Evidence for a regime shift after the 1997–1998 El Niño, based on 1995, 1998, and 2001 acoustic surveys in the Pacific Eastern Boundary Current

Increasing physical oceanographic evidence suggests that a regime shift, which featured increased equatoward surface winds and upwelling in the Eastern Pacific Boundary Current region, occurred following the 1997–1998 El Niño. We expect the signal of this change in biota to be stronger in euphausiids than in pelagic fishes such as Pacific hake (Merluccius productus) because lower trophic levels are more responsive to changes in upwelling and primary production. We used acoustic backscatter data from acoustic surveys in summer 1995, 1998, and 2001 to explore whether significant changes in abundance and distribution of euphausiids and fishes have occurred between 1995 and 2001. Graphical and statistical results show that the density of fish schools and euphausiid patches increased significantly south of Cape Blanco between 1995 and both 1998 and 2001. North of Cape Blanco there was no consistent change in euphausiid abundance, although both fish and euphausiid distributions in 2001 appear to be significantly higher nearshore than in the other years. Pelagic fish abundance distributions appeared more closely linked to El Niño than to regime shifts, with abundance shifted much farther north in 1998, an El Niño year, than in the other years, while 1995 and 2001 fish abundance distributions are similar to each other. The regime shift appears so far to have had a stronger effect on euphausiids than on pelagic fishes. The change in euphausiid abundance and distribution between the survey years was linked, through a conceptual model, to the relative strength of the poleward flowing California undercurrent and equatorward flowing California current and to the distribution and abundance of hake.     

Strike and dip of crustal boundaries — A method and its application

Summary If for a point on the earth's surface, the apparent (moveout) seismic velocities have been determined for a number of profiles in different directions, a possibility is offered to calculate true strike and dip of crustal boundaries. In the present paper the theory is developed, partly for a sloping Mohorovii discontinuity only, partly for sloping Conrad and Mohorovii discontinuities with parallel strikes. The theory for the sloping Mohorovii discontinuity is applied to field measurements at a place about 30 km west of Uppsala performed in June 1969. With a crustalP-wave velocity of 6.3 km/sec, a good fit to the observations is obtained for aPn-velocity of 8.55 km/sec and a downdip of Moho of 7.2° in the direction S 3.8° E. These numerical values are not the most important output of this paper. On the other hand, the testing of the method appears far more important, and also that improvements can be suggested in the experimental procedure in future applications of this method.     

Theory of the electromagnetic transient response of the ionosphere for plane wave excitation

Summary The reflection and transmission of a transient plane wave by an ionosphere with a linear electron density profile and constant collision frequency are calculated. The exact frequency domain solution is inverted numerically, and the resultant reflected and transmitted waveforms are found to satisfy causality. A linearly varying, nondispersive dielectric half-space is also considered, but the nonphysical nature of the model causes the reflected waveform to be noncausal. Using convolution the waveforms for a step modulated sinusoid excitation are also obtained from the impulse response of the ionosphere. The results provide insight into the physical mechanism involved in the ionospheric propagation of HF electromagnetic pulsed signals.     

An analysis of heat-, momentum-transports,and spectra for clear air turbulence in mid-stratosphere

Summary An analysis of the clear air turbulence in the mid-stratosphere indicates that the turbulence is characterized by an anisotropic field of turbulence with an intense lateral component of the turbulence, associated with strong thermally stable stratification. The Richardson number in the region of turbulence is generally smaller than , the Taylor's criterion for a stratified shear flow.The cospectra for the momentum transport by the streamwise, lateral, and vertical components of the velocity show similarity in their distribution, decreasing with increasing wavenumber. The cospectra for the heat transport by the streamwise, lateral, and vertical components of the velocity show similarity in the high and medium wavenumber ranges, but not in the low wavenumber range of the spectra.The power spectra of the temperature and wind speed are very similar, and are approximately proportional to the power of the wavenumber. The power spectra of the streamwise and lateral components of the velocity are approximately proportional to the –2 power of the wavenumber, whereas the spectrum of the vertical component of the velocity is approximately proportional to the –1 power of the wavenumber.sponsored by the National Science Foundation.     

