首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
A 100-year climatology of tropical storms and hurricanes within a 200-km buffer was developed to study their impacts on coral reefs of the Flower Garden Banks (FGB) and neighboring banks of the northwestern Gulf of Mexico. The FGB are most commonly affected by tropical storms from May through November, peaking in August–September. Storms approach from all directions; however, the majority of them approach from the southeast and southwest, which suggests a correlation with storm origin in the Atlantic and Gulf of Mexico. A storm activity cycle lasting 30–40 years was identified similar to that known in the Atlantic basin, and is similar to the recovery time for impacted reefs. On average there is 52% chance of a storm approaching within 200 km of the FGB every year, but only 17% chance of a direct hit every year. Storm-generated waves 5–25 m in height and periods of 11–15 s induce particle speeds of 1–4 m s?1 near these reefs. The wave–current flow is capable of transporting large (～3 cm) sediment particles, uplifting the near-bottom nepheloid layer to the banks tops, but not enough to break coral skeletons. The resulting storm-driven turbulence induces cooling by heat extraction, mixing, and upwelling, which reduces coral bleaching potential and deepens the mixed layer by about 20 m. Tropical storms also aid larvae dispersal from and onto the FGB. Low storm activity in 1994–2004 contributed to an 18% coral cover increase, but Hurricane Rita in 2005 reduced it by 11% and brought coral cover to nearly pre-1994 levels. These results suggest that the FGB reefs and neighboring reef banks act as coral refugia because of their offshore location and deep position in the water column, which shields them from deleterious effects of all but the strongest hurricanes.  相似文献   

2.
The Chilean Patagonian fjords region (41–56°S) is characterized by highly complex geomorphology and hydrographic conditions, and strong seasonal and latitudinal patterns in precipitation, freshwater discharge, glacier coverage, and light regime; all of these directly affect biological production in the water column. In this study, we compiled published and new information on water column properties (primary production, nutrients) and surface sediment characteristics (biogenic opal, organic carbon, molar C/N, bulk sedimentary δ13Corg) from the Chilean Patagonian fjords between 41°S and 55°S, describing herein the latitudinal pattern of water column productivity and its imprint in the underlying sediments. Based on information collected at 188 water column and 118 sediment sampling sites, we grouped the Chilean fjords into four main zones: Inner Sea of Chiloé (41° to ～44°S), Northern Patagonia (44° to ～47°S), Central Patagonia (48–51°S), and Southern Patagonia (Magellan Strait region between 52° and 55°S). Primary production in the Chilean Patagonian fjords was the highest in spring–summer, reflecting the seasonal pattern of water column productivity. A clear north–south latitudinal pattern in primary production was observed, with the highest average spring and summer estimates in the Inner Sea of Chiloé (2427 and 5860 mg C m?2 d?1) and Northern Patagonia (1667 and 2616 mg C m?2 d?1). This pattern was closely related to the higher availability of nutrients, greater solar radiation, and extended photoperiod during the productive season in these two zones. The lowest spring value was found in Caleta Tortel, Central Patagonia (91 mg C m?2 d?1), a site heavily influenced by glacier meltwater and river discharge loaded with glacial sediments. Biogenic opal, an important constituent of the Chilean fjord surface sediments (SiOPAL ～1–13%), reproduced the general north–south pattern of primary production and was directly related to water column silicic acid concentrations. Surface sediments were also rich in organic carbon content and the highest values corresponded to locations far away from glacier influence, sites within fjords, and/or semi-enclosed and protected basins, reflecting both autochthonous (water column productivity) and allochthonous sources (contribution of terrestrial organic matter from fluvial input to the fjords). A gradient was observed from the more oceanic sites to the fjord heads (west–east) in terms of bulk sedimentary δ13Corg and C/N ratios; the more depleted (δ13Corg ?26‰) and higher C/N (23) values corresponded to areas close to rivers and glaciers. A comparison of the Chilean Patagonian fjords with other fjord systems in the world revealed high variability in primary production for all fjord systems as well as similar surface sediment geochemistry due to the mixing of marine and terrestrial organic carbon.  相似文献   

