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Spatial variations of particle scavenging rates within the central and northern Adriatic Sea: use of U--Th disequilibria

Description: Dissolved and particlulate Thorium-234 ({sup 234}Th) were measured in near surface waters from the Central and Northern Adriatic Sea in order to gain an insight into the intensity and variability of active scavenging and particle removal processes. Dissolved {sup 234}Th to Uranium-238 ({sup 238}U) activity ratios vary from 0.21 to 0.75 and clearly imply that {sup 234}Th is being actively scavenged from sea water on a timescale of <3 months. The scavenging rate of dissolved {sup 234}Th with respect to scavenging onto particles appears to correlate with primary productivity measurements. Scavenging and rapid removal of particulate {sup 234}Th from the water column is best explained by a mechanism of zooplankton grazing and fecal pellet production. At one sight in the Jabuka Pit, particulate {sup 234}Th residence times below the pycnocline are long (30-40 days) suggesting that particles are being more readily recycled at these depths. By comparison, in a seawater profile collected near the Po outflow region, {sup 234}Th is depleted with respect to {sup 238}U through the entire water column. We conclude from this assessment that particles in waters near the Po River outflow will be more efficiently transported to bottom sediments compared with those in deeper waters over the Jabuka Pit.
Date: April 1, 1998
Creator: Hamilton, T. F.
Partner: UNT Libraries Government Documents Department

{sup 210}Pb dating of sediments from the central and northern Adriatic Sea: deposition and preservation of sedimentary organic carbon

Description: Lead-210 ({sup 21O}Pb) and organic C depth distribution profiles in sediments from the northern and central Adriatic Sea were measured as part of the EEC funded project on Eutrophic Limits of the Northern Adriatic (ELNA). {sup 210}Pb derived mass-accumulation rates decrease southward from between 0.15 and 0.2 g cm{sup -2}y{sup -1} close to the Po River outflow (> 24 m, water depth) to less than 0.04 g cm{sup -2}y{sup -1} in the Jabuka Pit (246 m, water depth) in the central Adriatic Sea. The mass- accumulation rates obtained in the Jabuka Pit correspond to mean sedimentation rates of about 0.03 cm y{sup -1} (ref. porosity = 0.5) and fall between 5 to 20 times lower than rates found for north Adriatic shelf cores. Estimated sedimentation rates are considered as upper limits because of the possible effects of bioturbation and physical disturbance on the {sup 21O}Pb sedimentary record but are consistent with data from previous work. Rates of sediment accumulation and carbon burial appear to be strongly influenced by the transport of fluvial materials from land and transport of fine-grained particles. First-order estimates of organic C burial rates into surface sediment ranged from 1 to 0.028 mMol cm{sup -2}y{sup -1} between the Po delta and the Jabuka Pit regions, respectively. We estimate that a maximum of 50% of organic C preserved in surface sediment may be derived from biological production in the overlying water column.
Date: April 1, 1998
Creator: Hamilton, T. F., LLNL
Partner: UNT Libraries Government Documents Department

{sup 210}Pb dating of sediments from the central and the northern Adriatic Sea: The deposition and preservation of sedimentary organic carbon

Description: A central goal of the ELNA project is to assess the carbon assimilation capacity of the Northern Adriatic Sea. This requires fundamental quantitative information on budgets and sinks of organic carbon. Any change in carbon production in the water column should be reflected in the underlying sediments. Moreover, the fraction of particulate organic carbon reaching the sea floor which is subsequently preserved in the sediment will be strongly coupled to sediment accumulation and mixing. In this study a series of box cores were collected in order to characterize a hypothetical eutrophication gradient extending from the Po River outflow region in the north down to the shallow meso-Adriatic depression (Jabuka Pit). The main tasks assigned to IAEA-MEL were to provide {sup 210}Pb derived sedimentation and dry-mass accumulation rates and to examine the possible correlations between sedimentary processes, the deposition and preservation of sedimentary organic carbon and pelagic primary productivity.
Date: April 1, 1996
Creator: Hamilton, T.; Fowler, S.; Miquel, J.C. & La Rosa, J.
Partner: UNT Libraries Government Documents Department

Spatial variations of scavenging rates within the central and northern Adriatic Sea: Use of U-Th disequilibria

Description: Dissolved and particulate concentrations of {sup 234}Th, {sup 210}Pb and {sup 210}Po were measured in surface waters of the central and northern Adriatic Sea. The half-lives and biogeochemistry of {sup 234}Th (t{sub 1/2} = 24.1 days), {sup 210}Po (t{sub 1/2} = 138.4 days) and {sup 210}Pb (t{sub 1/2} = 22.3 years) are ideal for studying particle dynamics in the upper water column. It has been observed that the residence time of dissolved {sup 234}Th with respect to removal on biogenic particles is inversely proportional to primary productivity and may be used to predict export from or new production in the euphotic zone. Model-derived {sup 234}Th fluxes can also be used to help validate sediment trap collections in the upper water column. The aim of the present study was to gain an insight into the intensity and variability of active scavenging and particle removal processes in the central and northern Adriatic Sea. This semi-enclosed sea is strongly influenced by fresh water runoff from rivers carrying large quantities of dissolved nutrients and suspended particles. These inputs of nutrients and suspended particles are expected to give rise to large biological and biogeochemical gradients across the Continental Shelf and offer a range of environments for studying particle scavenging and removal processes. The authors have examined the relationships between {sup 234}Th scavenging rates and residence times of dissolved and particulate phases as a function of other physical and biological parameters (primary productivity, POC, chlorophyll, C/N and total suspended matter). Particular reference is made to the two contrasting environments of the Adriatic--a eutrophic zone near the Po River outflow and in the largely oligotrophic waters of the Jabuka Pit.
Date: April 1, 1996
Creator: Hamilton, T.; Heilmann, J.; Fowler, S. & Miquel, J.C.
Partner: UNT Libraries Government Documents Department

A Preliminary Look at the Crust and Upper Mantle of North Africa Using Libyan Seismic Data

Description: In recent years, LLNL has been developing methods to jointly invert both surface wave dispersion data and teleseismic receiver functions. The technique holds great promise in accurately estimating seismic structure, including important tectonic parameters such as basin thickness, crustal thickness, upper mantle velocity, etc. We proposed applying this method to some recently available data from several Libyan stations, as we believe the technique has not been applied to any stations in Libya. The technique holds the promise of improving our understanding of the crust and upper mantle in Libya and North Africa. We recently requested seismic data from stations GHAR (Gharyan) and MARJ (Al Marj) in Libya for about 20 events. The events were large events at regional distances suitable for making dispersion measurements. An example of waveforms recorded at the two stations from an earthquake in Italy is shown in Figure 1. The paths traverse the Ionian Sea. Notice the slow short period group velocities of the surface waves across the Mediterranean, particularly to the easternmost station MARJ. However, because of data availability, signal-to-noise ratio, etc. we were unable to make measurements for every one of these events at both stations. Figure 2 shows a map of paths for 20 sec Rayleigh waves in the eastern Mediterranean region. Paths measured at the two Libyan stations are shown in green. Rayleigh wave dispersion measurements at 20 sec period are sensitive to velocities in the upper 20 km or so, and reveal sediment thickness, crustal velocity, and crustal thickness. Tomographic inversions reveal the sharp group velocity contrast between regions with deep sedimentary basins and those without. Figure 3, the result of an inversion made before adding the new dispersion measurements, shows slow group velocities in the Black Sea, Adriatic Sea, and Eastern Mediterranean. In general, these features correspond well with the ...
Date: August 5, 2005
Creator: Pasyanos, M
Partner: UNT Libraries Government Documents Department