Nd isotopic evidence for the position of southernmost Indian terranes within East Gondwana

Sm-Nd model ages of orthopyroxene-bearing massif charnockites from the Cardamom Hills Massif and adjoining supracrustal rocks from the Kerala Khondalite Belt in southernmost India are used to infer some of the relationships within these rocks and between them and neighboring areas. Most of these rocks have model ages of 2.1–2.8 Ga with most charnockites in the range 2.2–2.6 Ga. Thus, 3.0–3.4 Ga Archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks. The similarity of Sm-Nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks. A cordierite gneiss, representative of a relatively minor lithology in the supracrustal belt, has a model age of 1.3 Ga. The protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks. The source material of the gneiss protolith may have been located in the Wanni and Vijayan Complexes of Sri Lanka. The overlap of the model ages of rocks in this area and those in the Highland Complex of Sri Lanka supports the notion that these two sets of rocks were joined to each other in Gondwana. They belong to a belt that ran from Antarctica through Sri Lanka and India into Madagascar. This belt was involved in Pan-African tectono-metamorphism, as reflected in the 550 Ma age of the last, granulite-forming, event throughout the belt.     

Geochronology and geochemistry of eclogites from the Mariánské Lázně Complex,Czech Republic: Implications for Variscan orogenesis

The Mariánské Lázn complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated _Nd values and initial ⁸⁷Sr/⁸⁶Sr ratios. Five of the eight eclogite samples have high _Nd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower _Nd values (+ 5.4 to –0.8) and widely variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7096). Such low _Nd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.     

Gulf of Mexico hydrocarbon seep communities—IX. Sterol biosynthesis of seep mussels and its implications for host-symbiont association

The sterol analysis of six hydrocarbon seep mussels (mytilid II and mytilid Ib) from the Alaminos Canyon in the Gulf of Mexico is reported. The sterol composition of the mussel-symbiotic bacteria complex reflects a preponderant synthesis of 4α-methyl sterols (seep mytilid II), and a predominant biosynthesis of 4-desmethyl sterols with some amounts of 4α-methyl sterols (seep mytilid Ib). This suggests a methane-based symbiotic relationship between the mussels and methanotrophic bacteria. It also suggests that the biosynthesis of sterols in the mussel-bacteria complex is completed to the level of cholest-5-en-3ß-ol (mytilid II) or 5α-cholestan-3ß-ol (mytilid Ib).     

The seismicity of Kohistan, Pakistan: Source studies of the Hamran (1972.9.3), Darel (1981.9.12) and Patan (1974.12.28) earthquakes

Long period body waves are examined to show that the Hamran (1972.9.3), Darel (1981.9.12) and Patan (1974.12.28) earthquakes in Kohistan had focal depths of about 8–10 km. All involved high angle reverse faulting (thrusting) and had seismic moments of about 2.2 to 2.7·10²⁵ dyne cm. These shallow depths contrast with the deeper hypocentres found in the Hindu Kush and northeast Karakoram to the north and in Hazara to the south. The Hamran and Patan shocks were assigned depths of 45 km by the ISC, indicating that even well-recorded events in this region may have focal depths in error by 30 km     

Experimental melting of crustal xenoliths from Kilbourne Hole,New Mexico and implications for the contamination and genesis of magmas

