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1.
Analysis of sediment and aquatic vegetation samples collected along the Onkaparinga estuary, South Australia, revealed the distribution patterns of benthic and epi-benthic micro-organisms, specifically foraminifera and ostracods. The total assemblage provided an assemblage ‘snap shot,’ contemporaneous with the environmental conditions at the time of sampling, as well as seasonally and time-averaged distribution information. Species richness is low in the upper reaches of the estuary and favours species with a wide salinity tolerance. Observed species richness increases seawards. Species richness and abundance within species is greater where there is conspicuous aquatic vegetation. Thus, species richness and distribution appear to be related to the proximity to the sea and the provision of diverse habitable substrate. Foraminiferal species, Ammonia beccarii and Elphidium excavatum, and ostracods belonging to the genera Paracypria, Xestolebris and Leptocythere dominate the total assemblage. These species are characteristic of euryhaline conditions. The mutual maximum occurrences of Reophax barwonensis, Ammobaculites barwonensis and Trochammina inflata signify mid-estuarine conditions. In the lower regions of the estuary, the appearance of shallow marine species of foraminifera such as Elphidium crispum and Quinqueloculina poeyana, and ostracods Bairdoppilata sp., Hemicytherura spp. and Paranesidea spp. signifies interactions with the adjacent inner shelf coastal waters. Throughout the entire length of the estuary, the presence of vegetal substrate created a strong numerical bias towards live occurrences of the ostracods Paracypria sp. and Xestolebris cedunaensis. These species are potentially useful as proxies for paleoenvironmental interpretations of estuarine sediments.  相似文献   

2.
Nearly 6 m of uncompacted muddy sediment was recovered from the floor of the northern Coorong Lagoon in the core Coorong #5. Radiocarbon analyses of molluscan shells indicate that sedimentation at the core site commenced before 6830 ± 90 yr cal BP, and the presence of Pinus pollen confirms a modern age for the uppermost 0.5 m. Microfossils extracted from the core sediment samples, 2 cm slices at 10 cm intervals, included the foraminifera Ammonia sp., Elphidium excavatum and Elphidium gunteri; the ostracods Osticythere baragwanathi and Leptocythere lacustris; and charophyte oogonia. Shell fragments of the estuarine bivalve Spisula (Notospisula) trigonella in the lowermost 0.7 m of the core are evidence that these sediments were subject to some marine influence, but the absence of foraminifera and ostracods from this same interval indicates that at the core site salinity was not sufficient to support populations of these organisms. Thus, prior to 6830 ± 90 yr cal BP the Younghusband Peninsula was in place, in part isolating the northern lagoon from the Southern Ocean. The initial recorded salinity event is signified by abundant Ammonia sp. at a core depth of 5.2 m. The duration of this event was relatively brief; foraminifera were mostly absent in the immediately overlying 2 m, representing ca 700 yr of sedimentation. This observation is attributed to substantial inflow of freshwater from the River Murray. In the upper 3.0 m, Ammonia sp. was present in most core samples indicating that for most of the past 6000 years the Coorong Lagoon has been sufficiently saline to support a continuing population of this species. At a core depth of 1.3 m, the sediment sample yielded >2000 tests of Ammonia sp., and they were accompanied by maximum pre-modern numbers of E. excavatum, O. baragwanathi and oogonia. Taken together, these data signify the maximum pre-modern salinity event recorded in the core sediments, probably correlating in time with regional drought conditions at ca 3500 yr BP. Elphidium gunteri is confined to the modern sediments where it is abundant and accompanied by equally large numbers of Ammonia sp., E. excavatum, O. baragwanathi and L. lacustris. These data collectively indicate water conditions that are significantly changed from those that prevailed in the Coorong Lagoon for most of the Holocene.  相似文献   

