Physical,chemical, and biological conditions associated with the early stages of the Lake Michigan vernal thermal front

An investigation of the thermal front in southeastern Lake Michigan during April 1988 revealed a dynamic physical, chemical, and biological environment. The front was observed approximately 4 km from the coast as a distinct gradient separating cold open-lake waters from warmer nearshore waters. Surface isotherms near the front were generally parallel to one another but skewed with respect to shore. Comparison between April 22 and 29 showed that the surface isothern pattern was modulated by wind stress. The pattern from April 29 showed signs of flow instabilities with horizontal scales of 1 km to 5 km. Surface drifter trajectories provided estimates of horizontal convergence at the fron which varied from 7×10^?6 s^?1 to 20×10^?6 s^?1. Inferred rates of downwelling, which ranged from 9.5 m d^?1 to 20.7 m d^?1, were sufficient to move a water parcel from the surface to the bottom in 2 to 6 d at the front. Convergent circulation was observed on both sampling dates despite contrasts in wind stress. Concentrations of chloride, soluble silica, and chlorophyll, which were always higher inshore, were 5% to 82% larger than offshore mean values. The aquatic environment just inshore of the thermal front was characterized by chlorphyll concentrations which exceeded 5.0 μg 1^?1 while concentrations offshore were between 1.0 μg 1^?1 and 2.0 μg 1^?1. A relatively uniform vertical structure in chlorophyll concentrations in the frontal zone was consistent with the observed convergence and inferred downwelling near the front.     

Determination of part-per-trillion levels of atmospheric dimethyl sulfide by isotope dilution gas chromatography/mass spectrometry

Stable isotopic dilution was applied to the determination of dimethyl sulfide (DMS) in ambient air at the low part-per-trillion by volume (pptrv) levels. Perdeuterated DMS was used as an internal standard in the gas chromatography/mass spectrometry determination. The isotopically labelled internal standard provided insensitivity to possible losses of DMS in sampling and analysis. The lower limit of detection (LLD) was 1 pptrv with a sample acquisition time of 2 min.     

Eolian evidence for spatial variability of late Quaternary climates in tropical Africa

Study of the eolian fraction of late Quaternary sediments from the tropical Atlantic reveals that two modes of long-term climate variability have existed in tropical Africa during the last 150,000 yr. Tropical northwest Africa (i.e., the southwestern Sahara and Sahel) was driest during glaciations and stades, but wetter than at present during interglaciations and interstades. This may be a response to ice sheets at higher latitudes, via equatorward displacement of the westerlies and the subtropical high. In contrast, central equatorial Africa (southeast of the Sahara) was most arid during interstades and times of ice growth, and most humid during deglaciation. Wet periods in this area correspond to insolation maxima in northern hemisphere summer. A 23,000-yr precessional rhythm is suggested, supporting a direct link between African Monsoon intensity and orbitally modulated insolation. The late Holocene is the only time observed when both areas are arid during an interglacial episode. This may reflect, in part, anthropogenic disturbance of late Holocene climates.     

Response of the uppermost mantle to Indo-Eurasian collisional tectonics

The rheological law for the power law creep field of peridotite suggested by Chopra and Paterson (1981, 1984) is combined with theoretically modelled thermal and stress perturbations for continent-continent collisions (Bird et al., 1975; Bird, 1978a) to determine the effect of such collisions on the uppermost mantle below the zone of convergence. The model predicts a change from continental to oceanic geotherms during the oceanic subduction phase, followed by a rapid rise in differential stress during the collisional phase.These perturbations, when combined, cause a cyclical change in the steady state flow of the uppermost mantle ending with a re-equilibration of thermal and stress regimes. The thermal re-equilibration proceeds at a much slower rate, with a suggested total re-equilibration period of about 100 m.y. after collision has ceased. The combination of the flow law with the model predicts that the perturbations in the mantle are limited in depth with no changes in mantle behavior below about 100 km.     

Hydrologic and geochemical properties of the San Andreas fault at the Stone Canyon well

The Stone Canyon well penetrates 600 m of highly fractured and severely altered quartz diorite intimately associated with the creeping segment of the San Andreas fault of central California. Geophysical logs reveal a complex hydrology dominated by major fractures. Fluid pressure in some fractures is sufficient to prevent invasion of the formation by heavy drilling mud, implying pore pressures at least 10% higher than hydrostatic ones. At least three chemically distinct waters are encountered, including a chloride brine clearly segregated from the shallow, potable groundwater. Chemical alteration of the quartz diorite persists throughout the well, far below the depth where the water-rock reactions responsible for the ubiquitous chlorite and mixed-layer clays can be considered weathering. Whole-rock ¹⁸O analyses indicate significant interaction of the rocks with a low ¹⁸O fluid within some of the fractured and altered zones, whereas a deeper sample shows¹⁸O enrichment. High pore pressures encountered in Stone Canyon may be due to tectonic compression. Measurements of temporal variations in the pore pressure at the well may provide a means of predicting earthquakes along this segment of the San Andreas fault.     

