986 resultados para East Church (Salem, Mass.)


Relevância:

30.00% 30.00%

Publicador:

Resumo:

The scavenging of 231Pa and 230Th was investigated in the Atlantic Sector of the Southern Ocean by combining results from sediment trap and in situ filtration studies. We present the first high-resolution profile of dissolved 230Th and 231Pa in surface waters across the ACC, showing a dramatic southward increase of both radionuclides around the southern ACC Front at 533S. High dissolved 231Pa/230Th ratios combined with low 230Th/231Pa fractionation factors (F) in these surface waters result in extremely high 231Pa94/230Th94 ratios of material collected in the shallow traps. Particulate 231Pa94/230Th94 ratios in a shallow trap near Bouvet Island increase continuously during the productive period in austral summer, and drop back in the low flux period. This behavior, following the Rayleigh fractionation principle, is interpreted to be due to an increase in the dissolved 231Pa/230Th ratio in the euphotic zone resulting from preferential scavenging of 230Th relative to 231Pa, even in opal-dominated regions. In the post-bloom stage, the depleted radionuclide concentrations are replenished by upwelling of Circumpolar Deep Water. The high particulate 231Pa94/230Th94 signal is weakened during downward transport of the bloom particles in the water column by incorporation of deep suspended particles, which have a lower 231Pa94/230Th94 ratio. It is shown that under the special hydrographic conditions in the Southern Ocean scavenging from the upper water column significantly influences the budgets of 230Th and 231Pa in the sediment. Nevertheless, the budgets are still made up primarily by scavenging from the large standing stock of deep suspended particles.

Relevância:

30.00% 30.00%

Publicador:

Resumo:

Pore-water samples from the equatorial sedimentary bulge area show reversals in depth profiles of 87Sr/86Sr ratios at the sediment/basement interface. Results of this work support inferences made from previous pore-water data (from DSDP drilling in the area) that large-scale horizontal advection of seawater has occurred through the basement underlying the thick sedimentary sequence in this region. The area of apparent advection includes the eastern part of the equatorial high-productivity zone and part of the Guatemala Basin. We attempted to find links between the observed near-basement reversals in pore-water chemistry and sedimentary thickness, age, and topography of the area. Most of the sites that show horizontal advection have disturbed basement topography or outcrops within 10 to 20 km, suggesting that the cooling effects of outcrops may extend for at least 20 km horizontally. Heat-flow data from the area were compared to determine whether sites showing near-bottom chemistry reversals were consistent with areas of low conductive heat flow. This was generally true for the area of the sedimentary bulge and Guatemala Basin. Not enough pore-water data from the Nazca Plate were available to establish any reliable systematics. Because the high-productivity area is well-sealed from hydrothermal circulation, the missing heat must be lost by horizontal advective heat transport. From profiles of strontium isotopes and other elements that show departure from seawater values with increasing depth in the sediments, but return to seawater values near the basement, it appears that water flows relatively freely through much of the oceanic crust, even when sealed by considerable sedimentary cover.

Relevância:

30.00% 30.00%

Publicador:

Resumo:

Lithological, geochemical, stratigraphic, and paleoecological features of carbonaceous sediments in the Late Jurassic Volgian Basin of the East European Platform (Kostroma Region) are considered. The shale-bearing sequence studied is characterized by greater sedimentological completeness as compared with its stratotype sections in the Middle Volga region (Gorodishche, Kashpir). Stratigraphic position and stratigraphy of the shale-bearing sequence, as well as distribution of biota in different sedimentation settings are specified. It is shown that Volgian sediments show distinct cyclic structure. The lower and upper elements of cyclites consist of high-carbonaceous shales and clayey-calcareous sediments, respectively, separated by transitional varieties. Bioturbation structures in different rocks are discussed. Microcomponent composition and pyrolytic parameters of organic matter, as well as distribution of chemical elements in lithologically variable sediments are analyzed. Possible reasons responsible for appearance of cyclicity and accumulation of organic-rich sediments are discussed.