3.
Ross River flows through the Townsville/Thuringowa urban area in north Queensland, Australia, which has a dry tropical climate characterized by high inter-annual rainfall variation. Unregulated rivers in the Ross catchment basin deliver freshwater flows to their estuaries during both strong and weak wet seasons. The construction of a series of dams and weirs on Ross River means the wet-dry cycle is accentuated, leading to constant marine salinities throughout the estuary becoming the norm, with a lack of freshwater flow for five or more years at a time. The fish fauna of Ross River estuary was sampled in the post wet and dry seasons during an extremely dry climatic period (1994) and extremely wet climatic period (2000) using a small mesh (6 mm) pocket seine net. The fish fauna seemed to reflect seasonal differences. Catches from 1994 (dry period) were comprised entirely of 88 marine and euryhaline species, while the 69 species captured in 2000 (wet period) included 13 freshwater species. However, the freshwater species in the upper estuary were individuals washed over the weir, rather than part of a functional faunal gradient. During 1994 faunal composition was related more to site identity than to the position of the site along an upstream gradient. In contrast, during 2000 there were clear upstream faunal gradients with compositions in upstream sites heavily influenced by freshwater species, and marine and euryhaline species dominating downstream sites. Patterns of species dominance also varied between years. In contrast, trophic composition showed consistent shifts in both years, from high proportions of herbivores, carnivores and benthoplanktivores in May towards high proportions of benthivores in August. Not only do faunal composition, seasonal faunal change and ecological connectivity seem to be impaired, but ecological processes in the estuary that rely on seasonal freshwater flows are likely to be unable to operate normally in most years. The extreme seasonality in Ross River may serve as a model for many of the changes that will be experienced in dry tropics estuaries under global climate change scenarios of more extreme seasonality.  相似文献   

4.
Sediments from Rapid Lake document glacial and vegetation history in the Temple Lake valley of the Wind River Range, Wyoming over the past 11,000 to 12,000 yr. Radiocarbon age determinations on basal detrital organic matter from Rapid Lake (11,770 ± 710 yr B.P.) and Temple Lake (11,400 ± 630 yr B.P.) bracket the age of the Temple Lake moraine, suggesting that the moraine formed in the late Pleistocene. This terminal Pleistocene readvance may be represented at lower elevations by the expansion of forest into intermontane basins 12,000 to 10,000 yr B.P. Vegetation in the Wind River Range responded to changing environmental conditions at the end of the Pleistocene. Following deglaciation, alpine tundra in the Temple Lake valley was replaced by a Pinus albicaulis parkland by about 11,300 14C yr B.P. Picea and Abies, established by 10,600 14C yr B.P., grew with Pinus albicaulis in a mixed conifer forest at and up to 100 m above Rapid Lake for most of the Holocene. Middle Holocene summer temperatures were about 1.5°C warmer than today. By about 5400 14C yr B.P. Pinus albicaulis and Abies became less prominent at upper treeline because of decreased winter snowpack and higher maximum summer temperatures. The position of the modern treeline was established by 3000 14 C yr B.P. when Picea retreated downslope in response to Neoglacial cooling.  相似文献   

5.
Coupled measurements of δ18O and accelerator mass spectrometry (AMS) 14C in a particular species of planktonic foraminifera may be used to calculate sea-level estimates for the last deglaciation. Of critical importance for this type of study is a knowledge of the seasonality of foraminiferal growth, which can be provided by δ18O measurements of modern shells (core tops, plankton tows). Isotopic (δ18O, AMS-14C dating) and faunal records (transfer function sea surface temperature) were obtained from two cores in the North Atlantic at about 37°N. The locations were chosen to obtain high sedimentation rate records removed from the major ice-melt discharge areas of the last deglaciation. Based upon Globigerina bulloides data, four δ18O-based sea-level estimates were calculated: −67 ± 7 m at 12,200 yr B.P. and −24 ± 8 m at about 8200 yr B.P. for core SU 81-18; −83 ± 10 m at 12,200 yr B.P. and −13 ± 11 m at about 8500 yr B.P. for core SU 81-14. Using a second working hypothesis concerning the seasonability of G. bulloides growth, it is suggested that the sea-level rose by about 40 m during the millennium which followed 14,500 yr B.P.  相似文献   

6.
The paleoenvironmental history of the Beilun River estuary on the coast of Beibu Gulf in the northwest South China Sea is reconstructed based on fossil diatoms, isotopic dating, sedimentary grain size data, mineralogy and geochemistry in three sediment core samples. Results show that the estuary has experienced significant environmental changes since deposition began about 20,000 yr ago. Freshwater runoff of the Beilun River initially was strong. However, the freshwater runoff reduced significantly after a transgressive event. Subsequently the estuary’s position began to migrate to the northeast. At the end of the Late Pleistocene the estuary shifted gradually towards the southwest. In the Early-Mid Holocene, the estuary’s geomorphology was shaped by seawater transgressing into the ancient river channel. The basin was filled continuously but slowly to form the present Beilun River estuary. Holocene transgression in this area could be divided roughly into three stages, including oscillation period 1, the maximum transgression period, and oscillation period 2.  相似文献   