Experiments (P=6.9 kb; T=900–1000°C) on four crustal xenoliths from Kilbourne Hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region. Granodioritic gneisses (samples KH-8 and KH-11) yield little melt (<5–25%) by 925°C, but undergo extensive (30–50%) melting between 950 and 1000°C. A dioritic charnockite (KH-9) begins to melt, with the consumption of all modal K-feldspar, by 900°C. It is as fertile a melt source as the granodiorites at lower temperatures, but is outstripped in melt production by the granodiorite gneisses at high temperature, yielding only 26% melt by 1000°C. A pelitic granulite (KH-12) proved to be refractory (confirming earlier predictions based on geochemistry) and did not yield significant melt even at 1000°C. All melts have the composition of metaluminous to slightly peraluminous granites and are unlikely to be individually recognizable as magma contaminants on the basis of major element chemistry. However, the relative stability of K-feldspar during partial melting will produce recognizable signatures in Ba, Eu, K/Ba, and Ba/Rb. Melts of KH-11, which retains substantial K-feldspar throughout the melting interval, are generally low in Ba (<500–800 ppm), have high K/Ba and low Ba/Rb (est.) (62–124 and 1–3, respectively). Melts of KH-9, in which all K-feldspar disappears with the onset of melting, are Ba-rich [2000–2600 ppm, K/Ba=16–22; Ba/Rb (est.) =25–47]. Melts of KH-8 have variable Ba contents; <500 ppm Ba at low temperature but >900 ppm Ba in high-temperature melts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu suggests that the melts coexisting with K-feldspar will have strong negative Eu anomalies. Isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs. For example, the most radiogenic (and incompatible element-rich) sample examined here (the pelitic granulite,⁸⁷Sr/⁸⁶Sr=0.757) is refractory, while samples with far less radiogenic Sr (⁸⁷Sr/⁸⁶Sr=0.708-0.732) produced substantial melt. This suggests that, in this area, the isotopic signature of contamination may be more subtle than expected. The experimental results can be used to model the petrogenesis of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Hole, in the Black Range in the Mogollon-Datil volcanic field. The experimental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpretation is permitted by Sr isotopes. Curstal contamination of a mantle-derived magma best explains the chemical and isotopic characteristics of these tuffs. Both experimental and geochemical data suggest that the rhyolites of Moccasin John Canyon and Diamond Creek could represent direct melts of granodiorite basement similar, but not identical, to the Kilbourne Hole granodiorites, perhaps slightly modified by crystal fractionation. The absence of volcanic rocks having⁸⁷Sr/⁸⁶Sr>0.74 in the region is consistent with the refractory character of the pelitic granulite.     

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

The surface reactivity of biogenic, nanoparticulate UO₂ with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of the presence of surface-associated organic matter on the biogenic UO₂. Synthesis of biogenic UO₂ was accomplished by reduction of aqueous uranyl ions, by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) to remove surface-associated organic matter and soluble uranyl species (NAUO2), or (2) to remove only soluble uranyl species (BIUO2). A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO₂ as a function of particle size and surface-associated organic matter. The N₂-BET surface areas of the two biogenic UO₂ samples following the washing procedures are 128.63 m² g⁻¹ (NAUO2) and 92.56 m² g⁻¹ (BIUO2), and the average particle sizes range from 5-10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO₂ (both NAUO2 and BIUO2) over the pH range 3-9 is the same as that of bulk. The U L_III-edge EXAFS spectra for biogenic UO₂ (both NAUO2 and BIUO2) were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO₂. At pH 7, sorption of Zn(II) onto both bulk uraninite and biogenic, nanoparticulate UO₂ is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00 ± 0.20 μmol m⁻² UO₂ (bulk uraninite), 2.34 ± 0.12 μmol m⁻² UO₂ (NAUO2), and 2.57 ± 0.10 μmol m⁻² UO₂ (BIUO2). Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO₂ indicate that Zn(II) sorption is dependent on the washing protocol. Zn-U pair correlations were observed at 2.8 ± 0.1 Å for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn-U distance, coupled with an average Zn-O distance of 2.09 ± 0.02 Å, indicates that Zn(O,OH)₆ sorbs as bidentate, edge-sharing complexes to UO₈ polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite, which is consistent with a Pauling bond-valence analysis. The absence of Zn-U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO₂ surface. Surface-associated organic matter on the biogenic UO₂ particles also inhibited particle annealing at 90 °C under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO₂ surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.