Oxygen isotope partitioning between phosphate and carbonate in mammalian apatite

The oxygen isotope compositions of phosphate and structural carbonate in mammalian enamel and bone apatite are linked to that of body water at constant body temperature near 37°C, but the isotope systematics of oxygen in structural carbonate are not well understood. Using coupled measurements of the oxygen isotope composition of structural carbonate and phosphate from horse tooth enamel, the apparent oxygen isotope fractionation factor between structural carbonate and body water is estimated to be 1.0263 ± 0.0014. These estimates provide a quantitative basis for using the oxygen isotope composition of structural carbonate in mammalian biogenic apatite for ecological, climatological, and physiological reconstruction.     

High-Latitude Springtime Photochemistry. Part Ii: Sensitivity Studies of Ozone Production

A seven-year record of surface ozone measurements from Denali NationalPark, Alaska shows a persistent spring maximum. These data, combined withmeasurements of NO_x, hydrocarbons, O₃, and PANfrom a continental site in Alaska during the spring of 1995 are used as thebasis for a sensitivity study to explore tropospheric photochemistry in thisregion. Because of the relatively high concentrations of NO_x(mean of 116, median of 91 pptv), the net tendency was for photochemicalozone production. The range of net O₃ production for averageconditions measured at this site during spring is between 0.96–3.9ppbv/day depending on the assumptions used; in any case, this productionmust contribute to the observed springtime maximum in O₃.Model calculations showed that of the anthropogenic ozone precursors, onlyNO_x had a strong effect on the rate of ozone production; themeasured concentrations of anthropogenic hydrocarbons did not significantlyaffect the ozone budget. Naturally produced biogenic hydrocarbons, such asisoprene, may also have a significant effect on ozone production, even atconcentrations of a few 10's of pptv. An observed temperature-isoprenerelationship from a boreal site in Canada indicates that isoprene may bepresent during the Alaskan spring. Measurements of isoprene taken duringthe spring of 1996 suggest that reactive biogenic hydrocarbon emissionsbegin before the emergence of leaves on deciduous trees and that theconcentrations were sufficient to accelerate ozone production.     

The Miocene-Pliocene Macusani Volcanics,SE Peru

The Miocene-Pliocene Macusani volcanics, SE Peru, outcrop in three separate tectonic intermontane basins developed on a Paleozoic-Mesozoic volcano-sedimentary sequence. Several ignimbrite sheets are recognized and K-Ar dates record at least semi-continuous volcanic activity from 10 to 4 Ma in the Macusani field. The volcanics in the Macusani basin comprise crystal-rich (45% crystals) ash-flow tuffs and rare obsidians glasses, both with unusual mineralogy, similar to two-mica peraluminous leucogranites. The mineralogical assemblage (quartz, sanidine Or_69–75, plagioclase, biotite, muscovite and andalusite (both coexisting in the entire volcanic field), sillimanite, schörl-rich tourmaline, cordierite-type phases, hercynitic spinel, fluor-apatite, ilmenite, monazite, zircon, niobian-rutile) is essentially constant throughout the entire Macusani field. Two distinct generations of plagioclase are recognized, viz. group I (An_10–20) and group II (An_30–45). Sillimanite forms abundant inclusions in nearly all phases and is earlier than andalusite which occurs as isolated phenocrysts. Biotite (Al-, Ti-, Fe- and F-rich) shows pronounced deficiencies in octahedral cations. Muscovite is also F-rich and displays limited biotitic and celadonitic substitutions. There is no systematic variation in mineral chemistry with stratigraphic position. The mineralogical data provide a basis for distinction between an early magmatic and a main magmatic stage. The early stage corresponds to the magmatic evolution at or near the source region and includes both restites and early phenocrysts. Some biotites (with textures of disequilibrium melting to Fe — Zn spinel), part of the sillimanite, apatite and monazite, possibly some tourmaline and cordierite-type phases are restites. However, the restite content of the magma was low (5 vol. % maximum). The group II plagioclase are interpreted as early phenocrysts. During this stage, temperatures were as high as 800° C, pressure was no more than 5–7.5 kbar, was intermediate between WM and QFM and was low. The biotite melting textures and the coexistence of restites and early phenocrysts imply fast heating rates in the source region. The transition between the early and the main magmatic stage was abrupt (andalusite crystallization in place of sillimanite, group I vs. group II plagioclases) and suggests rapid ascent of the magma from its source region. During the main crystallization stage, temperature was 650° C or lower at a pressure of 1.5–2 kbar. (calculated from equilibrium between muscovite, quartz, sanidine and andalusite) are around 1, suggesting conditions close to H₂O-saturation. f _HF is around 1 bar but the ratios are significantly different between samples. ranges between 138 and 225 bar. This study shows that felsic, strongly peraluminous, leucogranitic magmas having andalusite and muscovite phenocrysts may be generated under H₂O-undersaturated conditions.CRPG Contribution n 769     

