Preliminary estimates of regolith generation and mobility in the Susquehanna Shale Hills Critical Zone Observatory,Pennsylvania, using meteoric 10Be

This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric ¹⁰Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain ¹⁰Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (～21 ka) and deforestation during early colonial land-use. Bulk soil samples (n = 16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples (n = 14) were also collected from a 24 m deep core drilled into the northern ridge top. All meteoric ¹⁰Be concentration profiles show a declining trend with depth, with most of the ¹⁰Be retained in the uppermost decimeters of the soil. Meteoric ¹⁰Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71 ± 0.02 × 10¹⁰ at/cm² and 3.69 ± 0.02 × 10¹⁰ at/cm², than at the ridge top site (1.90 ± 0.01 × 10¹⁰ at/cm²). The ¹⁰Be inventory at the convex ridge top site implies a minimum residence time of ～10.6 ka, or if erosion is steady, an erosion rate of 19.4 ± 0.2 m/My.     

Phyllosilicates synthesis: a way of accessing edges contributions in NMR and FTIR spectroscopies. Example of synthetic talc

A series of new-generation synthetic talcs were prepared by varying the hydrothermal synthesis duration from a few hours up to 2 months. Crystallinity and particle size analysis of the synthetic products were evaluated by photon correlation spectroscopy, transmission electron microscopy, differential thermal analysis and unoriented X-ray diffraction, and then analyzed by various spectroscopic methods such as Fourier transformed infrared spectroscopy and solid-state ¹H and ²⁹Si magic angle spinning nuclear magnetic resonance. The new process for preparing synthetic talcs allows to obtain single-phased particles which after few hours grow heterogeneously and simultaneously in both the c* direction and the (ab) plan. Fine particles, observed for whatever the synthesis duration, lead to the presence of numerous sheet edges surfaces due to particle size which represents the main difference with natural talc. Spectroscopy data show the influence of the fine particles on signals and highlight that synthetic talc characterization is a potential tool for better understanding crystal chemistry of natural talc.     

Remote sensing and the search for surface rupture, Haiti 2010

Minutes after the January 12, 2010 Haiti earthquake, most geologists and seismologists assumed that from its shallow teleseismic location and its largely strike-slip mechanism that a significant rupture must have occurred on the transform plate boundary south and west of Port au Prince. Within hours, plans were being made by geologists to map the anticipated rupture and, if possible, to trench it to obtain a record of paleoseismic slip. However, remote sensing images available a few days after the earthquake revealed raised corals and no significant plate boundary slip, and we now know that shallow slip was transpressive and that no surface rupture occurred. A week after the earthquake, it was clear that scientific visits to the region would be much delayed by the continuing needs of emergency response teams and military support who had commandeered access to the airport at Port au Prince. Serendipitously on 20 January, one of the authors accompanied a film crew on a chartered flight from nearby Santo Domingo with the quest to record the tectonic reasons for the disaster and to document the details of structural damage. At the time, there was still no clear idea of whether the transform boundary had a surface rupture, but there was abundant evidence for surface deformation from Google Earth images showing raised corals and collapsed coastlines along the Lêogáne coast. This article briefly describes communications between remote geologists and the ground-based crew who were guided to critical areas in the search for surface deformation using remote sensing data.     

The Low Compaction Grading Technique on Steep Reclaimed Slopes: Soil Characterization and Static Slope Stability

Since the Surface Mining and Control Reclamation Act of 1977, US coal mining companies have been required by law to restore the approximate ground contours that existed prior to mining. To ensure mass stability and limit erosion, the reclaimed materials have traditionally been placed with significant compaction energy. The Forest Reclamation Approach (FRA) is a relatively new approach that has been successfully used to facilitate the fast establishment of native healthy forests. The FRA method specifies the use of low compaction energy in the top 1.2–1.5 m of the contour, which may be in conflict with general considerations for mechanical slope stability. Although successful for reforestation, the stability of FRA slopes has not been fully investigated and a rational stability method has not been identified. Further, a mechanics-based analysis is limited due to the significant amount of oversize particles which makes the sampling and measurement of soil strength properties difficult. To investigate the stability of steep FRA slopes (steeper than 20°), three reclaimed coal mining sites in the Appalachian region of East Tennessee were investigated. The stability was evaluated by several methods to identify the predominant failure modes. The infinite slope method, coupled with the estimation of the shear strength from field observations, was shown to provide a rational means to evaluate the stability of FRA slopes. The analysis results suggest that the low compaction of the surface materials may not compromise the long-term stability for the sites and material properties investigated.     

L’ouverture de l’Atlantique et son influence sur les peuplements des grands foraminifères des plates-formes péri-océaniques au Mésozoïque

Résumé

— Au Jurassique et au Crétacé, le peuplement des plates-formes des marges atlantiques par les grands foraminifères benthi-ques s’est opéré d’Ouest en Est. Caractéristiques des zones tropicales ces organismes n’ont atteint la marge américaine de l’océan qu’au Callovien ou à l’Oxfordien. Jusqu’au Sénonien, les grands forammifères de cette marge étaient peu diversifiés; ils appartenaient à des espèces bien connues sur la marge européenne et africaine ainsi que dans la Néoléthys méditerranéenne où les grands foraminifères benthiques sont très diversifiés. Néanmoins un faible endémisme est apparu pendant l’Albien sur la marge américaine de l’océan.

