxide deposits, including the second most important ilmenite deposit in the world, the Tellnes deposit. The largest deposits are located in the Åna-Sira anorthosite massif. Others occur in the Håland-Helleren anorthosite massif, particularly along the deformed contact with the Egersund-Ogna massif, where they were previously considered formed by metasomatic processes. All deposits are now regarded as magmatic. The structure, mineralogy and geochemistry of 11 selected Fe-Ti deposits (Tellnes, Storgangen, Blåfjell, Laksedal, Kydlandsvatn, Kagnuden, Rødemyr, Hestnes, Eigerøy, Svånes, and Jerneld) are discussed in light of recent models proposed for the origin of Rogaland anorthosites and related rocks. Massif-type anorthosites result from the diapiric uprise of a plagioclase crystal mush which crystallized along a large P–T interval. Except for Tellnes, which is related to a post-deformation dyke, the Fe-Ti deposits in anorthosite massifs have been deformed by this movement during and after their crystallization. The differentiation process of the jotunitic parental magma has built up cumulates in the Bjerkreim-Sokndal layered intrusion and liquids in the Tellnes dyke and other jotunitic intrusions. Ilmenite is a liquidus mineral immediately after plagioclase in the sequence of crystallization of these jotunites, its interstitial character in the rocks resulting from subsolidus recrystallization. Ilmenite can thus accumulate early in the evolution of jotunitic magmas. This feature, together with high contents in Cr, V, Mg and Ni, links the Jerneld, Blåfjell and Svånes deposits (type?1) to the early evolution of a jotunitic magma. In the Bjerkreim-Sokndal intrusion, magnetite can appear with ilmenite at the very beginning of the sequence of crystallization, but normally crystallizes after orthopyroxene and before clinopyroxene and apatite. The early appearance of magnetite is a characteristic feature of type 2 deposits (Tellnes, Storgangen, Kydlandsvatn, Rødemyr I) and suggests conditions similar to the early magnetite cumulates in the Bjerkreim-Sokndal intrusion. Evidence of layering further favours gravity-controlled sorting processes to concentrate the oxides. Large-scale subsolidus segregation of the oxides due to high-temperature deformation can further concentrate these minerals in silicate-absent meter-sized masses. Type 3 deposits (Rødemyr II, Kagnuden, Hestnes and Eigerøy) could be derived from the more evolved stages of differentiation, as indicated by high REE in apatite, high Ti and Zn in magnetite and relatively low Cr, V, Mg, Ni contents in both oxides. The Cr content in both oxide minerals is however higher than in the equivalent cumulates of the Bjerkreim-Sokndal intrusion. Although immiscibility as the mechanism of enrichment leading to silicate-absent oxide-apatite veins, as in Hestnes and Eigerøy, cannot be precluded, there is no direct evidence in the veins, nor has any structural or geochemical evidence of immiscibility ever been found in jotunite dykes and Fe-Ti-P-rich rocks. Further investigations on the influence of subsolidus exchange of elements between the two oxides are needed to improve the use of trace elements as differentiation indexes.     

Strategies for waveform processing in sparker data

A sparker is a marine seismic impulsive source used for high-resolution seismic surveys. Sparker sources were very popular during the late 1960s and 1970s before being supplanted by small volume airguns. However, in the last 10 years there has been renewed interest in sparker technology because (1) it can be easily deployed at relatively low costs and (2) in certain areas the use of small airguns is restricted for environmental purposes. In this study a sparker source was used to assess the seismic stratigraphy of Quaternary deposits and to image the sediment/bedrock interface. Three different inverse filtering methods were tested (i.e., spiking deconvolution, match-filtering and vertical seismic profile (VSP) deconvolution) to correct the poor shot-to-shot repeatability of the source and to compress its reverberations. Results show that the matched-filter and VSP deconvolution methods, which design and apply one operator for each shot, produced comparable results, whereas the spiking deconvolution that used the same operator on all traces failed to compress the source signature properly.     

The crustal tongue melting model and the origin of massive anorthosites

Recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid-crust levels. Recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10–13 kbar, indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40–50 km depths. In the Sveconorwegian Province terne boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km. In Southern Norway, we suggest that a lithospheric-scale weakness zone (the Feda transition zone?) has channelled the Rogaland anorthosites through linear delamination, asthenospheric uprise and melting of a mafic lower crustal tongue.     

Isotopic compositions of S, N and C in soils and vegetation of three forest types in Québec, Canada

The concentrations and the isotopic compositions of S, N and C were studied in soils and in the dominant plant species of three forested watersheds (Québec, Canada) located along a latitudinal and atmospheric deposition gradient. Large increases in S, N and C isotope ratios (up to 3.9‰, 10‰, 2.6‰, respectively) were observed with increasing soil depth at the three watersheds. These increases were accompanied by a strong decrease in elemental concentrations resulting in a strong negative relationship between these two variables. Both S and N concentrations throughout the soil profile and δ³⁴S and δ¹⁵N in the mineral soil appeared to increase with increasing S and N deposition rates and decreasing latitude. A strong positive linear relationship was found between δ³⁴S and δ¹⁵N (R² = 0.72) values and between organic S and N concentrations (R² = 0.96) in soils. The slope of the linear relationship between δ³⁴S and δ¹⁵N (δ³⁴S = f(δ¹⁵N)) indicated that isotopic fractionation was almost 4 times higher for S than for N during transformations that occurred in soil. However, this difference might reflect a higher degree of openness of the S cycle compared to the N cycle rather than an isotope effect per se. Overall, the results suggest that N and S inputs significantly impact the isotope ratios and the concentrations of N and S in the soils, and that S and N were closely associated and subject to similar processes with the same isotopic effects throughout the soil profile. Contrary to most studies, δ³⁴S-SO₄ in stream water of the most northerly site with the lowest S deposition rate was significantly higher than δ³⁴S-SO₄ in atmospheric depositions but similar to the δ³⁴S of the bulk mineral soil. It suggests that the mineral soil actually contributes a large portion of the stream S-SO₄ for this site.     

