Nearshore currents on the southern Namaqua shelf of the Benguela upwelling system     

A.L. Fawcett  G.C. Pitcher  F.A. Shillington 《Continental Shelf Research》2008

Nearshore currents of the southern Namaqua shelf were investigated using data from a mooring situated three and a half kilometres offshore of Lambert's Bay, downstream of the Cape Columbine upwelling cell, on the west coast of South Africa. This area is susceptible to harmful algal blooms (HABs) and wind-forced variations in currents and water column structure are critical in determining the development, transport and dissipation of blooms. Time series of local wind data, and current and temperature profile data are described for three periods, considered to be representative of the latter part of the upwelling season (27 January–22 February), winter conditions (5–29 May) and the early part of the upwelling season (10 November–12 December) in 2005. Differences observed in mean wind strength and direction between data sets are indicative of seasonal changes in synoptic meteorological conditions. These quasi-seasonal variations in wind forcing affect nearshore current flow, leading to mean northward flow in surface waters early in the upwelling season when equatorward, upwelling-favourable winds are persistent. Mean near-surface currents are southward during the latter part of the upwelling season, consistent with more prolonged periods of relaxation from equatorward winds, and under winter conditions when winds were predominantly poleward. Within these seasonal variations in mean near-surface current direction, two scales of current variability were evident within all data sets: strong inertial oscillations were driven by diurnal winds and introduced vertical shear into the water column enhancing mixing across the thermocline, while sub-inertial current variability was driven by north–south wind reversals at periods of 2–5 days. Sub-inertial currents were found to lag wind reversals by approximately 12 h, with a tendency for near-surface currents to flow poleward in the absence of wind forcing. Consistent with similar sites along the Californian and Iberian coasts, the headland at Cape Columbine is considered to influence currents and circulation patterns during periods of relaxation from upwelling-favourable winds, favouring the development of a nearshore poleward current, leading to poleward advection of warm water, the development of stratification, and the creation of potentially favourable conditions for HAB development.  相似文献   

Recreating Ocean Space: Recreational Consumption and Representation of the Caribbean Marine Environment     

Carolyn Trist 《The Professional geographer》1999,51(3):376-387

This paper investigates the historical relationship between the development of marine-based tourism and representations of marine space. Over the past 100 years, the Caribbean Sea has been transformed from a transportation surface connecting exotic ports of call into a commodity itself. Imagined by early travelers as a two-dimensional space to be crossed, the Caribbean has became part of a new tropical marine aesthetic, a three-dimensional spectacle to be actively consumed and conserved. These changing representations of the marine environment as a space of consumption have been paralleled by changes in the relationships between marine and terrestrial political economies. As marine-based tourism has intensified, the balance of power among coastal resource users has shifted. The increasingly conflictive consequences of this shift are illustrated by the case of Soufrière in St. Lucia where new uses of marine space have re-configured social and economic relationships. Soufrière's coastal economy has been transformed and its marine space increasingly politicized as competing representations of the sea have been used to assert conflicting claims to resource access. This paper attempts to elucidate the power of these representations by investigating their historical relationship to the changing role of the Caribbean Sea in the international tourism industry.  相似文献   

Anisotropy of magnetic susceptibility investigations of the St Malo dyke swarm (Brittany, France): emplacement mechanism of doleritic intrusions     

Tahar Aïfa  Jean-Pierre Lefort  & Pol Guennoc 《Geophysical Journal International》1999,139(2):573-582