3.
The paper is focused on the global spatial structure, seasonal and interannual variability of the ～5-day Rossby (W1) and ～6-day Kelvin (E1) waves derived from the SABER/TIMED temperature measurements for 6 full years (January 2002–December 2007). The latitude structure of the ～5-day W1 wave is related to the gravest symmetric wave number 1 Rossby wave. The vertical structure of the ～5-day Rossby wave amplitude consists of double-peaked maxima centred at ～80–90 km and ～105–110 km. This wave has a vertically propagating phase structure from the stratosphere up to 120 km altitude with a mean vertical wavelength of ～50–60 km. The ～6-day E1 wave is an equatorially trapped wave symmetric about the equator and located between 20°N and 20°S. Its seasonal behaviour indicates some equinoctial and June solstice amplifications, while the vertical phase structure indicates that this is a vertically propagating wave between 20–100 km altitudes with a mean vertical wavelength of ～25 km.  相似文献   

4.
L-band (1.5 GHz) and VHF (244 MHz) amplitude scintillations observed from Kolkata (22.58°N lat, 88.38° E long, 32°N dip), over a solar cycle 1996–2006 are presented in this paper. Situated near the northern crest of the equatorial anomaly, it is an excellent platform for scintillation studies. Based on 11 years’ data, an attempt is made to develop models of hourly percentage occurrence of scintillations for the rising and declining phases of solar cycle using Neural Network. The relation between fading rate at VHF with the S4 index at L-band is also investigated.  相似文献   

5.
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), a limb sounding Fourier transform interferometer in the mid-infrared band, on board the polar-orbiting ENVIronmental SATellite (ENVISAT) was launched in March 2002 by the European Space Agency (ESA). For the MIPAS data validation, three balloon flights with MIPAS-B, a balloon version of MIPAS, were carried out on the 24th and 25th of September 2002 from Aire sur l’Adour (France 44°N, 0°E), on the 20th and 21th of March, and on the 2nd and 3rd of July 2003 from Esrange, Kiruna (Sweden 68°N, 21°E). The MIPAS operational data version 4.61 for the temperature vertical profiles were compared with the correlative MIPAS-B measurements via the coincident comparison and the trajectory comparison approaches, respectively. The precision of the MIPAS temperature was estimated to be 1.04–2.48 K in the region 123–8 hPa (15–33 km) and beyond the expectation. The systematic difference shows that the absolute accuracy of the MIPAS temperature is within the total combined errors between 356 and 3 hPa (8–39 km). The agreements in the middle stratosphere are better than in the lower stratosphere and upper troposphere. The maximum difference of 1.9 K (0.9%) was found at 228 hPa (11 km). The biases of MIPAS measurements are ?0.39 K (?0.19%) and +0.25 K (+0.12%) (“+” positive bias, “?” negative bias) corresponding to the altitude regions 143–31 hPa (14–24 km) and 31–5 hPa (24–37 km), respectively.  相似文献   

6.
Wind-driven processes exert an important impact on aquatic ecosystems, especially on shallow reservoirs. Flow and heat transport under wind in the Douhe reservoir in China were simulated by a two-dimensional mathematical model. Areas corresponding to different temperature rises were calculated for different wind speed conditions with high frequency. It is shown that high temperature rise areas increase for maximum wind speed conditions while low temperature rise areas keep constant for various wind speed conditions. The concentration of Chl.a decreases with the increase of wind speed, indicating that low wind speed is suitable for algae blooming in the Douhe reservoir. The effects of wind on Bacillariophyta biomass growth become more obvious with the increase of temperature rise areas. The influenced areas of lower temperature rise (0.2–1.49 °C) and higher temperature rise (1.5–2 °C) zone are 8.57 × 106 m2 and 5.18 × 106 m2, respectively, and corresponding total variation amounts of Bacillariophyta biomass are 2.24 × 105/m2 and 0.42 × 105/m2, respectively. Results show that wind has a significant impact on ecological effects due to thermal discharge from thermal power plant into shallow reservoirs.  相似文献   

7.
A geographically extensive investigation was carried out to analyze the concentrations of heavy metals, PCBs and OCPs in the sediments and marine organisms collected from the Liaohe Estuary. In order to determine the spatial distribution and potential ecological risk of heavy metals, the surface sediments were collected from 44 sites in the Liaohe Estuary. The results showed that the heavy metal contents in the sediments were observed in the following order: Cr (11.2–84.8 mg/kg) > Cu (1.7–47.9 mg/kg) > Pb (4.3–28.3 mg/kg) > As (1.61–12.77 mg/kg) > Cd (0.06–0.47 mg/kg) > Hg (0.005–0.113 mg/kg). In comparison with the concentrations of heavy metals and POPs in other regions, the concentrations of As, Pb and DDTs in the Liaohe Estuary were generally low, and other pollutant concentrations were inconsistent with those reported in other regions. The contamination factor (CF), the pollution load index (PLI), the geoaccumulation index and the potential ecological risk index were used to analyze the pollution situation, which showed that the heavy metal pollution in Liaohe Estuary is mainly dominated by Cd and Hg. The concentrations of the four heavy metals varied significantly in the three kinds of tested organisms (fish, mollusk and crustacean), indicating the different accumulative abilities of the species. The results obtained in this study provide useful information background information for further ecology investigation and management in this region.  相似文献   

