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1.
A study of an expanding seagrass bed on the south-west coast of British Columbia, Canada involved documentation and explanation of the pattern of expansion of the vegetation as well as documentation and experimental investigation of the accompanying changes in the distribution of infaunal invertebrates. Expansion followed a major environmental change, improved water clarity initiated in 1969 when a causeway blocked access to the site for silty Fraser River water. The original eelgrass (Zostera marina) bed expanded landward over 30 m year−1, probably causing more and more water to be retained in the bed at low tide and thus improving its own habitat. More rapid expansion occurred from 1979–1983 after Z.japonica colonized at the landward edge of the eelgrass bed and in a separate intertidal area. Expansion ceased around 1983 coincident with, but probably independent of, further construction.Burrowing shrimp (Callianassa californiensis) decreased in abundance between 1977 and 1984 in areas where the two seagrasses colonized. Short-term experiment over one summer and fall showed that removal of all shoots allowed adult shrimp and tube worms to colonize the sediment while addition of shoots of Z. japonica by transplanting caused temporary decreases in shrimp abundance. After a few weeks an established shrimp population destroyed the transplants. Among reasonable alternate hypotheses for the decline in Callianassa, the effect of sediment texture can be eliminated, but either direct negative effects of seagrass, i.e. inhibition of the burrowing of adult shrimp or of settlement of juveniles, or indirect effects, i.e. the harboring of more predators of shrimp in seagrass beds, deserve further study. 相似文献
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A long-term record of surface currents from a high-frequency radar system, along with near-surface hydrographic transects, moored current meter records, and satellite imagery, are analyzed to determine the relative importance of river discharge, wind, and tides in driving the surface flow in the Fraser River plume. The observations show a great deal of oceanographic and instrumental variability. However, averaged quantities yielded robust results. The effect of river flow, which determines buoyancy and inertia near the river mouth, was found by taking a long-term average. The resulting flow field was dominated by a jet with two asymmetric gyres; the anticyclonic gyre to the north had flow speeds consistent with geostrophy. The mean flow speed near the river mouth was 14.3?cm?s–1, while the flow further afield was 5?cm?s–1 or less. Wind stress and surface currents were highly coherent in the subtidal frequency band. Northwesterly winds drive a surface flow to the southeast at speeds of nearly 30?cm?s–1. Southeasterly winds drive a surface flow to the northwest at speeds reaching 20?cm?s–1; however, there is more spatial variability in speed and direction relative to the northwesterly wind case. A harmonic analysis was used to extract the tidally driven flows. Ellipse parameters for the major tidal constituents varied considerably in both alignment and aspect ratio over the radar domain, in direct contrast to a barotropic model which predicted rectilinear flow along the Strait of Georgia. This is a result of water filling and draining the shallow mud flats north of the Fraser's main channel. The M2 velocities at the surface were also weaker than their barotropic counterparts. However, the shallow water constituent MK3 was enhanced at the surface and nearly as strong as the mean flow, implying that non-linear interactions are important to surface dynamics. 相似文献
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Jacqueline Birnie 《第四纪科学杂志》1990,5(3):171-187
The Holocene environmental history of South Georgia is important because of the island's location in the Southern Westerlies in an oceanic zone of the world devoid of high resolution terrestrial records. This is the first attempt to interpret a palaeoenvironmental record from lake sediments in South Georgia. It is based on a wide variety of analyses undertaken on cores from two lakes. Both are in the same, unglaciated, drainage basin, but one is at 80 m above sea-level and near the altitudinal limit for vegetation growth, whereas the other is at 25 m and within the zone of continuous vegetation cover. Results from both lakes indicate shifts of vegetation boundaries, which, together with evidence for changing biotic productivity within the lakes themselves, are interpreted as indicating climatic changes. Radiocarbon dates on the main changes identify a climatic optimum, beginning before 5620 ± 290 14C yr BP, and ending at around 4815 ± 330 14C yr BP, when conditions in the upper part of the catchment were more conducive to plant growth than they are today. The record obtained from the lower lake was shorter, but indicates two periods of harsher climate relative to the present since 4000 yr BP. This interpretation of the lake evidence agrees with other dated evidence of environmental change from peat sections, glacial stratigraphy and geomorphology in South Georgia. Together the work allows an overall reconstruction of environmental change in the Holocene. 相似文献
