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201.
Russell A. Moll Alan Bratkovich William Y. B. Chang Peimin Pu 《Estuaries and Coasts》1993,16(1):92-103
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. 相似文献
202.
José María González-Jiménez Carlos Villaseca William L. Griffin Suzanne Y. O’Reilly Elena Belousova Eumenio Ancochea Norman J. Pearson 《Contributions to Mineralogy and Petrology》2014,168(2):1-24
Spinel lherzolite and wehrlite xenoliths from the Cenozoic Calatrava volcanic field carry the geochemical imprint of metasomatic agents that have affected the subcontinental lithospheric mantle beneath Central Iberia. Some xenoliths (mainly wehrlites) were enriched in REE, Sr, P, and CO2 by silicic-carbonate-rich metasomatic melts/fluids, while others record the effects of subduction-related hydrous silicate fluids that have precipitated amphibole and induced high Ti/Eu in primary clinopyroxene. The petrographic observations and geochemical data suggest that interstitial glass in the xenoliths represent the quenched products of Si-rich melts that infiltrated the mantle peridotite shortly before the entrainment of the xenoliths in the host magmas that erupted ca 2 million years ago. During their infiltration, the metasomatic melts reacted with peridotite, resulting in silica enrichment, while remobilizing grains of iron-rich monosulfide solid solution (Fe-rich Mss) initially enclosed in, or intergranular to, primary olivine and pyroxenes. In situ laser ablation inductively coupled plasma-mass spectrometry analysis of single sulfide grains reveals that the Fe-rich Mss in glass shows platinum-group element (PGE) patterns and 187Os/188Os compositions identical to the Fe-rich Mss occurring as inclusions in, or at grain boundaries of primary silicates. Moreover, independent of its microstructural position, Fe-rich Mss exhibits PGE and 187Os/188Os signatures typical of Mss either residual after partial melting or crystallized directly from sulfide melts. Our findings reveal that young metasomatic melt(s)/fluid(s) may carry remobilized sulfides with PGE and Os-isotopic signatures identical to those of texturally older sulfides in the peridotite xenolith. These sulfides thus still provide useful information about the timing and nature of older magmatic events in the subcontinental mantle. 相似文献
203.
Claudio Marchesi William L. Griffin Carlos J. Garrido Jean-Louis Bodinier Suzanne Y. O’Reilly Norman J. Pearson 《Contributions to Mineralogy and Petrology》2010,159(3):315-330
The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites
and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted
as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric
mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling.
The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that
of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial
melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides
in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric
mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement
of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic
episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent
with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model
ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites
reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent. 相似文献
204.
205.
Potential contractional folds on Jupiter's icy moon Europa have been identified. The best example is at the extensional band Astypalaea Linea, where a series of subtle topographic undulations, 25 km in wavelength, possess parasitic tectonic features that support a folding origin. A scenario has been qualitatively proposed, whereby folds form via unstable contraction of the icy lithosphere, compensate for extension elsewhere on Europa, and then subsequently relax. Here, we quantitatively address this scenario, applying a model for viscous-plastic buckling of planetary lithospheres and finite element simulations of topographic relaxation. Our results suggest that the lithosphere of Europa could indeed be unstable, but the low required surface temperatures limit fold formation to higher latitudes, and the high required driving stresses (9–10 MPa) are difficult to achieve on the satellite. The depth to the brittle–ductile transition is well constrained, and high thermal gradients are indicated, implying heat flows near 100 mW m−2. In addition, topographic relaxation progresses so slowly even at these heat flows that it is not a viable mechanism to eliminate such features over the age of Europa's surface. Given the paucity of identified folds, we conclude that the necessary conditions for their formation are rare and that lithospheric folding is a minor mechanism for compensating the large amounts of extension seen elsewhere on Europa. 相似文献
206.
Alexander S. Lloyd Terry Plank Philipp Ruprecht Erik H. Hauri William Rose 《Contributions to Mineralogy and Petrology》2013,165(1):129-153
We have investigated the loss of H2O from olivine-hosted melt inclusions (MIs) by designing an experiment using tephra samples that cooled at different rates owing to their different sizes: ash, lapilli, and bomb samples that were deposited on the same day (10/17/74) of the sub-Plinian eruption of Volcán de Fuego in Guatemala. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that MIs from ash and lapilli record the highest H2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H2O contents (loss of ~1 wt% H2O) and 10 % post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 min for a 3–4-cm radius bomb, assuming conductive cooling and the fastest H diffusivities measured in olivine (D~10?9 to 10?10 m2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, during magma ascent and degassing in the conduit. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H2O loss over the timescale of minutes to hours. The important implication of this study is that a significant portion of the published data on H2O concentrations in olivine-hosted MIs may reflect unrecognized H2O loss via diffusion. This work highlights the importance of reporting clast and MI sizes in order to assess diffusive effects and the potential benefit of using water loss as a chronometer of magma ascent. 相似文献
207.