8.
《Marine pollution bulletin》2012,64(5-12):195-200
Submarine groundwater discharge (SGD) on the reef flat of Bolinao, Pangasinan (Philippines) was mapped using electrical resistivity, 222Rn, and nutrient concentration measurements. Nitrate levels as high as 126 μM, or 1–2 orders of magnitude higher than ambient concentrations, were measured in some areas of the reef flat. Nutrient fluxes were higher during the wet season (May–October) than the dry season (November–April). Dissolved inorganic nitrogen (DIN = NO3 + NO2 + NH4) and soluble reactive phosphorus (SRP) fluxes during the wet season were 4.4 and 0.2 mmoles m−2 d−1, respectively. With the increase population size and anthropogenic activities in Bolinao, an enhancement of SGD-derived nitrogen levels is likely. This could lead to eutrophic conditions in the otherwise oligotrophic waters surrounding the Santiago reef flat.  相似文献   

9.
《Marine pollution bulletin》2009,58(6-12):473-478
Photosystem II (PSII) herbicides have been shown to affect the photosynthesis of corals at low, environmentally relevant concentrations. The recent detection of the PSII herbicide Irgarol-1051 in coastal waters of Hong Kong at concentrations above the EC50 for reduction of photosynthesis of corals prompted further investigation into the extent of PSII herbicide pollution in coral reefs of Hong Kong. Snap-shot and passive samples were taken from coral reef sites and evaluated via HPLC/MS–MS and a novel bioanalytical technique. Low concentrations (less than 10 ng L−1) of diuron and atrazine were found at all study sites. Extracts from these samples concentrated by a factor of 10 were found to reduce the photosynthetic yield of zooxanthellae. It appears unlikely that herbicide pollution is a key issue in isolation but may act synergistically with other stressors to reduce the viability of Hong Kong’s coral reefs. The study has also demonstrated the feasibility of combining sample extraction techniques with a coral specific bioanalytical technique for a sensitive assessment of risks associated with herbicide exposure in corals.  相似文献   

10.
New Late Cretaceous paleomagnetic results from the Okhotsk-Chukotka Volcanic Belt in the Kolyma-Omolon Composite Terrane yield stable and consistent remanent directions. The Late Cretaceous (86–81 Ma) ignimbrites from the Kholchan and Ola suites were sampled at 19 sites in the Magadan area (60.4° N, 151.0° E). We isolated the characteristic paleomagnetic directions from 16 sampled sites using an alternating field demagnetization procedure. The primary nature of these directions is ascertained by dual polarities and positive fold tests. A tilt-corrected mean direction (D = 42.8°, I = 84.7°, k = 46.0, α95 = 10.0°) yields a paleomagnetic pole of 66.7° N, 168.5° E (A95 = 18.8°) which appears almost identical to the 90–67 Ma pole reported from the Lake El’gygytgyn area of the Okhotsk-Chukotka Volcanic Belt (Chukotka Terrane). This consistency suggests that the Kolyma-Omolon Composite Terrane and Chukotka Terrane has acted as a single tectonic unit since 80 Ma without any significant internal deformation. Accordingly, we calculate a combined 80 Ma characteristic paleomagnetic pole (Long. = 164.7° E, Lat. = 68.0°, A95 = 10.9°, N = 12) for the Kolyma-Omolon-Chukotka Block which falls 16.5–17.5° south of the same age poles from Europe and East Asia. We ascribe this discrepancy in pole positions to tectonic activity in the area and infer a southward displacement of 1640 ± 1380 km for the Kolyma-Omolon-Chukotka Block with respect to the North American and Eurasian blocks since 80 Ma; more than 260 km of it is attributed to tectonic displacement in the Arctic Ocean due to the opening of the Canadian Basin.  相似文献   