7.
CSA mine exploits a ‘Cobar-type’ Cu–Pb–Zn±Au±Ag deposit within a cleaved and metamorphosed portion of the Cobar Supergroup, central New South Wales. The deposit comprises systems of ‘lenses’ that encompass veins, disseminations and semi-massive to massive Cu–Pb–Zn ores. The systems and contained lenses truncate bedding, are approximately coplanar with regional cleavage and similarly oriented shear zones and plunge parallel to the elongation lineation. Systems have extreme vertical continuity (>1000 m), short strike length (400 m) and narrow width (100 m), exhibit vertical and lateral ore-type variation and have alteration haloes. Models of ore formation include classical hydrothermalism, structurally controlled remobilisation and polymodal concepts; syntectonic emplacement now holds sway.Fluid inclusions were examined from quartz±sulphide veins adjacent to now-extracted ore, from coexisting quartz–sulphide within ore, and from vughs in barren quartz veins. Lack of early primary inclusions precluded direct determination of fluids associated with D2–D3 ore and vein emplacement. Similarly, decrepitation (by near-isobaric heating) of the two oldest secondary populations precluded direct determination of fluid phases immediately following D2–D3 ore and vein emplacement. Post-decrepitation outflow (late D3 to early post-D3) is recorded by monophase CH4 inclusions. Entrained outflow of deeply circulated meteoric fluid modified the CH4 system; modification is recorded by H2O+CH4 and H2O+(trace CH4) secondary populations and by an H2O+(trace CH4) primary population. The contractional tectonics (D2–D3) of ore emplacement was superseded by relaxational tectonics (D4P) that facilitated meteoric water penetration and return flow.Under D2 prograde metamorphism, entrapment temperatures (Tt) and pressures (Pt) for pre-decrepitation secondary inclusions are estimated as Tt300–330 °C and Pt1.5–2 kbar≈Plith (the lithostatic pressure). Decrepitation accompanied peak metamorphism (T350–380 °C) in mid- to late-D3, while in late-D3 to early post-D3, essentially monophase CH4 secondary inclusions were entrapped at Tt350 °C and Pt=1.5–2 kbar≈Plith. Subsequently, abundant CH4 and entrained meteoric water were entrapped as H2O+CH4 secondaries under slowly decreasing temperature (Tt330–350 °C) and constant pressure (Pt1.5–2 kbar). Finally, with increasingly dominant meteoric outflow, H2O+(trace CH4) populations record decreasing temperatures (Tt>300 to <350 down to 275–300 °C) at pressures of Phydrostatic<Pt (1 kbar) <Plith (1.5 kbar).The populations of inclusions provide insight into fluid types, flow regimes and P–T conditions during parts of the deposit's evolution. They indirectly support the role of basin-derived CH4 fluids in ore formation, but provide no insight into a basement-sourced ore-forming fluid. They fully support post-ore involvement of meteoric water. The poorly constrained entrapment history is believed to span 10 Ma from 395 to 385 Ma.  相似文献   

8.
During the middle Pleistocene Nome River glaciation of northwestern Alaska, glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals. The age of the Nome River glaciation is constrained by laser-fusion 40Ar/39Ar analyses of basaltic lava that overlies Nome River drift at Minnie Creek, central Seward Peninsula, that average 470,000 ± 190,000 yr (±1σ). Milligram-size subsamples of the lava were dated to identify and eliminate extraneous 40Ar enrichments that rendered the mean of conventional K---Ar dates on larger bulk samples of the same flow too old (700,000 ± 570,000 yr). While the 40Ar/39Ar analyses provide a minimum limiting age for the Nome River glaciation, maximum ages are provided by a provisional K---Ar date on a basaltic lava flow that underlies the Nome River drift at nearby Lave Creek, by paleomagnetic determinations of the drift itself at and near the type locality, and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the Anvilian marine transgression that underlie Nome River drift on the coastal plain at Nome. Taken together, the new age data indicate that the glaciation took place between 580,000 and 280,000 yr ago. The altitude of the Anvilian deposits suggests that eustatic sea level during the Anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression; by correlation with the marine oxygen-isotope record, the transgression probably dates to stage 11 at 410,000 yr, and the Nome River glaciation is younger still. Analyses of floor altitudes of presumed Nome River cirques indicate that the Nome River regional snowline depression was at least twice that of the maximum late Wisconsin. The cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern Alaska during the middle Pleistocene.  相似文献   