A high-quality aeromagnetic survey of northern Brittany (line spacing 250  m; flight elevation 150  m) has been used to delineate the Lower Carboniferous St Malo dyke swarm in detail. The dyke swarm fans at its northern and southern ends, and is affected by N60° sinistral transcurrent faults. After restoration of these offsets, the full structure trends in a N–S direction. Small dykes are not imaged, and only one-third of the swarm is evidenced by magnetism. Gravity and magnetic modelling shows that the swarm overlies a single N–S elongated magma chamber.
  The distribution of K _max inclinations of anisotropy of magnetic susceptibility (AMS) suggests that the dykes display a fanning magma flow in section. Computed K _max inclinations are usually shallower than the measured geological dips, probably because the flow becomes more disturbed as the dyke becomes shallower. We observe that the mean susceptibility values increase when the magma flow is steeper than about 70°.
  A detailed cross-section of St Briac dyke, which is part of the St Malo dyke swarm, reveals that the main carrier of magnetization is magnetite in the centre of the dyke and magnetite + maghaemite on its rims. The emplacement of the St Malo dyke swarm could have been contemporaneous with the N60° shearing which displaced the dyke swarm by about 20  km. The dyke swarm is cut at its southern end by an E–W-orientated fault which probably acted during Late Carboniferous times.  相似文献   

The Larval Fish Assemblage in Nearshore Coastal Waters off the St Lucia Estuary, South Africa   总被引：3，自引：0，他引：3  

S. A. Harris  D. P. Cyrus  L. E. Beckley 《Estuarine, Coastal and Shelf Science》1999,49(6):789

This paper reports on the composition, abundance and distribution of the larval fish assemblage in the nearshore coastal waters off the St Lucia Estuary mouth, South Africa. Ichthyoplankton samples were collected over a 12 month period from five stations located along a transect up to 2·5 km offshore, and from two stations north and south of the estuary mouth, respectively. In all, 6126 fish larvae, representing 89 families and 186 species, were collected. Larvae in the families Myctophidae and Tripterygiidae comprised 21% and 16% of the total catch, respectively. The most abundant species were an unidentified triplefin, Tripterygiid 1 and the lanternfish Benthosema fibulatum, together which contributed nearly 18% of the total catch. Larvae of marine spawners independent of estuaries dominated the catch both in terms of density (90%) and in terms of number of taxa (89%). Some larvae of estuarine-associated species were present, in addition to a few specimens of estuarine resident species. Overall the dominant environmental variable affecting larval densities was temperature, particularly for Trypterygiid 1 where temperature contributed to 9% of the variance model. Densities of fish larvae peaked in November and December 1990 (late spring and early summer) and were lowest from January to June 1991 (summer, autumn an early winter). Different taxa dominated the catch each month with reef- and shelf-associated species accounting for the peak in August and September 1990, oceanic species in November 1990 and a mixture of the two groups in December. Overall larval densities were significantly higher in bottom samples with a trend of increasing densities offshore for reef and shelf taxa. The larvae of reef and shore taxa were predominantly preflexion larvae, whilst the few estuarine spawner species that were collected were mainly postflexion. Ontogenetic patterns related to depth and distance offshore were evident for the dominant species in each estuarine-association category.The present study has shown that temporal and spatial variations in the larval fish assemblage off St Lucia are related to environmental conditions and ontogenetic behavioural patterns of certain species. The origin of many of the larvae in the assemblages off the coast of St Lucia is probably from both local spawning populations in the shelf waters off KwaZulu-Natal and spawning populations farther north in shelf waters off Mozambique. Additional studies with more detailed oceanographic measurements will further our understanding of the physical processes that supply larvae to the St Lucia region.  相似文献   

Contrasting changes between the northern and southern Gulf of St. Lawrence ecosystems associated with the collapse of groundfish stocks     

Lyne Morissette  Martin Castonguay  Claude Savenkoff  Douglas P. Swain  Denis Chabot  Hugo Bourdages  Mike O. Hammill  J. Mark Hanson 《Deep Sea Research Part II: Topical Studies in Oceanography》2009,56(21-22):2117