11.
For centuries, Bermuda has been challenged with wastewater management for the protection of human and environmental health. By quantifying the δ15N of the common sea fan Gorgonia ventalina sampled from 30 sites throughout Bermuda we show that sewage-derived nitrogen is detectable on nearshore coral reefs and declines across the lagoon to the outer rim. We also sampled gorgonians from two museum collections representing a 50y time-series (1958–2008). These samples revealed an increase in δ15N of > 4.0‰ until the mid-1970s, after which δ15N values slowly declined by ~ 2.0‰. A δ15N chronology from a gorgonian skeleton exhibited a similar decline over the last 30–40 years of approximately 0.6‰. We conclude that policies have been effective in reducing sewage impacts to Bermudian reefs. However, significant sources of sewage pollution persist and are likely have a strong impact on harbor and nearshore coral communities and human health.  相似文献   

12.
Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ～ 600 ka samples with an uncertainty of better than ± 15 ka (2σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the 238U–234U–230Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of 230Th and 234U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model.  相似文献   

13.
The results of experiments which characterise the optically stimulated luminescence (OSL) signals of an ash sample (BI07-TL-05) from Barren Island are presented. The infrared stimulated luminescence signal decreases to 5% of its initial value when preheated at 150 °C for 10 s, suggesting that the infrared stimulated luminescence signal associated with the 290–390 nm emission in this sample arises from a single trap evicted by heating to 150 °C. The post-IR blue stimulated luminescence emission has greater thermal stability and arises from traps which are emptied by heating to temperatures between 120 °C and 240 °C. Dose recovery experiments demonstrate that a laboratory dose can be reliably determined to within 5% for the post-IR blue stimulated luminescence signal. However, the fading rate for the post-IR blue stimulation is high, and the g-value is estimated to be (9.6 ± 3.5)% per logarithmic decade for BI07-TL-05.  相似文献   

14.
We have investigated the fate of Staphylococcus aureus by starving the cells and maintaining them in natural seawater at 22 and 4 °C. At 22 °C, cells developed a long-term survival state where about 0.037% of the initial population remained culturable over more than 7 months, whereas at 4 °C, bacteria lost culturability and transiently entered into the viable but non-culturable state (VBNC). However, after 22 days of entry into the VBNC state, the number of viable cells detected via the direct viable count method decreased significantly. We show here that mutational inactivation of catalase (KatA) or superoxide dismutase (SodA) rendered strains hypersensitive to seawater stress at 4 °C and consequently, part of the seawater lethality on S. aureus at low temperature is mediated by reactive oxygen species (ROS) during microcosm-survival process. Shifting the temperature from 4 to 22 °C of totally non-culturable wild-type cells induced a partial recovery of the population. However, deficiencies in catalase or superoxide dismutase prevent resuscitation ability.  相似文献   

15.
We present a catalog of moment tensor (MT) solutions and moment magnitudes, Mw, for 119 shallow (h ≤ 40 km) earthquakes in Greece and its surrounding lands (34°N–42°N, 19°E–30°E) for the years 2006 and 2007, computed with the 1D Time-Domain Moment Tensor inversion method (TDMT_INV code of Dreger, 2003). Magnitudes range from 3.2 ≤ Mw ≤ 5.7. Green's functions (GF) have been pre-computed to build a library, for a number of velocity profiles applicable to the broader Aegean Sea region, to be used in the inversion of observed broad band waveforms (10–50 s). All MT solutions are the outcome of a long series of tests of different reported source locations and hypocenter depths. Quality factors have been assigned to each MT solution based on the number of stations used in the inversion and the goodness of fit between observed and synthetic waveforms. In general, the focal mechanisms are compatible with previous knowledge on the seismotectonics of the Aegean area. The new data provide evidence for strike-slip faulting along NW–SE trending structures at the lower part of Axios basin, close to the heavily industrialized, and presently subsiding, region of the city of Thessaloniki. Normal faulting along E–W trending planes is observed at the Strimon basin, and in Orfanou Gulf in northern Greece. A sequence of events in the east Aegean Sea close to the coastline with western Anatolia sheds light on an active structure bounding the north coastline of Psara–Chios Islands about 20–25 km in length exhibiting right lateral strike-slip faulting.  相似文献   

