Heat capacity calculations for Al2O3 corundum and MgSiO3 ilmenite

We have used Kieffer's vibrational model to calculate heat capacities and entropies for Al₂O₃ corundum and MgSiO₃ ilmenite, using available vibrational and elastic data for these phases. The calculated heat capacity for corundum is within 1–2 percent of the experimental values between 100 K and 1,800 K, while that for MgSiO₃ ilmenite is within 1–2 percent of the experimental data between 350 K and 500 K. We have calculated the heat capacity for MgSiO₃ ilmenite from 50 K to 1,800 K, which extends the range of available heat capacity data for this phase. The results of this calculation suggest that there may be differences in the vibrational properties of corundum and MgSiO₃ ilmenite. Finally, we have used the results of our calculation to obtain a transition entropy of near -18.8 J/mol.K for the MgSiO₃ pyroxene-ilmenite reaction.     

Land cover change and soil organic carbon stocks in the Republic of Ireland 1851–2000

Using both historic records and CORINE land cover maps, we assessed the impact of land cover change on the stock of soil organic carbon (SOC) in the Republic of Ireland from 1851 to 2000. We identified ten principal land cover classes: arable land, forest, grassland, heterogeneous agricultural areas/other, nonvegetated semi-natural areas, peatland, suburban, urban, water bodies, and wetland. For each land cover class, the SOC stock was estimated as the product of SOC density and land cover area. These were summed to calculate a national SOC budget for the Republic of Ireland. The Republic of Ireland’s 6.94 million hectares of land have undergone considerable change over the past 150 years. The most striking feature is the decrease in arable land from 1.44 million ha in 1851 to 0.55 million ha in 2000. Over the same time period, forested land increased by 0.53 million ha. As of 2000, agricultural lands including arable land (7.85%), grassland (54.33%), and the heterogeneous agricultural areas/other class (7.91%) account for 70.09% of Irish land cover. We estimate that the SOC stock in the Republic of Ireland, to 1 m depth, has increased from 1,391 Tg in 1851 to 1,469 Tg in 2000 despite soil loss due to urbanization. This increase is largely due to the increase of forested land with its higher SOC stocks when compared to agricultural lands. Peatlands contain a disproportionate quantity of the SOC stock. Although peatlands only occupy 17.36% of the land area, as of 2000, they represented 36% of the SOC stock (to 1 m depth).     

Computation of the vertical gradient of gravity for two dimensional bodies

Summary A method of computation of the vertical gradients of gravity, at different elevations, from the residual gravity profile due to a localised two dimensional causative body is presented. The vertical gradient at an elevation above a station has been expressed as a weighted summation of the gravity values at equally spaced stations along a profile. These weights (coefficients) depend only upon the positions of the individual stations, relative to the station of computation, and upon the ratio of the elevation at which the vertical gradient value is sought to the separation of the stations. Functional forms of these coefficients have been derived analytically and they have been computed for different relative positions of the individual stations and for different values for the ratio of elevation to separation.An analytical expression has been derived for the remainder term which is necessary when the above summation is performed over a finite length of the profile only. As this expression involves the parameters, (i) total mass per unit length and (ii) the positions of the centre of gravity of the causative body, a method has been developed separately for their accurate determination from the anomaly profile of finite length.The efficacy of the method has been assessed by applying it to an arbitrary theoretical model.     

Post‐orogenic thermal history and exhumation of the northern Appalachian Basin: Low‐temperature thermochronologic constraints

Apatite fission‐track (AFT) thermochronology and (U‐Th)/He (AHe) dating, combined with paleothermometers and independent geologic constraints, are used to model the thermal history of Devonian Catskill delta wedge strata. The timing and rates of cooling determines the likely post‐orogenic exhumation history of the northern Appalachian Foreland Basin (NAB) in New York and Pennsylvania. AFT ages generally young from west to east, decreasing from ~185 to 120 Ma. AHe single‐grain ages range from ~188 to 116 Ma. Models show that this part of the Appalachian foreland basin experienced a non‐uniform, multi‐stage cooling history. Cooling rates vary over time, ~1–2 °C/Myr in the Early Jurassic to Early Cretaceous, ~0.15–0.25 °C/Myr from the Early Cretaceous to Late Cenozoic, and ~1–2 °C/Myr beginning in the Miocene. Our results from the Mesozoic are broadly consistent with earlier studies, but with the integration of multiple thermochronometers and multi‐kinetic annealing algorithms in newer inverse thermal modeling programs, we constrain a Late Cenozoic increase in cooling which had been previously enigmatic in eastern U.S. low‐temperature thermochronology datasets. Multi‐stage cooling and exhumation of the NAB is driven by post‐orogenic basin inversion and catchment drainage reorganization, in response to changes in base level due to rifting, plus isostatic and dynamic topographic processes modified by flexure over the long (~200 Myr) post‐orogenic period. This study compliments other regional exhumation data‐sets, while constraining the timing of post‐orogenic cooling and exhumation in the NAB and contributing important insights on the post‐orogenic development and inversion of foreland basins along passive margins.     

New political violence in Africa – Secular sectarianism in Sierra Leone

Mary Douglas (1993) has indicated some of the shared concerns that keep voluntary enclaves together as ‘social movements’. The present paper describes a rather different kind of enclave politics, being concerned with the ‘new violence’ of groups enclaved by social exclusion and force. The paper focuses on the emergence and subsequent development of dissidents in the civil war in Sierra Leone. The RUF was at the outset a tiny but conventional guerrilla force, modelled on one of the militias in the Liberian civil war, and seeking to overthrow a decayed neo-patrimonial ‘one-party’ regime. Gaining little popular support, its members became marooned in forested isolation on the Liberia-Sierra Leone border. Recruits were mainly school children seized by force and ‘converted’ through having to take part in atrocities against rural civilians. Unable to return to the larger society on pain of summary execution, captives have had little option but to adjust to the political fantasies of a violent and unstable leadership. They were enclaved by force. Indicating the predictive strength of Douglas' arguments about the cultural dynamics of the enclave, the subsequent atrocity-drenched story of the RUF suggests that it is not the content of belief that determines institutional culture but vice versa. To survive, the enforced enclave develops the concerns of the classic ‘sect’ - a doctrinaire ‘rationality’ (expressed in crude acts of ‘subtractive’ violence), rejection of magic, decision making by lots, preoccupation with defection, otiose leadership style. If the rest of the world is to come to terms with violent enclave organizations such as the RUF and Algerian GIA it may have to pay more careful attention to the way enclave institutions think. They may not respond ‘rationally’ to the kinds of incentives offered by mainstream groups organised around hierarchical and/or individualist systems of social accountability. This revised version was published online in August 2006 with corrections to the Cover Date.