In order to have a global view of ecosystem changes associated with the collapse of groundfish species in the Gulf of St. Lawrence during the early 1990s, Ecopath mass-balance models were constructed incorporating uncertainty in the input data. These models covered two ecosystems (northern and southern Gulf of St. Lawrence; NAFO divisions 4RS and 4T), and two time periods (before the collapse, in the mid-1980s, and after it, in the mid-1990s). Our analyses revealed that the ecosystem structure shifted dramatically from one previously dominated by piscivorous groundfish and small-bodied forage species during the mid-1980s to one now dominated only by small-bodied pelagic species during the mid-1990s in both southern and northern Gulf. The species structure in the northern Gulf versus southern Gulf was different, which may explain why these two ecosystems did not recover the same way from the collapse in the early 1990s. Productivity declined in the northern Gulf after the collapse but increased in the southern Gulf. The collapse of groundfish stocks resulted in declines in the mean trophic level of the landings in both the northern and the southern Gulf. Even though fishing mortality was then intentionally reduced, this part of the total mortality was taken up by predation. The temporal changes in the internal structure of both ecosystems are reflected in their overall emergent properties.  相似文献   

Radium and radon radioisotopes in regional groundwater, intertidal groundwater, and seawater in the Adelaide Coastal Waters Study area: Implications for the evaluation of submarine groundwater discharge     

Sbastien Lamontagne  Corinne Le Gal La Salle  Gary J. Hancock  Ian T. Webster  Craig T. Simmons  Andrew J. Love  Julianne James-Smith  Anthony J. Smith  Jochen Kmpf  Howard J. Fallowfield 《Marine Chemistry》2008,109(3-4):318

The input of groundwater-borne nutrients to Adelaide's (South Australia) coastal zone is not well known but could contribute to the ongoing decline of seagrass in the area. As a component of the Adelaide Coastal Waters Study (ACWS), the potential for using the radium quartet (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) and ²²²Rn to evaluate submarine groundwater discharge (SGD) was evaluated. Potential isotopic signatures for SGD were assessed by sampling groundwater from three regional aquifers potentially contributing SGD to the ACWS area. In addition, intertidal groundwater was sampled at two sand beach sites. In general, the regional groundwaters were enriched in long-lived Ra isotopes (²²⁶Ra and ²²⁸Ra) and in ²²²Rn relative to intertidal groundwater. Radium activity (but not ²²²Rn activity) was positively correlated to salinity in groundwater from one of the regional aquifers and in intertidal groundwater. Radium isotope ratios (²²³Ra/²²⁶Ra, ²²⁴Ra/²²⁶Ra and ²²⁸Ra/²²⁶Ra) were less variable than individual Ra isotope activities within potential SGD sources. Recirculated seawater (estimated from the intertidal groundwater samples with seawater-like salinities) also had distinctly higher Ra isotope ratios than the regional groundwaters. The activities for all radioisotopes were relatively low in seawater. The activity of the short-lived ²²³Ra and ²²⁴Ra were highest at the shoreline and declined exponentially with distance offshore. In contrast, ²²⁸Ra and ²²⁶Ra activities had a weak linear declining trend with distance offshore. Rn-222 activity was at or near background in all seawater samples. The pattern of enrichment in short-lived Ra isotopes and the lack of ²²²Rn in seawater suggest that seawater recirculation is the main contributor to SGD in the ACWS area. Preliminary modeling of the offshore flux of ²²⁸Ra and ²²⁶Ra suggest that the SGD flux to the ACWS area ranges between 0.2 and 3 · 10^− 3 m³ (m of shoreline)^− 1 s^− 1.  相似文献   

CAT-scan analysis of sedimentary sequences: An ultrahigh-resolution paleoclimatic tool     

Guillaume St-Onge  Bernard F. Long   《Engineering Geology》2009,103(3-4):127-133

Paleoclimate research is essential to determine the natural variability of climate and to place the current climate change into its natural context. The current need is to generate the highest temporal resolution paleoclimatic reconstructions possible in order to assess the natural variability of the climate system, but also to test the ability of numerical models to simulate conditions different from the ones observed with the relatively short instrumental records. In this paper, we show that CAT-scan analysis of sedimentary sequences, with its 1 mm downcore resolution, can be used to identify millennial to seasonal cycles in sedimentary sequences. In examples from the St. Lawrence Estuary, Eastern Canada, spectral analysis of the CAT-scan data from Holocene postglacial sediments revealed millennial- to centennial-scale oscillations possibly associated with either solar variability, changes in relative sea-level or tidal amplitude. Similarly, spectral analysis of Holocene and Sangamonian glaciomarine sequences revealed decadal- to annual-scale oscillations with periods close to the one previously associated with the North Atlantic Oscillation (NAO), whereas spectral analysis of the CAT-scan data from the Sangamonian rhythmites possibly revealed seasonal cycles.  相似文献   

Architecture and geodynamic evolution of the St Ives Goldfield,eastern Yilgarn Craton,Western Australia     

R.S. Blewett  R. Squire  J.M. Miller  P.A. Henson  D.C. Champion 《Precambrian Research》2010

A new structural evolution consisting of both extensional and contractional events has been defined for the St Ives Goldfield in the south-central Kalgoorlie Terrane of the eastern Yilgarn Craton in Western Australia. These events shaped the development of the fault architecture, which controlled the location of the regional anticlines, the magmatic centres, and the deposition of the Archaean greenstone successions. The fundamental grain of the St Ives Goldfield is north-northwest-trending. This trend is marked by faults which developed during D₁ extension, which was oriented east-northeast–west-southwest. Across these faults we map major stratigraphic changes in the thickness and composition of units, especially of the previously undivided Black Flag Group volcaniclastic rocks. The centre of the St Ives Goldfield is dominated by the Kambalda Anticline. This north-northwest-trending regional fold was likely established early during the D₁ extensional history, and was fully established during subsequent east-northeast-oriented D₂ contraction. The regional anticline is an important architectural element because (1) magmatism and gold mineralising fluids were focussed into this domed region, and (2) deformation was partitioned across the limbs and crest of this structure. The D₃ event involved regional uplift and extension, resulting in the formation of late basins (Merougil Conglomerate locally) and the emplacement of granitoids sourced from a metasomatised mantle wedge (Mafic-type porphyries). The most significant gold event in terms of endowment occurred during D_4b sinistral strike-slip shearing and associated thrusting (e.g., Tramways and Republican thrusts). These thrusts were previously interpreted as the first contractional structures to deform the area (‘D₁’), but are here reinterpreted as relatively late (D_4b). In this D_4b event, the north-northwest-trending faults underwent sinistral strike-slip shearing and were linked across the Kambalda Anticline by accommodation structures represented by generally east- to east-northeast-trending thrusts. Reactivation of D₁ transfer structures may have influenced the location of these later accommodation structures. Late-stage mineralisation during D₅ was the result of dextral strike-slip brittle shearing.  相似文献   

The ‘Roxolany Tephra’ (Ukraine) − new evidence for an origin from Ciomadul volcano,East Carpathians          下载免费PDF全文

Sabine Wulf  Stanisław Fedorowicz  Daniel Veres  Maria Łanczont  Dávid Karátson  Ralf Gertisser  Marc Bormann  Enikö Magyari  Oona Appelt  Ulrich Hambach  Petro F. Gozhyk 《第四纪科学杂志》2016,31(6):565-576

  相似文献   

Rapidly spreading mangroves at Port Gawler,South Australia: an update     

J. H. Cann 《Australian Journal of Earth Sciences》2018,65(2):243-247

The previously reported rapid spread of mangroves at Port Gawler on the northeastern shore of Gulf St Vincent, South Australia has continued. This is a tidal-dominated coastline where reduced incoming tidal currents have led to increased sedimentation on the tidal flat and the development of an area of samphire and mangroves that have split the tidal flat in two. We predict that the remaining sections of the tidal flat will eventually become a mature mangrove forest, although human activity may delay this process.  相似文献