9.
I examined the relative importance of beds of tapegrass (Vallisneria americana) and adjacent unvegetated habitats to juvenile and adult (6–35 mm standard length) rainwater killifish (Lucania parva) over a large spatial scale within the St. Johns River estuary, Florida. Abundance of rainwater killifish did not differ between oligohaline and tidal freshwater portions of the estuary and this species was relatively rare at opposite ends of the St. Johns River estuary. The presence of rainwater killifish at a given site was determined in part by large-scale variation in environmental factors such as habitat complexity and salinity. When present at a site, rainwater killifish were found almost exclusively in structurally complex beds of tapegrass. Behavioral observations in the laboratory indicated that rainwater killifish preferred vegetated over unvegetated habitats in the absence of both potential prey and predators and that use of vegetated habitats increased further upon addition of predatory largemouth bass (Micropterus salmoides). A laboratory predation experiment indicated that survival of rainwater killifish exposed to largemouth bass was significantly higher in vegetation than over open sand. Strong preferences for structurally complex vegetation likely reflect an evolved or learned behavioral response to risk of predation and help explain habitat use of rainwater killifish in the St. Johns River estuary.  相似文献   

10.
Five stations on the lower Saint John River, a complex multibasin estuary, were sampled semiquantitatively for zooplankton at biweekly intervals for one year, and qualitatively over a 4-year period. Planktonic Crustacea were dominated by the true estuarine copepods,Acartia tonsa andEurytemora affinis and the euryhaline marine copepodsOithona similis andPseudocalanus minutus. Atypical estuarine forms, confined to a lower fiord-like basin with salinity of 20‰, were the amphipod,Parathemisto abyssorum and the mysidErythrops erythrophthalma. River flows were highly variable from year to year. Certain basins function as lakes in some years and estuaries in other years, causing extreme zooplankton community fluctuations, and succession patterns dependent on salinity rather than season. On occasion freshwater zooplankters maintained viable populations at unusually high salinities (ca. 5‰). Vertical and horizontal distributions of zooplankters indicate that the estuary in fact comprises two systems: a true estuary in the upper reaches and the surface waters at the lower end, and a fiord in a subsidiary basin in the lower end.  相似文献   

11.
The late Quaternary vegetation of northern Isla de Chiloé is inferred from palynological analysis of a section in the Río Negro drainage (42°03′S, 73°50′W). At ca. 30,500 yr B.P., maxima of Astelia and Donatia occurred, suggesting wetland development. From that time until ca. 27,000 yr B.P., steppe indicators such as Compositae/Gramineae dominated, suggesting drier conditions. After 27,000 yr B.P., the moorland shrub Dacrydium gradually increased, reaching a maximum by 18,000 yr B.P. At this time Astelia increased again, suggesting development of cushion bog during cold and wet conditions. The glacial-postglacial transition is characterized by a marked change from peaty sediments to clays, a decrease in the cushion bog flora, and the prevalence of Gramineae/ Compositae and swamp taxa. This vegetation prevailed until ca.7000 yr B.P. when forest taxa became dominant. The floristic pattern inferred from the pollen spectra of the Rio Negro section suggests that the late Pleistocene vegetation of Chiloé resembled modern Magellanic Moorland vegetation (52°–56°lat S). Based on climatic conditions presently associated with Magellanic Moorland, its occurrence in Chiloé at low elevations during the late Pleistocene implies a decrease in average temperature of at least 4°C and an increase in annual precipitation of at least 1500 mm.  相似文献   

12.
The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique,  NW–SE convergence between Africa and Eurasia at a rate of  5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran–Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (MW 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ1 at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW–ENE extension (σ3 at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)–Eurasian plate convergence are currently accommodated at the N-Algerian margin,  2 mm/yr in the Moroccan Atlas, and  2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as  2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran–Betic–Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the  3.5 mm/yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.  相似文献   