16.
Explosion deep seismic sounding data sections of high quality had been obtained with RV Meteor in the Reykjanes Iceland Seismic Project (RRISP77 [Angenheister, G., Gebrande, H., Miller, H., Goldflam, P., Weigel, W., Jacoby, W.R., Pálmason, G., Björnsson, S., Einarsson, P., Pavlenkova, N.I., Zverev, S., Litvinenko, I.V., Loncarecic, B., Solomon, S., 1980. Reykjanes Ridge Iceland Seismic Experiment (RRISP 77). J. Geophys. 47, 228–238]) which close an information gap near 62°N. Preliminary results were presented by Weigel [Weigel, W., 1980. Aufbau des Reykjanes Rückens nach refraktionsseismischen Messungen. In: Weigel, W. (Ed.), Reykjanes Rücken, Island, Norwegischer Kontinentalrand. Abschlusskolloquium, Hamburg zur Meteor-Expedition, vol. 45. DFG, Bonn, pp. 53–61], and here we report on the data and results of interpretation. Clear refracted phases to 90 km distance permit crustal and uppermost mantle structure to be modelled by ray tracing. The apparent P-wave velocities are around 4.5, 6–6.5, 7–7.6 and 8.2–8.7 km/s, but no wide-angle reflections have been clearly seen. Accompanying sparker reflection data reveal thin sediment ponds in the axial zone and up to 400 m thick sediments at 10 Ma crustal age. Ray tracing reveals the following model below the sediments: (1) a distinct, 1–2 km thick upper crust (layer 2A) with Vp increasing with age (to 10 Ma) from <3.4 to 4.9 km/s and with a vertical gradient of 0.1–0.2 km/s/km, (2) a lower crust or layer 3 beginning at depths of 2 (axis) to 4 km (10 Ma age) below sea level with 6.1–6.8 km/s and similar vertical gradients as above, (3) the lower crust bottoms at 5.2–9.5 km depth below sea level (0–10 Ma) with a marked discontinuity, underneath which (4) Vp rises from about 7.5–7.8 km/s (0–10 Ma) with a positive vertical gradient of, again, 0.1–0.2 km/s/km such that 8 km/s would be reached at 12 km and deeper near the axis. Our preferred interpretation is that the mantle begins at the distinct discontinuity (“Moho”), but a deeper “Moho” of Vp ≈ 8 km/s cannot be excluded. From Iceland southward to 60°N several experiments show a decrease of crustal thickness from 14 to 8 km. Velocity trends with age across the ridge reflect cooling and filling of cracks, and thickness trends probably suggest volcanic productivity variations as previously suggested.Gravity inversion concentrates on a profile across the ridge with the above seismic a priori information; with 0.2–0.5 km depth uncertainty it leads to a good fit (±2.5 mGal where seismic data exist). Best fitting densities are (in kg/m3) for sediments, 2180; upper crust, 2450–2570; lower crust, 2850–2940; mantle lithosphere, 3215–3240 with a deficit for an asthenospheric wedge of no more than −100 kg/m3. The morphological ridges and troughs superimposed on the SE ridge flank are partly correlated, partly anti-correlated with the Bouguer anomaly and suggest that variable crustal density variations accompany the morphology variations.  相似文献   

17.
In the central Western Alps, a combined structural, petrological and 40Ar–39Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0 ± 0.1 GPa and 350 ± 30 °C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65 GPa and between 220 and 350 °C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an 40Ar–39Ar age of 37.12 ± 0.39 Ma, while D2 phengites yield ages of 35.42 ± 0.38 (sample M173) and 31.60 ± 0.33 Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37 Ma at the beginning of decompression, and that D2 lasted until at least ∼32 Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6 ± 0.2 GPa and 500–540 °C. D1 deformation occurred during exhumation until 0.7–10.5 GPa and 370 ± 30 °C. Published ages suggest that D1 deformation possibly started at ∼50 Ma and lasted until ∼37 Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2 ± 0.1 GPa and 280 ± 30 °C at ∼28 Ma. The gap of 0.6 ± 0.3 GPa and 150 ± 130 °C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50 Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45 Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37 Ma (D1). Between 37 and 31 Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50 Ma to ∼31 Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed.  相似文献   

