No description available.

No description available.

No description available.

No description available.

No description available.

A compilation of high quality post-rift sediment isopach data has been used in conjunction with the observed free-air gravity anomaly to determine segmentation of the long term mechanical properties of the lithosphere at the Eastern USA passive margin. This segmentation is represented by a process-oriented analysis in which the flexural response of the margin to post-rift sediment loading is controlled by spatial variations in effective elastic thickness (T_e) of the underlying lithosphere. Existing Eastern USA margin T_e estimates range from less than 10km to more than 30km. In this study it is shown that high strengths of 10 - 40km T_e are confined to structural arches dividing the broadest marginal basins, while low strengths of less than 10km T_e are typically found in structural embayments and beneath the deep basins. The hinge zone, across which the degree of continental thinning increases rapidly, marks the transition between high and low strength. Yield strength envelope models support an argument that regions of low strength were created by lithospheric thinning during rifting, and sustained by thermal insulation and flexural curvature associated with voluminous post-rift sediment deposition. Along-strike T_e variations - reflected in the alternation of basement platforms and embayments - are attributed to inheritance of lithospheric segmentation from earlier tectonic events. Along-strike segmentation of the margin has previously been observed as a 300 - 500km wavelength spectral energy peak in the shelf break Airy isostatic gravity anomaly (IGA) high. That this segmentation is explained by variations in the underlying lithospheric strength is demonstrated by a flexural IGA high in which the equivalent spectral peak is absent. The spectral energy of the along-strike T_e distribution peaks in the same waveband. Removal of process-oriented components from the observed free-air gravity anomaly reveals other contributions that were not resolved in earlier studies. In particular, the (previously unknown) Carolina Trough Isostatic Gravity Anomaly, has been identified and attributed to an extrusive (syn-rift) volcanic source. Detailed study of this anomaly suggests that the margin is segmented in terms of its volcanic character, and argues against recent estimates of the volume of new igneous material emplaced during rifting.

Missisquoi Bay, Lake Champlain is a eutrophic, northern shallow freshwater bay that experiences toxic cyanobacteria blooms during the summer months, largely as result of high nutrient (P and N) loading from the agricultural watershed. The sediments, which contain minerals that readily sorb P, can act as a sink or source of water column nutrients. Phosphorus, both inorganic and some organic forms, sorbs to metal oxides at neutral pH in the sediment, thus P release into overlying and pore water can be significantly affected by the reduction and subsequent solubilization of these oxides. This study addresses novel aspects of nutrient cycling in lake sediments as part of a larger study to better understand the link between phosphorus forms, mobility, and cyanobacteria blooms. These aspects include: 1) diel and seasonal sediment redox fluctuations and 2) the role of organic P (Porg) in overall P mobility within sediments as a function of depth and time. Missisquoi Bay sediment porewater redox chemistry was monitored across diel and seasonal cycles over the course of two summers (May-October, 2007 and 2008) by using in-situ voltammetry. Redox chemistry was monitored at the sediment-water interface (SWI) continuously over diel cycles, and the vertical concentration profiles of several key redox species (O2, Mn2+, Fe2+, and FeS(aq)) were obtained from cores collected at different times. The sediments were then analyzed for Total P (TP), Reactive P (RP), Porg, Mn, Fe, Ca, Al, Total Organic C and N. A bloom did not occur in Missisquoi Bay during the summer of 2007, but did in summer of 2008, providing an opportunity to compare the sediment chemistry between non-bloom and bloom conditions. Increasingly anoxic SWI conditions across summer 2008 were observed but the SWI remained oxic for the duration of summer 2007. Significant changes in diel cycle redox chemistry at the SWI were also detected in both summers. Reactive P in the surface sediments decreased across the 2008 season but not in 2007. A strong correlation found between RP and RFe (operationally defined as Fe(III)OOH) suggests that a significant portion of sediment P (30-40%) is closely associated with Fe(III)OOHs, which are susceptible to reduction in anoxic conditions. Phosphorus mobility from the sediment into the water column can be limited by the amount of Fe(III)OOH at the surface, thus P flux from the sediments would be greatest when reducing conditions promote solubilization of these minerals. Completely anoxic surface sediments were only observed during the presence of a bloom, explaining the loss of RP in the surface sediments in 2008 in the late summer. Organic P species represent 18-26% of the P in sediments and the lack of a definite, consistent trend of Porg fractionation across the season suggests that there is variable mobility and degradation of these complex organic compounds on small timescales. The loss of RP from the sediment in 2008 could have contributed to an estimated water column P increase on the order of thousands of μg/L, which in addition to measured increases in NH4+ gradients and subsequent N flux estimates in the upper sediment, could have sustained the bloom for an extended period of time. The relationship between the bloom and reducing sediment conditions suggest that bloom dynamics enhance nutrient release from the sediments, allowing for proliferation and sustainability of the bloom.

Graduation date: 1985 / Best scan available for tables. The original is a blurry photocopy.

Graduation date: 1968

The abundance and distribution of biogenic, terrigenous and volcanic particles in the Panama Basin are markedly dependent on bottom topography and dissolution of calcite in the deeper parts of the basin. Of the coarse fraction (>62μ), foraminiferal tests and acidic volcanic glass shards are concentrated on the Cocos and Carnegie Ridges as lag deposits. Foraminiferal fragments are found on these ridge flanks and on the Malpelo Ridge due to reworking by bottom currents accentuated by dissolution of calcite with increasing depth. The finest calcite, probably coccoliths with fine foraminiferal fragments, together with the hydrodynamically light radiolarian skeletons are concentrated by bottom currents in the basin adjacent to the ridges. The foraminiferal calcite compensation depth in the basin is 3400 m. This relatively shallow depth probably reflects the high surface water productivity over the basin, although the pattern of productivity is not reflected in the pattern of biogenic sediments. Acidic volcanic glass appears to have been carried into the basin from Costa Rica, Colombia and Ecuador by easterly winds at altitudes of 1500 to 6000 m. Basaltic shards from the Galapagos Islands have been dispersed only over short distances to the west. Terrigenous sand-sized material is found on the edge of the continental shelf, where associated glauconite points to a relict origin, and along the northern Cocos Ridge, where contour currents may act as the dispersal mechanism. / Graduation date: 1973