The relative density of bacterial colonization of small (5 to 40 μ diameter) suspended and sedimented detritus particles in salt marsh creeks was studied using an europium chelate-fluorescent brightener stain coupled with epifluorescent microscopy. Particles were obtained from four substations at each of three sites in salt marshes bordering the York River, Virginia. Coverage of detrital particles by bacteria was uniformly low, averaging 1.54%±1.83% of the particle surface area. Suspended particles larger than 10 μ diameter had lower bacterial densities than did sedimented particles in the same size ranges. It is speculated that microbial growth may be enhanced on sedimented particles both through lack of motion and access to greater nutrient concentrations. 相似文献
208.
William A. Bonner Neal E. Blair Richard M. Lemmon Jose J. Flores Glenn E. Pollock 《Geochimica et cosmochimica acta》1979,43(11):1841-1846
The optically pure d- and l-enantiomers of isovaline (I), which cannot be racemized by ordinary chemical mechanisms involving α-hydrogen removal, and which has been isolated in apparently racemic form from the Murchison meteorite, have been subjected to partial radiolysis by the ionizing radiation from a 3000 Ci 60Co γ-ray source. Both in the anhydrous and hydrated solid states and as solid sodium or hydrochloride salts each enantiomer suffered significant radioracemization of the undestroyed residue during its partial radiolysis. The sodium salt of isovaline in 0.1 M aqueous solution suffered extensive radiolysis with relatively small radiation doses, but showed no detectable radioracemization. The significance of these observations with respect to the primordial enantiomeric composition of the isovaline (and other amino acids) indigenous to meteroties is discussed. 相似文献
209.
Assessing future risk: quantifying the effects of sea level rise on storm surge risk for the southern shores of Long Island, New York 总被引:1,自引:1,他引:1
Christine C. Shepard Vera N. Agostini Ben Gilmer Tashya Allen Jeff Stone William Brooks Michael W. Beck 《Natural Hazards》2012,60(2):727-745
Sea level rise threatens to increase the impacts of future storms and hurricanes on coastal communities. However, many coastal
hazard mitigation plans do not consider sea level rise when assessing storm surge risk. Here we apply a GIS-based approach
to quantify potential changes in storm surge risk due to sea level rise on Long Island, New York. We demonstrate a method
for combining hazard exposure and community vulnerability to spatially characterize risk for both present and future sea level
conditions using commonly available national data sets. Our results show that sea level rise will likely increase risk in
many coastal areas and will potentially create risk where it was not before. We find that even modest and probable sea level
rise (.5 m by 2080) vastly increases the numbers of people (47% increase) and property loss (73% increase) impacted by storm
surge. In addition, the resulting maps of hazard exposure and community vulnerability provide a clear and useful example of
the visual representation of the spatial distribution of the components of risk that can be helpful for developing targeted
hazard mitigation and climate change adaptation strategies. Our results suggest that coastal agencies tasked with managing
storm surge risk must consider the effects of sea level rise if they are to ensure safe and sustainable coastal communities
in the future. 相似文献
210.
Stable cosmogenic isotopes such as 3He and 21Ne are useful for dating of diverse lithologies, quantifying erosion rates and ages of ancient surfaces and sediments, and for assessing complex burial histories. Although many minerals are potentially suitable targets for 3He and 21Ne dating, complex production systematics require calibration of each mineral–isotope pair. We present new results from a drill core in a high-elevation ignimbrite surface, which demonstrates that cosmogenic 3He and 21Ne can be readily measured in biotite and hornblende. 21Ne production rates in hornblende and biotite are similar, and are higher than that in quartz due to production from light elements such as Mg and Al. We measure 21Nehbl/21Neqtz = 1.35 ± 0.03 and 21Nebio/21Neqtz = 1.3 ± 0.02, which yield production rates of 25.6 ± 3.0 and 24.7 ± 2.9 at g? 1 yr? 1 relative to a 21Neqtz production rate of 19.0 ± 1.8 at g? 1 yr? 1. We show that nucleogenic 21Ne concentrations produced via the reaction 18O(α,n)21Ne are manageably small in this setting, and we present a new approach to deconvolve nucleogenic 21Ne by comparison to nucleogenic 22Ne produced from the reaction 19F(α,n)22Ne in F-rich phases such as biotite. Our results show that hornblende is a suitable target phase for cosmogenic 3He dating, but that 3He is lost from biotite at Earth surface temperatures. Comparison of 3He concentrations in hornblende with previously measured mineral phases such as apatite and zircon provides unambiguous evidence for 3He production via the reaction 6Li(n,α)3H → 3He. Due to the atypically high Li content in the hornblende (~ 160 ppm) we estimate that Li-produced 3He represents ~ 40% of total 3He production in our samples, and must be considered on a sample-specific basis if 3He dating in hornblende is to be widely implemented. 相似文献