13.
Differences in phytoplankton community composition along a riverine to, freshwater tidal continuum was an important factor affecting the primary productivity and quantity of phytoplankton biomass available to the San Francisco Estuary food web downstream. The relative contribution of riverine and freshwater tidal phytoplankton was determined using measurements of primary productivity, respiration, and phytoplankton species composition along a riverine to freshwater tidal gradient in the San Joaquin River, one of two major rivers that flow into, the San Francisco Estuary. Chla-specific net primary productivity was greater in the freshwater tidal habitat and was correlated with both a higher growth efficiency and maximum growth potential compared with the river upstream. Cluster analysis indicated these differences in growth parameters were associated with differences in species composition, with greater percent diatom and green algal species biomass upstream and flagellate biomass downstream. Correlation between the chla specific net productivity and phytoplankton species composition suggested the downstream shift from riverine diatom and green algal species to flagellate species contributed to the seaward increase in net primary productivity. Environmental conditions, such as specific conductance and water transparency, may have influenced primary productivity along the riverine to freshwater tidal continuum through their effect on both species composition and growth rate. Data suggest light was not the sole controlling factor for primary productivity in this highly turbid estuary; phytoplankton growth rate did not increase when riverine plankton communities from low light conditions upstream were exposed to higher light conditions downstream. This study suggests that the availability of phytoplankton biomass to the estuarine food web may be influenced by management of both phytoplankton growth and community composition along the riverine to freshwater tidal continuum.  相似文献   

14.
The Suwannee River watershed is one of the least developed in the eastern United States, but with increasing urbanization it is facing potential long-term alterations in freshwater flow to its estuary in the Gulf of Mexico. The purpose of this study was to develop biological indicators of oyster reef state along a natural salinity gradient in the Suwannee River estuary in order to allow the rapid assessment of the effect of changing freshwater input to this system. Percent cover and density of three size classes of living oysters, as well as the abundance of several predominant reef-associated invertebrates, were measured along a broad salinity gradient in the estuary and were correlated with salinity estimates from a long-term database for the preceding 12–24 mo. All eastern oyster,Crassostrea virginica, parameters (percent cover and density of three size classes) were significantly and negatively related to salinity. Data from samples collected near the lower intertidal were more closely dependent upon salinity than were samples from the higher intertidal at the same sites. Salinity differences were most closely reflected in differences in total oyster cover. This relationship corresponded with a general decline in oyster habitat with increasing distance from the mouth of the Suwannee River. Species richness was significantly and positively correlated with allC. virginica parameters (percent cover and density of three size classes), but the relationship explained only about half the variability. Density data of the hooked mussel,Ischadium recurvum, and a mud crab,Eurypanopeus depressus, were positively and strongly correlated withC. virginica parameters, likely reflecting the abundance of habitat provided byC. virginica shells. All of the biological indicators measured responded similarly along the salinity gradient, indicating they provide reliable indices of the effect of changing salinities in the Suwannee River estuary over the previous 1 or 2 yr. Some areas of positive relief defined as reefs 30 years ago are no longer oyster habitat, suggesting an ongoing decline, but nearshoreC. virginica were abundant. *** DIRECT SUPPORT *** A02BY003 00002  相似文献   

15.
The objective of this study was to determine if exploitative competition between between juvenile Chinook salmon (Oncorhynchus tshawytscha) and threespine stickleback (Gasterosteus aculeatus) reduces the foraging opportunity of juvenile Chinook salmon in tidal channels of the Columbia River estuary. We sampled Chinook salmon and stickleback diets monthly and over a diel cycle in spatially distinct emergent marshes of the Columbia River estuary. Diets of the two fish species did not differ among marsh systems, but both fish species exhibited diel and seasonal differences in diet composition. Diet overlap between the two fish species was greatest in March and June. Exploitative competition was unlikely based on a comparison between consumption rates and estimated invertebrate production.  相似文献   

16.
An ecological transfer function based on the distribution of planktonic foraminifera in 66 Mediterranean and 8 North Atlantic surface-sediment samples is used to estimate sea-surface temperatures and salinities for the eastern Mediterranean during the last glacial maximum (18,000 yr B.P.). The present-day distribution of planktonic foraminifera can be explained by four faunal assemblages, each of which has diagnostic environmental preferences. Factor 1 is a tropical-subtropical assemblage; factor 2 is a transitional assemblage; factor 3 is a low-salinity assemblage; and factor 4 is a subpolar assemblage. The geographic distribution of these faunal assemblages reflect the variation in overlying hydrographic conditions. The 18,000-yr B.P. samples were selected based on total faunal stratigraphy, oxygen-isotope stratigraphy, and previously determined radiometric dates for eastern Mediterranean volcanic ash layers. Estimated temperature and salinity patterns show that the greatest change between present-day and 18,000-yr B.P. sea-surface conditions existed in the Aegean Sea and immediately south of Crete. The winter temperature anomaly (18,000 yr B.P.-present) within the Aegean Sea is 6°C cooler than present. In contrast to this, the maximum summer temperature anomaly exists to the south of Crete, where sea-surface temperatures were 4°C cooler than present. Estimated sea-surface salinities also show that the greatest change took place within the Aegean Sea, being 5‰ less saline than present. The estimated temperature and salinity patterns seem to reflect changing drainage patterns during glacial times and the diversion of cool, low-salinity water into the Aegean Sea. The source of this glacial runoff appears to be large freshwater lakes that existed during this time over parts of eastern Europe and western Siberia.  相似文献   