18.
Seismic analysis and geochemical interpretations provide evidence that two separate hydrothermal cells circulate within the greater Lassen hydrothermal system. One cell originates south to SW of Lassen Peak and within the Brokeoff Volcano depression where it forms a reservoir of hot fluid (235–270 °C) that boils to feed steam to the high-temperature fumarolic areas, and has a plume of degassed reservoir liquid that flows southward to emerge at Growler and Morgan Hot Springs. The second cell originates SSE to SE of Lassen Peak and flows southeastward along inferred faults of the Walker Lane belt (WLB) where it forms a reservoir of hot fluid (220–240 °C) that boils beneath Devils Kitchen and Boiling Springs Lake, and has an outflow plume of degassed liquid that boils again beneath Terminal Geyser. Three distinct seismogenic zones (identified as the West, Middle, and East seismic clusters) occur at shallow depths (< 6 km) in Lassen Volcanic National Park, SW to SSE of Lassen Peak and adjacent to areas of high-temperature (≤ 161 °C) fumarolic activity (Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell) and an area of cold, weak gas emissions (Cold Boiling Lake). The three zones are located within the inferred Rockland caldera in response to interactions between deeply circulating meteoric water and hot brittle rock that overlies residual magma associated with the Lassen Volcanic Center. Earthquake focal mechanisms and stress inversions indicate primarily N–S oriented normal faulting and E–W extension, with some oblique faulting and right lateral shear in the East cluster. The different focal mechanisms as well as spatial and temporal earthquake patterns for the East cluster indicate a greater influence by regional tectonics and inferred faults within the WLB. A fourth, deeper (5–10 km) seismogenic zone (the Devils Kitchen seismic cluster) occurs SE of the East cluster and trends NNW from Sifford Mountain toward the Devils Kitchen thermal area where fumarolic temperatures are ≤ 123 °C. Lassen fumaroles discharge geothermal gases that indicate mixing between a N2-rich, arc-type component and gases derived from air-saturated meteoric recharge water. Most gases have relatively weak isotopic indicators of upper mantle or volcanic components, except for gas from Sulphur Works where δ13C–CO2, δ34S–H2S, and δ15N–N2 values indicate a contribution from the mantle and a subducted sediment source in an arc volcanic setting.  相似文献   

19.
《Continental Shelf Research》2008,28(18):2574-2583
Horizontal distribution patterns of jack mackerel Trachurus japonicus larvae were investigated extensively in the East China Sea (ECS) along the shelf-break region between 26° and 30°N during February–March based on fine-scale larval sampling in 2002 and 2003. A total of 2363 T. japonicus ranging from 1.2 to 12.4 mm body length (BL) were collected at 310 bongo net sampling stations, of which larvae <10 mm BL accounted for 99.1%. In both years, newly hatched larvae (<3 mm BL) were concentrated in the shelf-break region mainly in the southern part of ECS between 26° and 27°N in warm water of 21–23 °C, suggesting that their primary spawning ground existed in and around this area. With growth, larvae were transported in two different directions, i.e., northward and northeastward, corresponding closely with the direction of the Kuroshio Branch Current north of Taiwan (KBCNT) and the Kuroshio, respectively. Replicate sampling cruises at 2 week intervals were conducted in 2003, and the larval distribution pattern changed significantly between the sampling cruises, suggesting that the transport process fluctuates over relatively short periods in relation to oceanographic processes. The transport speed by the KBCNT was estimated to be 0.13–0.28 knots based on the larval distribution, which is one order of magnitude slower than that by the Kuroshio (1.5–3 knots). Habitat temperature gradually declined with growth in both the Kuroshio and KBCNT, but in the KBCNT it was 1–2 °C lower than in the Kuroshio. Our results suggest that the two different larval transport processes lead to a significant difference in the transport route, habitat conditions (such as temperature and food), and site where young fish recruit to the demersal habitat, which will result in different survival and recruitment processes.  相似文献   

20.
Outcrops of the Cretaceous Upper sandstone formation some 375 km to the East of Addis Ababa on the motor Highway to Harar was paleomagnetically investigated. About seventy core samples were collected at various stratigraphic levels from 250–300 meters thick sedimentary formation. After standard sample preparations in the laboratory the resulting specimens were subjected to routine paleomagnetic demagnetization protocol. In the first steps of demagnetizations process the recent and viscous magnetizations were removed by heating until a temperature of level of 300 °C. Further demagnetization of the samples resulted in the isolation of the final magnetization with stable line segments that is directed towards the origin, which is interpreted as Characteristic Remanent Magnetization (ChRM). Rock – magnetic experiments have identified goethite (αFeOOH), hematite (αFe2O3), detritial hematite, and magnetite as the magnetic mineral phases carrying the remanence. The ChRM identified resulted in an average value of (Ds = 0.5°, Is = ?0.7°, α95 = 4.3°, N = 34) for the red sandstones while an average value of (Ds = 335.8°, Is = ?31.8°, α95 = 4.7°, N = 14) for the limestone intercalations. The former ChRM in the red sandstone is determined to be secondary while the latter ChRM is known to be primary. Comparison of these directional results and their pole equivalents with the African plate Apparent Polar Wander Path curve established by Besse and Courtillot (2003) give ages of between 115–130 Million years for the limestone intercalation and ages of 30 million years for red sandstone unit. These are interpreted respectively as estimates of the age of deposition and a later remagnetization respectively.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号