The flux of charcoal to the troposphere during the period of agricultural burning in Panama

Although extensive areas of forests and grasslands are burned in the tropics, relatively little scientific attention has been focused on this phenomenon. In order to determine the land area burned and estimate the charcoal (elemental or graphitic carbon) produced, I monitored agricultural burning in a 1145 km² area in central Panama during the 1981 dry season. Over 10% of the land surface was burned in that year. Charcoal concentrations in the aerosol were also measured and reached values of 3.1 gC/m³ during the peak in burning. Off-peak values of aerosol charcoal are less than 1 gC/m³. The high charcoal concentration reflects the massive amounts of vegetational burning occurring in the area.The charcoal advected by the air mass flowing over the area has been estimated using a box model. Assuming an average aerosol concentration of charcoal of 1 gC/m³ for a three-month burning period, a 2 km atmospheric mixed layer, a 14 km/h wind velocity to the south, and a 150 km wide zone across the western Gulf of Panama watershed, I estimate that, during the dry season, 9×10⁹ g charcoal are mobilized by the troposphere. If 4.1×10¹² g phytomass are annually burned in this region, then the charcoal emission factor to the troposphere is 2.2×10^–3.     

Estimating the consequences of CO2-induced climatic change on north american grain agriculture using general circulation model information

A procedure to estimate the potential climatic effects of a doubling of atmospheric carbon dioxide concentration on agricultural production is illustrated. The method combines use of atmospheric general circulation models (GCMs) and process-oriented crop models. Wheat and corn (maize) yields in three important North American grain cropping regions are treated. Combined use of these two types of models can provide insights into the impacts of climate changes at the level of plant physiology, and potential means by which agricultural production practices may adapt to these changes.Specific agronomic predictions are found to depend critically on the details of the projected climate change. Uncertainties in the specification of the doubled-CO₂ climate by the GCM, particularly with respect to precipitation, dictate that agricultural predictions derived from them at this time must be regarded only as illustrative of the impact assessment method.     

Microcrystalline textures resulting from rapid crystallization in a pseudotachylite melt in a meta-anorthosite

