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81.
Charles T. Roman Norbert Jaworski Frederick T. Short Stuart Findlay R. Scott Warren 《Estuaries and Coasts》2000,23(6):743-764
Geographic signatures are physical, chemical, biotic, and human-induced characteristics or processes that help define similar or unique features of estuaries along latitudinal or geographic gradients. Geomorphologically, estuaries of the northeastern U.S., from the Hudson River estuary and northward along the Gulf of Maine shoreline, are highly diverse because of a complex bedrock geology and glacial history. Back-barrier estuaries and lagoons occur within the northeast region, but the domiant type is the drowned-river valley, often with rocky shores. Tidal range and mean depth of northeast estuaries are generally greater when compared to estuaries of the more southern U.S. Atlantic coast and Gulf of Mexico. Because of small estuarine drainage basins, low riverine flows, a bedrock substrate, and dense forest cover, sediment loads in northeast estuaries are generally quite low and water clarity is high. Tidal marshes, seagrass meadows, intertidal mudflats, and rocky shores represent major habitat types that fringe northeast estuaries, supporting commercially-important fauna, forage nekton and benthos, and coastal bird communities, while also serving as links between deeper estuarine waters and habitats through detritus-based pathways. Regarding land use and water quality trends, portions of the northeast have a history of over a century of intense urbanization as reflected in increased total nitrogen and total phosphorus loadings to estuaries, with wastewater treatment facilities and atmospheric deposition being major sources. Agricultural inputs are relatively minor throughout the northeast, with relative importance increasing for coastal plain estuaries. Identifying geographic signatures provides an objective means for comparing the structure, function, and processes of estuaries along latitudinal gradients. 相似文献
82.
A new high-resolution magnetostratigraphic record from the eastern Arctic Ocean has yielded further evidence for the existence of the Laschamp excursion (37–35 ka), the Mono Lake excursion (27–25.5 ka) and possibly another very short excursion (22 ka) inferred from steep negative inclinations. Ages are based on nine AMS (accelerator mass spectrometry) 14 C dates, oxygen isotope stratigraphy and correlation with ODP site 983. Estimates of relative palaeointensity variations for the time interval from 80 to 10 ka have revealed that the documented geomagnetic excursions are linked to large fluctuations of the relative palaeointensity. The lowest values were obtained for the two excursions and the normal–reversed (N–R) and reversed–normal (R–N) transitions of the Laschamp polarity excursion, which itself is characterized by a slight increase of relative palaeointensity during its reversed state. The results are in general agreement with palaeointensity studies from other regions, indicating that these fluctuations could be global phenomena and that the geomagnetic field of the Brunhes Chron was very variable in amplitude as well as in geometry. The new result is one of the rare records comprising large directional as well as large relative palaeointensity variations. 相似文献
83.
Geophysical data collected on three U.S. Naval Oceanographic Office cruises to the Galapagos Rise are presented. These data allow definition of the morphology and structure of the Galapagos Rise.A postulated “hot spot” beneath the Galapagos platform is suggested as the cause of: (1) decreased seismicity along the spreading center for a 400 km E—W distance from the islands; (2) distinctive petro-chemistry of tholeiites from the islands and adjacent oceanic crust generated by the Galapagos Rise; (3) high-amplitude magnetic anomalies in a 1,000 km E—W band including and just north of the Galapagos platform; and (4) morphologic shape and the regionally elevated sea floor of the Galapagos Rise as it approaches the insular platform. 相似文献
84.
Stephan Steinke Till J.J. Hanebuth Christoph Vogt Karl Stattegger 《Marine Geology》2008,250(3-4):199-210
Variations in clay mineral composition of sediment cores from the margin and continental slope of the Sunda Shelf (southern South China Sea, SE Asia) covering the past 17,000 yr reflect changing influences of sediment sources together with clay mineral partitioning processes in shallow waters. We identify the deglacial sea level rise as the principal factor driving these changes. During the late glacial, high values of kaolinite are interpreted to reflect a higher contribution of clays from soils that have formed on the exposed Sunda Shelf and in the southern archipelagos of Indonesia. At this time core sites were located in close proximity to the mouths of the Sunda Shelf palaeo-drainage systems on the emerged shelf (“Sundaland”). The progressive landward displacement of the coastline and breakdown of these vast drainage systems during deglaciation led to a decrease in influence of the kaolinite-rich southern sources. When the coastline had retreated closely to its present-day position in mid-Holocene times, the former dominance of southern sources was replaced by a stronger influence of illite-rich sources (e.g. Borneo). The overriding control of sea level changes on the clay mineral distribution patterns precludes a definite climatic interpretation of clay mineral data in terms of climatic/monsoonal changes in such highly dynamic sedimentary environment. 相似文献
85.
D. H. Garrison D. D. Bogard A. A. Albrecht S. Vogt G. F. Herzog J. Klein D. Fink B. Dezfouly-Arjomandy R. Middleton 《Meteoritics & planetary science》1992,27(4):371-381
Abstract— Core samples were obtained from various locations of the ~ 105-kg Chico, NM, L6 chondrite in order to study the effects of large shielding on the production rates of cosmic-ray-produced nuclides. Relations between measured abundances of cosmogenic nuclides (10Be, 26Al, and stable isotopes of He, Ne, and Ar) and the cosmogenic 22Ne/21Ne ratio were determined and compared with recent model predictions of production rates. The measured 22Ne/21Ne ratios (1.06-1.08) and significant variations observed in concentrations of cosmogenic 21Ne and 3He suggest an ~40-cm shielding gradient across Chico and irradiation within a large object (> 100-cm radius). Noble gas data indicate that Chico experienced greater shielding than chondrites Knyahinya or Keyes and similar to Jilin. Values of 10Be (average = 20.7 dpm/kg) and 26Al (average = 71.1 dpm/kg) are nearly constant, however, and show no correlation with either 22Ne/21Ne or 21Ne. Activities of 10Be and 26Al suggest irradiation in a smaller object (~40–80 cm radius). The 26Al activity and the 26Al/10Be ratio (average value = 3.42) are both significantly larger than values for most other chondrites. These results could indicate a two-stage irradiation with t1 ~ 104 Ma and t2 ~ 4 Ma and a second-stage body the size of Knyahinya. The single stage, 10Be/21Ne exposure age for Chico is 65 Ma. The 22Ne/21Ne ratio apparently becomes insensitive to shielding for objects the size of Chico. No substantial evidence exists for chondrites with 22Ne/21Ne ratios significantly less than ~ 1.055. 相似文献
86.
87.
88.
Norbert A. Jaworski Peter M. Groffman Aimee A. Keller Jan C. Prager 《Estuaries and Coasts》1992,15(1):83-95
Nitrogen and phosphorus mass balances were estimated for the portion of the Potomac River basin watershed located above Washington, D.C. The total nitrogen (N) balance included seven input tource terms, six sinks, and one “change-in-storage” term, but was simplified to five input terms and three output terms. The phosphorus (P) balance had four input and three output terms. The estimated balances are based on watershed data from seven information sources. Major sources of nitrogen are amimal waste and atmospheric deposition. The major sources of phosphorus are animal waste and fertilizer. The major sink, for nitrogen is combined denitrification, volatilization, and change-in-storage. The major sink for phosphorus is change-in-storage. River exports of N and P were 17% and 8%, respectively, of the total N and P inputs. Over 60% of the N and P were volatilized or stored. The major input and output terms on the budget are estimated from direct measurements, but the change-in-storage term is calculated by difference. The factors regulating retention and storage processes are discussed and research needs are identified. 相似文献
89.
90.