17.
The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (dp=18°, dm=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (dp=5.3°, dm=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (dp=2.4°, dm=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation.  相似文献   

18.
Holocene high-resolution cores from the margin of the Arctic Ocean are rare. Core P189AR-P45 collected in 405-m water depth on the Beaufort Sea slope, west of the Mackenzie River delta (70°33.03′N and 141°52.08′W), is in close vertical proximity to the present-day upper limit of modified Atlantic water. The 5.11-m core spans the interval between ∼6800 and 10,400 14C yr B.P. (with an 800-yr ocean reservoir correction). The sediment is primarily silty clay with an average grain-size of 9 φ. The chronology is constrained by seven radiocarbon dates. The rate of sediment accumulation averaged 1.35 mm/yr. Stable isotopic data (δ18O and δ13C) were obtained on the polar planktonic foraminifera Neogloboquadrina pachyderma (s) and the benthic infaunal species Cassidulina neoteretis. A distinct low-δ18O event is captured in both the benthic and planktonic data at ∼10,000 14C yr B.P.—probably recording the glacial Lake Agassiz outburst flood associated with the North Atlantic preboreal cold event. The benthic foraminifera are dominated in the earliest Holocene by C. neoteretis, a species associated with modified Atlantic water masses. This species decreases toward the core top with a marked environmental reversal occurring ∼7800 14C yr B.P., possibly coincident with the northern hemisphere 8200 cal yr B.P. cold event.  相似文献   

19.
The detection of long-term shifts in species composition and spatial structuring of aquatic communities may be obscured by high levels of interannual variation. Estuarine fish communities are likely to exhibit high levels of variation owing to the influence of riverine forcing and the importance of anadromous and transient species, whose abundances may not be locally controlled. We describe patterns of interannual variation and long-term shifts in the nearshore fish community of the mesohaline Hudson River estuary based on 21 yr of beach seine sampling conducted annually between late August and mid November. Of the 60 species encountered, the most abundant were Atlantic silversides (Menidia menidia), striped bass (Morone saxatilis), white perch (Morone americana), American shad (Alosa sapidissima), and blueback herring (Alosa aestivalis). Relationships between annual community composition and seasonal flow and temperature regimes were examined with canonical correspondence analysis. Annual variation was most closely correlated with river flows in the 3-mo period preceding fish sampling, indicating a persistent effect of environmental conditions on community structure. Despite significant interannual variation in composition, longer-term trends in community structure were observed. These included declines in catch rates of freshwater and estuarine species and a dramatic increase in the catch of Atlantic silversides, an annual marine species. Associated with these changes were declines in community diversity and increased compositional variation. These results indicate that analyses of temporal changes in community structure need to account for the multiple time scales under which forcing factors and community composition vary.  相似文献   

20.
Ferrous granulites in the area of Tidjénouine (Central Hoggar) exhibit a remarkable mineralogical composition characterized by the association orthoferrossilite–fayalite–quartz. These granulites are metamorphosed mafic igneous rocks showing the juxtaposition of different metamorphic parageneses. Peak paragenesis with garnet–clinopyroxene–amphibole–plagioclase–quartz reach to assemblage with orthopyroxene–plagioclase2. Secondary orthopyroxene reacted with garnet to produce symplectites with fayalite + plagioclase + quartz. The latest stage corresponds to an orthopyroxene–fayalite–quartz–plagioclase assemblage. The metamorphic history of the ferrous granulites is inferred by combining the study of phase relations with the construction of a petrogenetic grid and pseudosection in the CFMASH and CFAS systems using the Thermocalc program of [J. Metamorph. Geol. 6 (1988) 173]. The evolution of paragenetic minerals indicates a metamorphic P–T path through the following conditions: 7.1 ± 1 kbar at 880 °C, 4.9 ± 1.6 kbar at 750 °C and 3–4 kbar at 700 °C, which is consistent with a clockwise P–T path recorded throughout the area.  相似文献   

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