Rate and mechanism in prograde metamorphism

For a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature (disequilibrium) depends on the slowest of three sequential steps: (a) surface detachment of reactant minerals, (b) transport of material to the site of mineral growth, and (c) nucleation and growth of the product mineral. We have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism. The dissolution and growth steps are linear functions of the driving force (– G) of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself.While little is known about heterogeneous nucleation, we argue from field observations that the rate of nucleation is not in general rate limiting. Additionally, if a fluid phase is present true grain boundary diffusion cannot be the mechanism which transports material over the mm to cm distances observed between reactant and product minerals.Simple models of contact (200° C temperature rise in 10,000 years) and regional (10° C per million years) metamorphic events lead to several conclusions concerning the rate determining step. Firstly, growth and dissolution are extremely rapid, dehydration reactions at 500° C going to completion in 2×10² years (contact) and 1×10⁴ years (regional), if all solutes are readily transported. Secondly, the effect of substantial fracture flow of fluid is to divert the transporting medium away from the grain boundary region and hence to retard the transport step. Under most such circumstances it appears that diffusive transport of aqueous SiO₂ or Mg species will be rate controlling. Despite this retardation of reaction rates, the extent of disequilibrium is rarely more than a few degrees C. Extensive disequilibrium (40° C) can only occur for reactions such as the andalusite sillimanite transformation which have very small entropy changes and which occur in rapid metamorphic events.     

Comparison of precipitation in the regional climate model BALTIMOS to radar observations

Observational data and simulations of the regional climate system Baltic integrated model system (BALTIMOS) were used to study precipitation in the Baltic Sea and its drainage basin with a special focus on the diurnal cycle. The study includes a general evaluation of BALTIMOS precipitation, showing that BALTIMOS has too many light rain events causing an overestimation of the total annual precipitation amount. The diurnal cycle as well as its spatial distribution was analysed. BALTIMOS captures the broad characteristics: a significant diurnal variability with an afternoon peak above land and weak variability with a nocturnal peak above sea. An algorithm to distinguish between frontal and convective precipitation was applied to examine the diurnal cycle more thoroughly. The local solar time of maximum rain in summer is about 1 to 2 h earlier in BALTIMOS than in radar observations of precipitation.     

Mineral magnetic record of paleoclimate variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina)

Environmental magnetic techniques were applied to a loess–paleosol sequence of the Chacopampean plain (Buenos Aires, Argentina). Mineral magnetic carriers and their grain size were identified in order to detect magnetic mineral fluctuations associated with climatic changes. Multidomain magnetite of detrital origin dominates the record. In paleosols, a high coercivity fraction was identified. Horizons with no visual evidence of pedogenesis, but showing magnetic behavior analogous to that of paleosols were observed and are thought to represent environmental conditions similar to those prevailing during paleosol formation.The results suggest that the magnetic signal yielded by paleosols in these South American loess deposits is different from that in the Chinese loess. This may be due to differences in parent materials, diagenetic processes and/or differences in paleoclimatic conditions in both regions.     

Effects of mixed physical barrier on residual saltwater removal and groundwater discharge in coastal aquifers

Physical barriers are widely used to control seawater intrusion (SWI). Amongst different kinds of physical barriers, mixed physical barriers (MPBs) are shown to be an effective approach to prevent SWI. However, the system may hinder the discharge of fresh groundwater and the removal of residual saltwater trapped in the inland aquifers of MPBs. Herein, using the validated numerical model, for the first time, we investigated the dynamics of residual saltwater and groundwater discharge after the installation of MPBs. For examining the applicability of MPB and its response to structural variations and hydraulic gradient, the comparison with traditional physical barriers and sensitivity analysis was also carried out. The MPB increased the mixing area of freshwater and saltwater at the beginning of the removal process, resulting in the reduction of the saltwater wedge length (R_L) by 74.6% and the removal of total salt mass (R_M) by 62.6% within the 4% of the total removal time. Meanwhile, the groundwater discharge (Q') rose rapidly after a sharp decline from 100% to 40% in the first stage. As the residual saltwater wedge was retreated, the mixing intensity and removal efficiency decreased substantially in the second stage. Similarly, Q' raised with a declining rate at this stage. The removal efficiency was positively correlated with wall depth and hydraulic gradient and there were optimal distance of the middle spacing and height of lower dam to reach the highest efficiency. The groundwater discharge reduced monotonously with the increase of dam height and wall depth as well as the decrease of barrier spacing and hydraulic gradient. Under certain conditions, the efficiency of MPB in removing residual saltwater could be 40%–100% and 0%–56% higher than that of traditional subsurface dam and cutoff wall, respectively. The laboratory scale conclusions provide valuable physical insight for the real field applications regarding dynamic mechanism and regularity. These findings will always help decision makers choose proper engineering measures and protect groundwater resources in coastal areas.     

Geologie und Erdöl von Comodoro Rivadavia (Patagonien)

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