D’une façon générale, la faible diversité des formes américaines est sans doute liée à la faible largeur qu’avait alors l’Océan Atlantique petit bassin océanique entre deux super continents, et aux instabilités climatiques que cette situation pouvait engendrer.

Un important changement est intervenu au Campanien où le peuplement des plates-formes carbonatées américaines a montré une grande diversité et un fort taux d’endémisme. Ceci peut être lié pour partie à la largeur notable qu’atteignait alors l’Atlantique aussi bien dans sa partie centrale que méridionale, isolant ainsi la marge américaine de la marge européenne et africaine.

S’agissant du régime climatique, la distribution des grands foraminifères au Jurassique supérieur-Crétacé inférieur montre qu’un même régime tropical régnait alors du Guatemala à Terre Neuve et du Sénégal à la Bretagne sur les deux rivages. Au Crétacé supérieur, les grands foraminifères ne sont connus qu’aux basses latitudes sur la rive américaine (Texas, Floride, Bahamas) tandis qu’ils sont relativement abondants aux latitudes élevées sur la marge européenne (Hollande, Suède méridionale). Il est probable qu’un paléo-Gulstream réchauffait alors les côtes septentrionales de l’Europe.     

Using Mg Isotopes to Trace Cyanobacterially Mediated Magnesium Carbonate Precipitation in Alkaline Lakes

This study assesses the potential use of Mg isotopes to trace Mg carbonate precipitation in natural waters. Salda Lake (SW Turkey) was chosen for this study because it is one of the few modern environments where hydrous Mg carbonates are the dominant precipitating minerals. Stromatolites, consisting mainly of hydromagnesite, are abundant in this lake. The Mg isotope composition of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments were measured. Because Salda Lake is located in a closed basin, mass balance requires that the Mg isotopic offset between Lake Salda water and precipitated hydromagnesite be comparable to the corresponding offset between Salda Lake and its water inputs. This is consistent with observations; a ??²⁶Mg offset of 0.8?C1.4??? is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a ??²⁶Mg 0.9?C1.1??? more negative than its corresponding fluid phase. This isotopic offset also matches closely that measured in the laboratory during both biotic and abiotic hydrous Mg carbonate precipitation by cyanobacteria (Mavromatis, V., Pearce, C., Shirokova, L. S., Bundeleva, I. A., Pokrovsky, O. S., Benezeth, P. and Oelkers, E.H.: Magnesium isotope fractionation during inorganic and cyanobacteria-induced hydrous magnesium carbonate precipitation, Geochim. Cosmochim. Acta, 2012a. 76, 161?C174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg₅(CO₃)₄(OH)₂·5(H₂O)) and hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) with morphological features similar to those of natural samples. Concurrent abiotic control experiments did not exhibit carbonate precipitation demonstrating the critical role of cyanobacteria in the precipitation process.     

Efficient methods for computing observation impact in 4D-Var data assimilation

This paper presents a practical computational approach to quantify the effect of individual observations in estimating the state of a system. Such a methodology can be used for pruning redundant measurements and for designing future sensor networks. The mathematical approach is based on computing the sensitivity of the analyzed model states (unconstrained optimization solution) with respect to the data. The computational cost is dominated by the solution of a linear system, whose matrix is the Hessian of the cost function, and is only available in operator form. The right-hand side is the gradient of a scalar cost function that quantifies the forecast error of the numerical model. The use of adjoint models to obtain the necessary first- and second-order derivatives is discussed. We study various strategies to accelerate the computation, including matrix-free iterative solvers, preconditioners, and an in-house multigrid solver. Experiments are conducted on both a small-size shallow-water equations model and on a large-scale numerical weather prediction model, in order to illustrate the capabilities of the new methodology.     

Carbonate Mineral Saturation State as the Recruitment Cue for Settling Bivalves in Marine Muds

After a pelagic larval phase, infaunal bivalves undergo metamorphosis and transition to the underlying sediments to begin the benthic stage of their life history, where they explore and then either accept or reject sediments. Although the settlement cues used by juvenile infaunal bivalves are poorly understood, here we provide evidence that carbonate saturation state is a significant chemical cue in both direct observation laboratory studies and field manipulations. In the laboratory, plantigrade-stage Mercenaria mercenaria (200 μm shell height) showed a significant positive relationship between percent burrowed and Ω_aragonite, with an increasing probability of settlement with increasing saturation state. In the field, we increased bivalve recruitment by a factor of three in a 30-day field study by raising the pH (～0.3) and saturation state of surface sediments by buffering sediments with crushed shell (CaCO₃). The susceptibility of infaunal bivalves to dissolution mortality and the tight coupling of other sedimentary biogeochemical processes with carbonate dynamics suggest that mineral thermodynamics may be an overarching cue new settlers are responding to.