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Lake Balboa is a deep sinkhole lake in south Georgia. Subbottom acoustic profiling and long sediment cores reveal four stratigraphic units within >20 m of lacustrine fill above acoustically stratified and faulted Miocene/Pliocene overburden and basement Eocene limestone which hosts the Floridan Aquifer system (FAS). Fill consists of thin, rhythmically bedded, peaty sediments (Unit I), indicative of slow sedimentation in a shallow swamp until 13,110–12,680 cal BP, a mix of desiccated and over-consolidated clay and silty sand (Unit II) implying periodic subaerial exposure of the lake bed until 9470–9025 cal BP, and gyttja associated with a rapid 8 m rise in water level by 9120–9020 cal BP (Unit III), as well as an additional rise of 10 m to modern lake levels (Unit IV). Accumulation of nearly 4.5 m of low permeability gyttja, and higher lake levels have fundamentally changed exchange between surface water and groundwater, reducing by 40% or more the rate of vertical leakage to the FAS and increasing shallow aquifer discharge to the lake. 相似文献
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Linking invertebrate burrow distributions (neoichnology) to physicochemical stresses on a sandy tidal flat: implications for the rock record 总被引：1，自引：0，他引：1
SHAHIN E. DASHTGARD 《Sedimentology》2011,58(6):1303-1325
Invertebrate burrow distributions (neoichnology) across a modern tidal flat are presented as an analogue for interpreting the ichnology of palaeo‐tidal‐flat successions. Burrow distributions are linked to physical and chemical (physicochemical) stresses to establish the main controls on the distribution of biosedimentary structures. Across the tidal flat, there is clear heterogeneity in both the diversity of traces and the intensity of burrowing. This heterogeneity reflects a myriad of physicochemical stresses, with the sedimentation rate dominating burrow distributions. Across all substrates, the total area occupied by organisms rarely exceeds 3% of the tidal‐flat surface, and is commonly <1%; this equates to a bioturbation index value of one. To reach bioturbation index values of two to six, sediments must be available to biogenic reworking and/or recolonization. With an increasing sedimentation rate, substrates are rapidly buried and re‐exposed, which limits the time when a substrate is available to colonization. For palaeo‐ichnological studies, this research presents several key results. (i) Burrow cross‐cutting relationships in tidal‐flat successions commonly reflect natural heterogeneities in the areal distribution of infaunal communities, rather than infaunal tiering. (ii) Ichnofabric analysis of palaeo‐tidal flats with a high sedimentation rate would yield fabrics that reflect heterogeneities in the areal distribution of infaunal communities rather than variability in the physicochemical stresses of the environment. (iii) The composite trace‐fossil assemblage of tidal flats cannot be attributed to a single ichnofacies, but instead comprises elements typical of multiple ichnofacies. (iv) The main controls on trace assemblages across tidal flats in fully marine settings are sedimentological and include the sedimentation rate and, to a lesser extent, grain size. 相似文献
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Variscan to Alpine magmatic activity on the North Tethys active Eurasian margin in the Caucasus region is revealed by 40Ar/39Ar ages from rocks sampled in the Georgian Crystalline basement and exotic blocs in the Armenian foreland basin. These ages provide insights into the long duration of magmatic activity and related metamorphic history of the margin, with: (1) a phase of transpression with little crustal thickening during the Variscan cycle, evidenced by HT-LP metamorphism at 329–337 Ma; (2) a phase of intense bimodal magmatism at the end of the Variscan cycle, between 303 and 269 Ma, which is interpreted as an ongoing active margin during this period; (3) further evolution of the active margin evidenced by migmatites formed at ca. 183 Ma in a transpressive setting; (4) paroxysmal arc plutonic activity during the Jurassic (although the active magmatic arc was located farther south than the studied crystalline basements) with metamorphic rocks of the Eurasian basement sampled in the Armenian foreland basin dated at 166 Ma; (5) rapid cooling suggested by similar within-error ages of amphibole and muscovite sampled from the same exotic block in the Armenian fore-arc basin, ascribed to rapid exhumation related to extensional tectonics in the arc; and finally (6) cessation of ‘Andean’-type magmatic arc history in the Upper Cretaceous. Remnants of magmatic activity in the Early Cretaceous are found in the Georgian crystalline basement at c. 114 Ma, which is ascribed to flat slab subduction of relatively hot oceanic crust. This event corresponds to the emplacement of an oceanic seamount above the N Armenian ophiolite at 117 Ma. The activity of a hot spot between the active Eurasian margin and the South Armenian Block is thought to have heated and thickened the Neo-Tethys oceanic crust. Finally, the South Eurasian margin was uplifted and transported over this hot oceanic crust, resulting in the cessation of subduction and the erosion of the southern edge of the margin in Upper Cretaceous times. Emplacement of Eocene volcanics stitches all main collisional structures. 相似文献