Pressure-temperature conditions in the Wadi Kid metamorphic complex: Implications for the pan-african event in SE Sinai

Geothermobarometry using continuous equilibria in pelitic rocks of the central and northeastern Wadi Kid areas, Sinai, yields pressure estimates of 320 MPa (3.2 kbar) and temperature estimates of 565° C and >620° C for staurolite-andalusite zone assemblages and anatectic rocks, respectively. This corresponds to geothermal gradients of 47° C/km and 50° C/km. TheseP-T conditions are shown to be compatible with burial under a volcanic cover. The principal heat source for high-grade metamorphism and anatexis is thought to be rising magmatic diapirs. Uplift was relatively slow and the final stages of heating and early stages of cooling probably occurred isobarically. The data imply a high heat flow regime for the Pan-African event in Sinai.     

Lead isotopic evidence for evolutionary changes in magma-crust interaction,Central Andes,southern Peru

Lead isotopic measurements were made on Andean igneous rocks of Jurassic to Recent age in Moquegua and Tacna Departments, southernmost Peru, to clarify the petrogenesis of the rocks and, in particular, to investigate the effect of crustal thickness on rock composition. This location in the Cordillera Occidental is ideal for such a study because the ca. 2 Ga Precambrian basement rocks (Arequipa massif) have a distinct Pb isotopic signature which is an excellent tracer of crustal interaction, and because geomorphological research has shown that the continental crust was here thickened drastically in the later Tertiary.Seven samples of quartz diorites and granodiorites from the Ilo and Toquepala intrusive complexes, and seven samples of Toquepala Group subaerial volcanics were analyzed for Pb isotopic compositions. The plutonic rocks range in age from Jurassic to Eocene; the volcanic rocks are all Late Cretaceous to Eocene. With one exception, the Pb isotopic ratios are in the ranges ²⁰⁶Pb/²⁰⁴Pb= 18.52–18.75, ²⁰⁷Pb/²⁰⁴Pb= 15.58–15.65, and ²⁰⁸Pb/²⁰⁴Pb= 38.53–38.74. The data reflect very little or no interaction with old continental material of the Arequipa massif type.Lead from four Miocene Huaylillas Formation ash-flow tuffs, two Pliocene Capillune Formation andesites and five Quaternary Barroso Group andesites has lower ²⁰⁶Pb/²⁰⁴Pb than that in the pre-Miocene rocks, but relatively high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb (²⁰⁶Pb/²⁰⁴Pb= 18.16–18.30, ²⁰⁷Pb/²⁰⁴Pb= 15.55–15.63, ²⁰⁸Pb/²⁰⁴Pb= 38.45–38.90). Tilton and Barreiro [9] have shown that contamination by Arequipa massif granulites can explain the isotopic composition of the Barosso Group lavas, and the new data demonstrate that this effect is evident, to varying degrees, in all the analysed Neogene volcanic rocks. The initial incorporation of such basement material into the magma coincided with the Early Miocene uplift of this segment of the Cordillera Occidental [32], and thus with the creation of a thick crustal root. The data strongly imply a relationship between crustal thickness and degree of crustal contamination of magmas in this area, but a rigorous relationship is not yet established.     

Provincialism and correlations between some properties of vitrinites

Optical and chemical properties for hand-picked samples of vitrinite from a number of British coalfields are correlated and the correlations compared with previously published data. The form of the relationship found for the properties of British vitrinites is generally similar to that exhibited by a number of other sets of analyses, but some significant differences also exist in several of the correlations. For maximum reflectance as a function of carbon content, the present data indicate that at medium and low ranks, maximum reflectance is lower than the level suggested by most previous studies. The relationships of volatile-matter yield to reflectance and carbon content suggest that at low ranks, volatile-matter yield is strongly dependent upon the nature of the coalification history of the vitrinite. Furthermore, these correlations are likely to show provincialism, in that correlations which hold for one coalfield may not be representative of the relationship in other coalfields. Similarly, major differences in relationships involving bireflectance are associated with the tectonic setting at the time of effective coalification. The data presented here indicate that for low-rank coals at least, correlations between properties of vitrinites must take account of provincialism if they are to be sufficiently reliable to be useful. The measurement of a number of rank-sensitive variables can yield additional information about rank, as compared with the use of a single rank-sensitive variable.