Aragonitic dendritic prismatic microstructure of the anomalodesmatan bivalve Thracia

The shells of most anomalodesmatan bivalves are composed of an outer aragonitic prismatic granular or columnar layer and inner nacreous layers. The family Thraciidae is one of the very few which lack nacreous layers. In particular, the genus Thracia is exceptional in its possession of a very distinctive, but previously unreported microstructure, which we term ‘dendritic prisms’. Dendritic prisms, consist of slender fibres of aragonite which radiate perpendicular to, and which stack along, the axis of the prism. SEM and TEM study of the periostracum and its intervening mantle, as well as of the shell, in three species of Thracia, have been used to reconstruct the mode of shell calcification and to unravel the crystallography of the dendritic units. The periostracum is composed of an outer dark layer and an inner translucent layer. During the free periostracum phase the dark layer grows at the expense of the translucent layer but at the position of the shell edge, the thickness of the translucent layer which has not previously transformed into the dark layer mineralizes and produces the units typical to the dendritic prismatic layer. Within each unit, the caxis is oriented along the prismatic axis, whereas the a-axis of aragonite runs parallel to the long axis of the fibres. The six-rayed alignment of the latter implies that prisms are formed by {110} polycyclically twinned crystals. We conclude that, despite its distinctive appearance, the dendritic prismatic layer of Thracia is homologous to the outer granular prismatic or prismatic layer of other anomalodesmatans, while the nacreous layer present in most anomalodesmatans has been suppressed

Convective dissolution of carbon dioxide in saline aquifers

Geological carbon dioxide (CO2) storage is a means of reducing anthropogenic emissions. Dissolution of CO2 into the brine, resulting in stable stratification, increases storage security. The dissolution rate is determined by convection in the brine driven by the increase of brine density with CO2 saturation. We present a new analogue fluid system that reproduces the convective behaviour of CO2‐enriched brine. Laboratory experiments and high‐resolution numerical simulations show that the convective flux scales with the Rayleigh number to the 4/5 power, in contrast with a classical linear relationship. A scaling argument for the convective flux incorporating lateral diffusion from downwelling plumes explains this nonlinear relationship for the convective flux, provides a physical picture of high Rayleigh number convection in a porous medium, and predicts the CO2 dissolution rates in CO2 accumulations. These estimates of the dissolution rate show that convective dissolution can play an important role in enhancing storage security

The effect of a fissure on storage in a porous medium

We consider the two-dimensional buoyancy driven flow of a fluid injected into a saturated semi-infinite porous medium bounded by a horizontal barrier in which a single line sink, representing a fissure some distance from the point of injection, allows leakage of buoyant fluid. Our studies are motivated by the geological sequestration of carbon dioxide (CO2) and the possibility that fissures in the cap rock may compromise the safe long-term storage of CO2. A theoretical model is presented that accounts for leakage through the fissure using two parameters, which characterize leakage driven both by the hydrostatic pressure within the overriding fluid and by the buoyancy of the fluid within the fissure. We determine numerical solutions for the evolution of both the gravity current within the porous medium and the volume of fluid that has escaped through the fissure as a function of time. A quantity of considerable practical interest is the efficiency of storage, which we define as the amount of fluid remaining in the porous medium relative to the amount injected. This efficiency scales like t−1/2 at late times, indicating that the efficiency of storage ultimately tends to zero. We confirm the results of our model by comparison with an analogue laboratory experiment and discuss the implications of our two-dimensional model of leakage from a fissure for the geological sequestration of CO2

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction