Four new bathymetric maps of the Tonga Trench and forearc between 14 °S and 27 °S display the important morphologic and structural features of this dynamic convergent margin. The maps document a number of important geologic features of the margin. Major normal faults and fault lineaments on the Tonga platform can be traced along and across the upper trench slope. Numerous submarine canyons incised in the landward slope of the trench mark the pathways of sediment transport from the platform to mid- and lower-slope basins. Discontinuities in the trench axis and changes in the morphology of the landward slope can be clearly documented and may be associated with the passage and subduction of the Louisville Ridge and other structures on the subducting Pacific Plate. Changes in the morphology of the forearc as convergence changes from normal in the south to highly-oblique in the north are clearly documented. The bathymetric compilations, gridded at 500- and 200-m resolutions and extending along 500 km of the landward trench slope and axis, provide complete coverage of the outer forearc from the latitude of the Louisville Ridge-Tonga Trench collision to the northern terminus of the Tonga Ridge. These maps should serve as a valuable reference for other sea-going programs in the region, particularly the Ocean Drilling Program (ODP) and the National Science Foundation MARGINS initiative. 相似文献
-In this work, on the basis of the characteristics of coastal erosion near Xizhuang (Penglai, Shandong, China) and the in-situ measured data and theoretical calculation, the causes of coastal erosion there are obtained: (1) natural erosion, (2) beach sand borrowing, and (3) sand borrowing at the Dengzhou shoal near Xizhuang, which results in the loss of the function of the wave force resistance of the shoal. Since the wave energy is increased, the longshore sediment transport rate is increased. Coastal erosions are more and more serious. A scientific basis to reduced the calamity of coastal erosion in local area is presented. Some experiences are accumulated for studying coastal erosion. 相似文献
Data are presented indicating the complexity and highly variable response of beaches to cold front passages along the northern Gulf of Mexico, in addition to the impacts of tropical cyclones and winter storms. Within the past decade, an increase in the frequency of tropical storms and hurricanes impacting the northern Gulf has dramatically altered the long-term equilibrium of a large portion of this coast. A time series of net sediment flux for subaerial and nearshore environments has been established for a section of this coast in Florida, and to a lesser extent, Mississippi. The data incorporate the morphological signature of six tropical storms/hurricanes and more than 200 frontal passages.
Data indicate that (1) barrier islands can conserve mass during catastrophic hurricanes (e.g., Hurricane Opal, a strong category 4 hurricane near landfall); (2) less severe hurricanes and tropical storms can promote rapid dune aggradation and can contribute sediment to the entire barrier system; (3) cold fronts play a critical role in the poststorm adjustment of the barrier by deflating the subaerial portion of the overwash terrace and eroding its marginal lobe along the bayside beach through locally generated, high frequency, steep waves; and (4) barrier systems along the northern Gulf do not necessarily enter an immediate poststorm recovery phase, although nested in sediment-rich nearshore environments. While high wave energy conditions associated with cold fronts play an integral role in the evolution and maintenance of barriers along the northern Gulf, these events are more effective in reworking sediment after the occurrence of extreme events such as hurricanes. This relationship is even more apparent during the clustering of tropical cyclones.
It is anticipated that these findings will have important implications for the longer term evolution of barrier systems in midlatitude, microtidal settings where the clustering of storms is apparent, and winter storms are significant in intensity and frequency along the coast. 相似文献
Studies of the concentrations of particulate and dissolved organic carbon in the Duplin River, of the tidal exchange of POC and DOC in the marsh, of the standing stock and movement of Spartina alterniflora wrack in the Duplin, and of the removal of carbon from the surface of the marsh by rain were conducted at Sapelo Island, Georgia in order to test three hypotheses about export of carbon from the Duplin River watershed. We found that the gradients in POC and DOC concentrations are such that carbon is being transported down the Duplin River throughout the year, although in smaller quantities than previously believed. In contrast, almost all tidal exchanges within the marsh result in deposition of carbon. Most of this deposited carbon is subsequently eroded as a result of rain falling on the exposed marsh surface, and is washed back into the tidal creeks. This cycle of deposition and erosion is a possible mechanism keeping POC in the thin aerobic surface layer of the marsh, thus increasing its availability to detritivores and aerobic microbes. The standing stock of wrack is only a fraction of the S. alterniflora produced each year, and its export is a negligible term in the carbon balance equation. 相似文献
Morphodynamic modeling is employed in the present work to predict the long-term evolution (over the next 100 years) of typical sedimentary coasts in the western Russian Arctic. The studied objects are the coasts of Varandey (the Barents Sea), Baydaratskaya Bay and Harasavey (the Kara Sea). The model developed takes into account both the short-term processes (storm events) and long-term factors (for example, changes in sea level, inter-annual variations in gross sediment flux, lack or excess of sediment supply). Predicted and observed morphological changes in coastal profiles are shown to agree well for time scales ranging from weeks to decades. It is revealed that under given environmental conditions, the morphological evolution is strongly influenced by storm surges and associated wind-driven circulation. The water level gradient created by a surge generates a seaward flow at the bed. This outflow is shown to be an important destructive mechanism contributing to the erosion and recession of Arctic coasts. The rate of change is found to depend on both the exposure of the coast (relative to the direction of dominant winds) and its height above the sea. The open coast of Varandey is expected to retreat as much as 300–500 m over 100 years, while recession of the less exposed coasts of Baydaratskaya Bay would not exceed about 100 m/century. If long-term sediment losses are insignificant, the rate of erosion decays with time and the morphodynamic system may tend toward equilibrium. It is concluded that the expected relative sea-level rise (up to 1 m over the nearest 100 years) is non-crucial to the future coastal evolution if an erosion activity is already high enough. 相似文献
The south-flowing waters of the Kamchatka and Oyashio Currents and west-flowing waters of the Alaskan Stream are key components of the western sub-Arctic Pacific circulation. We use CTD data, Argo buoys, WOCE surface drifters, and satellite-derived sea-level observations to investigate the structure and interannual changes in this system that arise from interactions among anticyclonic eddies and the mean flow. Variability in the temperature of the upstream Oyashio and Kamchatka Currents is evident by warming in mesothermal layer in 1994–2005 compared to 1990–1991. A major fraction of the water in these currents is derived directly from the Alaskan Stream. The stream also sheds large anticyclonic (Aleutian) eddies, averaging approximately 300 km in diameter with a volume transport significant in comparison with that of the Kamchatka Current itself. These eddies enclose pools of relatively warm and saline water whose temperature is typically 4 °C warmer and salinity is 0.4 greater than that of cold-core Kamchatka eddies in the same density range. Aleutian eddies drift at approximately 1.2 km d−1 and retain their distinctive warm and salty characteristics for at least 2 years. Selected westward pathways during 1990–2004 are identified. If the shorter northern route is followed, Aleutian eddies remain close to the stream and persist sufficiently long to carry warm and saline water directly to the Kamchatka Current. This was observed during 1994–1997 with substantial warming of the waters in the Kamchatka Current and upstream Oyashio. If the eddies take a more southern route they detach from the stream but can still contribute significant quantities of warm and saline water to the upstream Oyashio, as in 2004–2005. However, the eddies following this southern route may dissipate before reaching the western boundary current region. 相似文献