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An experimental study of temperature cycles and the heat budget in the Duplin River, a tidal creek bordered by extensive intertidal salt marshes, was carried out in late summer of 2003 and spring of 2004 near Sapelo Island on the central Georgia coast in the southeastern US. Three water masses are identified with differing temperature and salinity regimes, the characteristics of which are dictated by channel morphology, tidal communication with the neighboring sound, ground water hydrology, the extent of local intertidal salt marshes and side channels and the spring–neap tidal cycle (which controls both energetic mixing and, presumably, ground water input). For the first experiment, heat budgets are constructed for the upper (warmer) and lower (cooler) areas of the Duplin River showing the diminishing importance of tidal advection away from the mouth of the creek along with the concomitant increase in the importance of both direct atmospheric fluxes and of interactions with the marsh and side creeks. The second experiment, in the spring of 2004, reexamines the heat budget on seasonal and daily averaged scales revealing the decreased importance of advective fluxes relative to direct atmospheric fluxes on this scale but the constant importance of marsh/creek interactions regardless of time scale or season. Short period temperature fluctuations which affect larval development are examined and analogies are drawn to use heat to understand the marsh as a source of sediment, carbon and other nutrients. 相似文献
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M. Alber 《Estuarine, Coastal and Shelf Science》2000,50(6):805
Suspended particle dynamics were investigated in the Ogeechee River Estuary during neap tide in July 1996. Samples were operationally separated into ‘ truly suspended ’ (settling velocity <0·006 cm s−1) and ‘ settleable ’ (settling velocity >0·006 cm s−1) fractions over the course of a tidal cycle to determine whether these two fractions were comprised of particles with differing biological and chemical characteristics. Total suspended sediment, organic carbon and nitrogen, chlorophyll a and phaeopigment concentrations were measured in each fraction, as well as rates of bacterial hydrolytic enzyme activity [β-1,4-glucosidase (βGase) and β-xylosidase (βXase)]. The majority of the suspended sediment (by weight) was in the truly suspended fraction; all measured parameters were largely associated with this fraction as well. When compared to the settleable material, the truly suspended material was significantly higher in % POC (5·7±0·6 vs. 3·9±1·8), % chlorophyll (0·07±0·02 vs. 0·03±0·01), % phaeopigment (0·030±0·006 vs. 0·018±0·012), and weight-specific maximal uptake rates (Vmaxper mg suspended sediment) of both enzymes (1·8±0·4 vs. 0·7± 0·2 nmol mg−1 h−1βGase and 1·1±0·3vs . 0·3±0·2 nmol mg−1 h−1βXase), providing clear evidence for a qualitative distinction between the two fractions. These results are interpreted to mean that the more organic-rich, biologically active material associated with the suspended fraction is likely to have a different fate in this Estuary, as ‘ truly suspended ’ sediments will be readily transported whereas ‘ settleable ’ sediments will settle and be resuspended with each tide. These types of qualitative differences should be incorporated into models of particle dynamics in estuaries. 相似文献
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Nancy Soontiens Susan E. Allen Doug Latornell Kate Le Souëf Idalia Machuca Jean-Philippe Paquin 《大气与海洋》2016,54(1):1-21
The Strait of Georgia is a large, semi-enclosed body of water between Vancouver Island and the mainland of British Columbia connected to the Pacific Ocean via Juan de Fuca Strait at the south and Johnstone Strait at the north. During the winter months, coastal communities along the Strait of Georgia are at risk of flooding caused by storm surges, a natural hazard that can occur when a strong storm coincides with high tide. This investigation produces storm surge hindcasts using a three-dimensional numerical ocean model for the Strait of Georgia and the surrounding bodies of water (Juan de Fuca Strait, Puget Sound, and Johnstone Strait) collectively known as the Salish Sea. The numerical model employs the Nucleus for European Modelling of the Ocean architecture in a regional configuration. The model is evaluated through comparisons of tidal elevation harmonics and storm surge with observations. Important forcing factors contributing to storm surges are assessed. It is shown that surges entering the domain from the Pacific Ocean make the most significant contribution to surge amplitude within the Strait of Georgia. Comparisons between simulations and high-resolution and low-resolution atmospheric forcing further emphasize that remote forcing is the dominant factor in surge amplitudes in this region. In addition, local wind patterns caused a slight increase in surge amplitude on the mainland side of the Strait of Georgia compared with Vancouver Island coastal areas during a major wind storm on 15 December 2006. Generally, surge amplitudes are found to be greater within the Strait of Georgia than in Juan de Fuca Strait. 相似文献