Multiple magnetization events in the Red River carbonates, Williston Basin, Canada: Evidence for fluid-flow?

Samples collected from the Upper Ordovician Red River carbonates in a well at the centre of the Williston Basin revealed two paleomagnetic components with different inclinations, 60.3 ± 3.9° (k = 70.7, N = 12) and 20.4 ± 3.3° (k = 141.2, N = 8), but similar declination values in individual specimens. Inclination-only analysis indicates two possible scenarios for the age of these two magnetizations: in scenario (a) the timing of magnetization happened sometime between Late Ordovician to Devonian; and in scenario (b) there are two different remagnetizations, one that overlaps Pennsylvanian to Permian time while the other can have either a Late Jurassic or a Tertiary age. Whereas dolomitization and some isotopic data tend to support scenario (a), previous paleomagnetic data from the Williston Basin and from younger units in the same well, the tectonic evolution of the basin, and the hydrocarbon maturation pattern in the Red River carbonates all favour chemical remagnetization(s) driven by orogenic fluids during the Alleghenian and Laramide orogenies.     

Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period

The Holocene climate evolution in Northern Africa is studied in a 9000-yr-long transient simulation with a coupled atmosphere–ocean–vegetation model forced by changes in insolation and atmospheric greenhouse gas concentrations. The model simulates in the monsoonal domains a significant decrease in precipitation under influence of the orbitally forced reduction in summer insolation. In the Western Sahara region, the simulated mid-Holocene transition from humid to arid conditions (the termination of the African Humid Period) is highly non-linear with the occurrence of centennial-scale climate fluctuations due to the biogeophysical feedback between precipitation and vegetation cover. This result is in agreement with proxy data from the Western Sahara region. The other monsoonal regions experience a more gradual climate evolution that linearly follows the insolation forcing, which appears in disagreement with available lake level records.     

Variations in water clarity and chlorophylla in Tampa Bay, Florida, in response to annual rainfall, 1985–2004

In the Tampa Bay region of Florida, extreme levels of annual and seasonal rainfall are often associated with tropical cyclones and strong El Niño episodes. We used stepwise multiple regression models to describe associations between annual and seasonal rainfall levels and annual, bay-segment mean water clarity (as Secchi depth [m]), chlorophylla (μg I^?1), color (pcu), and turbidity (ntu) over a 20-yr period (1985–2004) during which estimated nutrient loadings have been dominated by non-point sources. For most bay segments, variations in annual mean water clarity were associated with variations in chlorophylla concentrations, which were associated in turn with annual or seasonal rainfall. In two bay segments these associations with annual rainfall were superimposed on significant long-term declining trends in chlorophylla. Color was significantly associated with annual rainfall in all bay segments, and in one segment variations in color were the best predictors of variations in water clarity. Turbidity showed a declining trend over time in all bay segments and no association with annual rainfall, and was significantly associated with variations in water clarity in only one bay segment. While chlorophylla, color, and turbidity a affected water clarity to varying degrees, the effects of extreme rainfall events (El Niño events in 1998 and 2003, and multiple tropical cyclone events in 2004) on water clarity were relatively short-lived, persisting for periods of months rather than years. During the 20-yr period addressed in these analyses, declining temporal trends in chlorophylla and turbidity, produced in part by a long-term watershed management program that has focused on curtailing annual loadings of nitrogen and other pollutants, may have helped to prevent the bay as a whole from responding more adversely to the high rainfall periods that occurred in 1998 and 2003–2004.     

Late Holocene sea-level changes and isostasy in western Denmark

Cores and exposed cliff sections in salt marshes around Ho Bugt, a tidal embayment in the northernmost part of the Danish Wadden Sea, were subjected to ¹⁴C dating and litho- and biostratigraphical analyses to reconstruct paleoenvironmental changes and to establish a late Holocene relative sea-level history. Four stages in the late Holocene development of Ho Bugt can be identified: (1) groundwater-table rise and growth of basal peat (from at least 2300 BC to AD 0); (2) salt-marsh formation (0 to AD 250); (3) a freshening phase (AD 250 to AD 1600?), culminating in the drying out of the marshes and producing a distinct black horizon followed by an aeolian phase with sand deposition; and (4) renewed salt-marsh deposition (AD 1600? to present). From 16 calibrated AMS radiocarbon ages on fossil plant fragments and 4 calibrated conventional radiocarbon ages on peat, we reconstructed a local relative sea-level history that shows a steady sea-level rise of 4 m since 4000 cal yr BP. Contrary to suggestions made in the literature, the relative sea-level record of Ho Bugt does not contain a late Holocene highstand. Relative sea-level changes at Ho Bugt are controlled by glacio-isostatic subsidence and can be duplicated by a glacial isostatic adjustment model in which no water is added to the world's oceans after ca. 5000 cal yr BP.     

