Composition of carbonate overgrowths produced on Iceland spar calcite crystals buried in bahamian carbonate-rich sediments

High-magnesium calcite overgrowths were precipitated on Iceland spar calcite crystals buried in Bahamian sediments. Their composition was within the range of carbonate cements in this area. The precipitation rate of the overgrowths was slower and the magnesium content higher than predicted from laboratory experiments, possibly due to adsorption of organic compounds.     

Sulfate reduction in deep coastal marine sediments

Sulfate reduction rates in sediments of four deep stations in the Saguenay Fjord and the Laurentian Trough, Gulf of St. Lawrence, are among the lowest reported for the coastal environment. Maximum rates were 0.4–7.0 nmol cm⁻³ day⁻¹. The low rates are due to relatively low sedimentation rates and continuously low temperatures. Regional differences in both integrated and maximum sulfate reduction rates in the sediment correlate with sediment trap measurements of sedimentation rate and organic carbon flux.Sulfate reduction accounts for the degradation of 5–26% of the estimated downward flux of organic matter to these sediments. Unlike the absolute rate of sulfate reduction, the relative proportion of the carbon flux that is degraded via sulfate reduction is not directly correlated with the sedimentation rate but is a function of organic matter composition, intensity of bioturbation, and the abundance of sub-oxic electron acceptors. Thus, the lowest proportion of carbon degradation via sulfate reduction occurred at a Gulf site, where a combination of low sedimentation and bioturbation rates allowed a long residence time for organic matter near the sediment surface and, in consequence, a low flux of labile carbon into the sulfate reduction zone. The highest proportion was observed at a station with a similar organic carbon flux but with higher rates of sedimentation and bioturbation. At a third site, with the highest rates of sulfate reduction as well as the highest rates of sedimentation and bioturbation, the contribution of sulfate reduction to organic matter degradation was only intermediate. This is attributable to the exhaustion of the supply of porewater sulfate. In deep coastal environments the proportion of organic matter degraded via sulfate reduction can be highly variable both spatially and temporally.     

Comparative study of digestion methods EPA 3050B (HNO₃--H₂O₂--HCl) and ISO 11466.3 (aqua regia) for Cu, Ni and Pb contamination assessment in marine sediments

Knowing the metal extraction capacity of a digestion method is crucial for a better environmental interpretation of metal concentrations determined in sediments. One of the main problems at the present is the lack of harmonization of information obtained by two of the most popular sediment partial digestion methods: ISO 11466.3 (aqua regia) and EPA 3050B (HNO?--H?O?--HCl). In the present work, the amount of Cu, Ni and Pb leached by using both methods was compared with the total content of those elements in marine sediments collected, as an example, from the Cienfuegos Bay, Cuba. Similar amounts of Cu were extracted by both methods; while leaching of Ni and Pb were different. Generally, the EPA method extracted more Ni than the ISO method. In contrast, Pb was extracted in a larger amount by the ISO method. Some explanations are given for the observed results. X-ray Diffraction, X-ray Fluorescence, Particle Induced X-ray Emission Spectrometry and Energy Dispersive X-ray coupled to Scanning Electron Microscopy were employed for this purpose. On the other hand, none of the methods studied extracted simultaneously the fraction of all the metals, probably provided by human activity (Theoretical Anthropogenic Fraction) in both sediments studied. The use of ISO 11466.3 or EPA 3050B is recommended since the analytical performance parameters of both, in combination with Flame Atomic Absorption Spectrometry, are adequate. For a better environmental interpretation of the analytical results, information on the extraction efficiency of the selected method for specific elements and sediments under study should also be provided, together with the determined concentrations.     

Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen Gulf region,Canadian Arctic

To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The C_ORG:N_TOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ¹³C_ORG and δ¹⁵N_TOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (C_ORG:N_TOT ratio) and isotopic (δ¹³C_ORG and δ¹⁵N_TOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.     

Quantitative reconstruction of Late Cenozoic landscapes: a case study in the Sierra de Atapuerca (Burgos,Spain)

We have developed a method to reconstruct palaeorelief by means of detailed geomorphological and geological studies, geostatistical tools, GIS and a DEM. This method has been applied to the Sierra de Atapuerca (NE Duero Basin, Burgos, Spain), allowing us to model a three‐dimensional reconstruction of the relief evolution from the Middle Miocene to the present. The modelling procedure is based on geostatistical recovery of the palaeosurfaces characteristic of each geomorphological evolution stage, using polynomial regressions, trend surfaces and kriging. The modelling of morphology trends has been useful in establishing new geological and geomorphological relationships in the geodynamic evolution of this basin, such as uplift quantification, correlation of erosion surfaces and sedimentary units, and the evolution of fluvial base levels. The palaeosurface reconstruction together with an analysis of the slope retreat have allowed us to reconstruct the palaeoreliefs that define the Late Cenozoic landscape evolution of this area, where the Lower and Middle Pleistocene archaeopalaeontological sites of the Sierra de Atapuerca are located. Copyright © 2007 John Wiley & Sons, Ltd.