The very fine-grained (1 m) polygonal microcrystalline texture occurring in the groundmass of pseudotachylite veins in the Harris meta-anorthosite and its variation with position are described and an origin by crystallization from a melt at very large undercooling suggested. The intrusive nature of the veins is shown by their geometry and internal structures. Clasts, which are almost always only plagioclase, are generally concentrated towards the centres of veins. Flow of tens of millimetres can account for this concentration in millimetre-thick veins as a result of the Bagnold effect. The veins are generally thin (5 m to 5 mm or more), are frequently zoned and always contain transparent granules of high relief (probably Al-rich pyroxene) and opaque granules of magnetite up to a few micrometres in size. The granules are either uniformly distributed in microcrystalline textures or concentrated locally giving cellular textures. In some veins, spherulitic or bow-tie textures occur. The coarsest textures are found in the centres of the thickest veins. The groundmass of the pseudotachylite is never completely isotropic but consists of a mosaic of transparent plagioclase crystals decreasing in size from the centres of thick veins to less than 1 m in thin veins or in the margins of thicker veins. This fine microcrystalline texture was studied by both scanning and transmission electron microscopy and consists of polyhedral crystals of regular size in the range 0.2–1.5 m, which show little sign of deformation. The local composition of the pseudotachylites varies little from the average compositions of the rocks in optically homogeneous veins, the variation being within the compositional space defined by the minerals of the host rock. This shows that homogenization of the pseudotachylite has occurred. In cellular veinsdifferentiation has occurred as the compositions of the cell centres lie outside those of the minerals of the host rock. This was produced by segregation of the granules, pyroxene being absent from the host rock. The plagioclase in the pseudotachylite is more disordered than that in the host rock. All the microtextures described are absent from the associated cataclasites and cannot be due to recrystallization of a fine-grained and intensely strained rock powder. The physical state on and after intrusion was that of a melt and injection was followed by crystallization. The melt was produced by more or less total fusion of the host rock minerals at shallow depth by heat produced during local faulting and perhaps during crack propagation. The fine microcrystalline texture very closely resembles that produced during hypercooling of molten metals and alloys. It thus possibly formed not by devitrification but from a melt at much greater degrees of undercooling than the spherulitic and bow-tie textures.     

Compression of lageos laser data

Since the first generation of satellite laser tracking stations, techniques have continuously improved; passes containing several thousands of range data are now common. For various reasons, such as data transmission problems or difficulties in processing the whole data set, quick-look data or samples are often used, though they lead to a loss-of information. In this paperm, we have investigated a way of compressing LAGEOS laser data into normal points using a simple reference orbit. For assessing their accuracy, these normal points have been compared, by the way of a global processing, to those derived by the University of Texas in a different way. Finally the whole set of raw data obtained during the three months of the MERIT short campaign have been compressed into normal points using this method. Evaluation of the external precision of these normal points has been performed by comparison with another series; the pole component evaluations show precisions similar to those obtained from raw data solutions.     

“Crustal splitting and the emplacement of Pyrenean lherzolites and granulites”—A reply to M.W. Fischer

The emplacement of Pyrenean lherzolites is a fascinating problem to tackle because of the unusual location of the lherzolites within sediments and the numerous constraints which are currently available on the Pyrenean belt (plates kinematics, petrology, structural geology, geochemistry, geophysics). Our model [1] represents an attempt to integrate the emplacement of lherzolites within the geodynamic evolution of the Pyrenees. However, on the basis of imprecise statements and observations, and neglecting or quoting incorrectly several previous works. Fischer [2] considers that we failed to address a number of problems relevant to the evolution of the North Pyrenean Fault Zone. Most of his comments find an answer in a careful reading of our article; nevertheless the following points must be clearly emphasized.     

Crustal splitting and the emplacement of Pyrenean lherzolites and granulites

Pyrenean lherzolites representing fragments of the upper mantle are embedded in Mesozoic sediments along the 500-km-long, 20-km-wide “North Pyrenean Zone”. These ultramafics and the nearby occurrence of granulites, the presence of undersaturated rocks intrusive in the Cretaceous, and the development of a narrow elongate zone of “Pyrenean metamorphism”, confer a fundamental orogenic and geodynamic significance to this zone. These phenomena and the emplacement of the lherzolites may be attributed to an important phase of crustal thinning followed by a succession of compressive stages. The crustal thinning involved in the North Pyrenean Zone is possibly related to a transcurrent movement mainly during Albian and Senonian times, indicating a transform zone [24]. The ascent and emplacement of lherzolites in Albian sediments may have occurred in response to alternating tensional and compressional stresses operating in transcurrent zones. The Pyrenean metamorphism in Mesozoic sediments may be related to convective (emplacement of lherzolites) as well as conductive (thermal anomaly in connection with the rifting) processes. This model is compared with existing transform domains. Finally, it is suggested that crustal thinning may permit the incorporation of lower crust-upper mantle associations in the upper levels of the crust during subsequent compressive stages.