Bayesian estimation of intensity–duration–frequency curves and of the return period associated to a given rainfall event

Intensity–duration–frequency (IDF) curves are used extensively in engineering to assess the return periods of rainfall events and often steer decisions in urban water structures such as sewers, pipes and retention basins. In the province of Québec, precipitation time series are often short, leading to a considerable uncertainty on the parameters of the probabilistic distributions describing rainfall intensity. In this paper, we apply Bayesian analysis to the estimation of IDF curves. The results show the extent of uncertainties in IDF curves and the ensuing risk of their misinterpretation. This uncertainty is even more problematic when IDF curves are used to estimate the return period of a given event. Indeed, standard methods provide overly large return period estimates, leading to a false sense of security. Comparison of the Bayesian and classical approaches is made using different prior assumptions for the return period and different estimation methods. A new prior distribution is also proposed based on subjective appraisal by witnesses of the extreme character of the event.     

A Comparison of Higher-Order Vertical Velocity Moments in the Convective Boundary Layer from Lidar with In Situ Measurements and Large-Eddy Simulation

We have analyzed measurements of vertical velocity w statistics with the NOAA high resolution Doppler lidar (HRDL) from about 390 m above the surface to the top of the convective boundary layer (CBL) over a relatively flat and uniform agricultural surface during the Lidars-in-Flat-Terrain (LIFT) experiment in 1996. The temporal resolution of the zenith-pointing lidar was about 1 s, and the range-gate resolution about 30 m. Vertical cross-sections of w were used to calculate second- to fourth-moment statistics of w as a function of height throughout most of the CBL. We compare the results with large-eddy simulations (LES) of the CBL and with in situ aircraft measurements. A major cause of the observed case-to-case variability in the vertical profiles of the higher moments is differences in stability. For example, for the most convective cases, the skewness from both LES and observations changes more with height than for cases with more shear, with the observations changing more with stability than the LES. We also found a decrease in skewness, particularly in the upper part of the CBL, with an increase in LES grid resolution.     

Observations and Large-Eddy Simulations of Entrainment in the Sheared Sahelian Boundary Layer

At the top of the planetary boundary layer, the entrainment of air, which incorporates dry and warm air from the free troposphere into the boundary layer, is a key process for exchanges with the free troposphere since it controls the growth of the boundary layer. Here, we focus on the semi-arid boundary layer where the entrainment process is analyzed using aircraft observations collected during the African Monsoon Multidisciplinary Analysis experiment and large-eddy simulations. The role of the entrainment is specifically enhanced in this region where very large gradients at the planetary boundary-layer top can be found due to the presence of the moist, cold monsoon flow on which the dry, warm Harmattan flow is superimposed. A first large-eddy simulation is designed based on aircraft observations of 5 June 2006 during the transition period between dry conditions and the active monsoon phase. The simulation reproduces the boundary-layer development and dynamics observed on this day. From this specific case, sensitivity tests are carried out to cover a range of conditions observed during seven other flights made in the same transition period in order to describe the entrainment processes in detail. The combination of large-eddy simulations and observations allows us to test the parametrization of entrainment in a mixed-layer model with zero-order and first-order approximations for the entrainment zone. The latter representation of the entrainment zone gives a better fit with the conditions encountered in the Sahelian boundary layer during the transition period because large entrainment thicknesses are observed. The sensitivity study also provides an opportunity to highlight the contribution of shear stress and scalar jumps at the top of the boundary layer in the entrainment process, and to test a relevant parametrization published in the recent literature for a mixed-layer model.     

The Lac Cornu retrograded eclogites (Aiguilles Rouges massif,Western Alps,France): evidence of crustal origin and metasomatic alteration

Metabasic rocks interbedded in amphibolite facies supracrustal gneisses outcrop around Lac Carnu in the Aiguilles Rouges massif (Western Alps). The cores of the thickest boudinaged lenses are made up of eclogitic amphibolites grading outwards into amphibolites. The common assemblage is unzoned garnet + symplectitic clinopyroxebe + hornblende + plagioclase. In a slightly amphibolitized sample, minimum P and T conditions of equilibrium between garnet and omphacite inclusions are 780°C and 11 Kb. A polymetamorphic pre-Alpine evolution of the massif is thus demonstrated. Eclogitic amphibolites show variations in major element composition similar to the Skaergaard evolution in the Al₂O₃ ? FeO + Fe₂O₃ ? MgO triangle. Variation diagrams in which Zr is taken as differentiation index also indicate magmatic trends for Mn, Ti, P, Ni, Co, Y, V and Cr, Ca, K, Na, Sr, Rb and Si were mobile during the evolution of the rocks. The igneous trend can be described by a quantitative model of fractional crystallization in which a noritic assemblage separated in the initial stage. This provides evidence of crustal P, T conditions of differentiation for the original materoal and in situ evolution for the eclogites. A second group of amphibolites shows banded structure and transitional terms with the surrounding gneisses. A volcano-sedimentary origin is suggested though the process cannot be modelled. The rocks may result from complex interactions of magnetic, metasomatic and sedimentary processes. It is concluded that Lac Cornu metabasites were originally continental tholeiites, though several geochemical criteria tend to indicate an oceanic origin. This casts some doubt on the validity of these criteria, when applied to metamorphic rocks.