Two-stage metamorphic evolution of the Bemarivo Belt of northern Madagascar: constraints from reaction textures and in situ monazite dating

New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ～900–950 °C and pressures of ～9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al₂O₃) pointing to even higher temperatures of ～970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.     

Powder factors in asymmetric blast patterns

A cell powder factor defines the local powder factor in a simple geometric procedure that accounts for the charge in each blast hole and the triangulation formed by blast hole collars in a blast pattern. In the case of asymmetric blast patterns, the cell powder factor deviates from the global powder factor. Poor drilling of blast hole collars produces significant variation in the cell powder factor, more so, than does poor blast hole charging. The cell powder factor does not account for timing within a blast and does not easily handle non-parallel blast hole configurations. The cell powder factor is best used as a design or audit tool before blast holes are loaded. It gives the opportunity to correct for poor drilling and/or poor charging practices prior to firing a blast.     

The structure and dynamics of 2-dimensional fluids in swelling clays

The interlayer pores of swelling 2:1 clays provide an ideal 2-dimensional environment in which to study confined fluids. In this paper we discuss our understanding of the structure and dynamics of interlayer fluid species in expanded clays, based primarily on the outcome of recent molecular modelling and neutron scattering studies. Counterion solvation is compared with that measured in bulk solutions, and at a local level the cation-oxygen coordination is found to be remarkably similar in these two environments. However, for the monovalent ions the contribution to the first coordination shell from the clay surfaces increases with counterion radius. This gives rise to inner-sphere (surface) complexes in the case of potassium and caesium. In this context, the location of the negative clay surface charge (i.e. arising from octahedral or tetrahedral substitution) is also found to be of major importance. Divalent cations, such as calcium, eagerly solvate to form outer-sphere complexes. These complexes are able to pin adjacent clay layers together, and thereby prevent colloidal swelling. Confined water molecules form hydrogen bonds to each other and to the clays' surfaces. In this way their local environment relaxes to close to the bulk water structure within two molecular layers of the clay surface. Finally, we discuss the way in which the simple organic molecules methane, methanol and ethylene glycol behave in the interlayer region of hydrated clays. Quasi-elastic neutron scattering of isotopically labelled interlayer CH₃OD and (CH₂OD)₂ in deuterated clay allows us to measure the diffusion of the CH₃- and CH₂-groups in both clay and liquid environments. We find that in both the one-layer methanol solvates and the two-layer glycol solvates the diffusion of the most mobile organic molecules is close to that in the bulk solution.     

Compositional trends among IID irons; their possible formation from the P-rich lower magma in a two-layer core

Group IID is the fifth largest group of iron meteorites and the fourth largest magmatic group (i.e., that formed by fractional crystallization). We report neutron-activation data for 19 (of 21 known) IID irons. These confirm earlier studies showing that the group has a relatively limited range in Ir concentrations, a factor of 5. This limited range is not mainly due to incomplete sampling; Instead, it seems to indicate low solid/liquid distribution coefficients reflecting very low S contents of the parental magma, the same explanation responsible for the limited range in group IVA. Despite this similarity, these two groups have very different volatile patterns. Group IVA has very low abundances of the volatile elements Ga, Sb and Ge whereas in group IID Ga and Sb abundances are the highest known in a magmatic group of iron meteorites and Ge abundances are the second highest (after group IIAB). Group IID appears to be the only large magmatic group having high volatile abundances but low S. In the volatile-depleted groups IVA and IVB it is plausible that S was lost as a volatile from a chondritic precursor material. Because group IID seems to have experienced minimal loss of volatiles, we suggest that S was lost as an early melt having a composition near that of the Fe–FeS eutectic (315 mg/g S). When temperatures had risen 400–500 K higher P-rich melts formed, became gravitationally unstable, and drained through the first melt to form an inner core that was parental to the IID irons. As discussed by [Kracher, A., Wasson, J.T., 1982. The role of S in the evolution of the parental cores of the iron meteorites. Geochim. Cosmochim. Acta 46, 2419–2426], it is plausible that a metal-rich inner core and a S-rich outer core could coexist metastably because stratification near the interface permitted only diffusional mixing. The initial liquidus temperature of the inner, P-rich core is estimated to have been 1740 K; after >60% crystallization the increase in P and the decrease in temperature may have permitted immiscibility with the S-rich outer core. We have not recognized samples of the outer core.