Sediment core samples, Upper

Comparative elemental analyses of the upper and lower sections of two sediment cores collected on the MAP show that organic concentrations decreased at both locations from values of 0.93-1.02 wt.% OC below the oxidation front to values 0.16-0.21 wt.% within the surface-oxidized layer (Figure 9). Pollen abundances decreased in the same samples from —1,600 grains g below the oxidation front to zero above it. Overall, 80% of the organic matter and essentially all of the pollen that has been stable for 140 kyr in the presence of pore-water sulfate was degraded in the upper section of the MAP cores as a result of long-term exposure to dissolved O2. 

Four sediment samples were taken in 1998 and 1999 from three locations at the Teltow Canal in Berlin, as indicated in Fig. 1. In addition to the surface sediment samples T1 and T3 taken by means of a 4L Ekman-Birge grab sampler a short sediment core T2 was obtained by using a tube coring device. The sediment core was subdivided into an upper part T2a (0-3 cm) and a lower part T2b (3-10 cm), that represents an older accumulation time between 1980 and 1990. 

An earlier study (11) of alkanes, cycloalkanes, and phenanthrenes in another sediment core from this location showed an interesting trend of decreasing concentrations between the upper 2 cm and 54-58 cm, which pointed towards fossil fuel combustion as the principal source of hydrocarbons in these surface sediments. The core was collected in August 1975 with a 21-cm-diame-ter, 1-m-long sphincter corer (14). Three sections of the core were used for this study Sample 4, the top 4 cm Sample 5, 20-24 cm and Sample 6, 38-42 cm. There was no sulfide present in the top 8 cm. An oxic, bioturbation zone of about 4 cm is indicated by the 210Pb depth profile and benthic ecology studies (12,15). 

Plutonium concentrations found in Lake Michigan sediments were reported to range from 35 to 250 pCi/kg dry sediment (9.5x10 to 6.8x10 Bq/kg) (Edgington et al. 1976). It was estimated in this report that radioactivity in the sediments was confined to the upper 6 cm of the sediments, and in many of the core samples, no radioactivity was detected below a depth of 3 cm. 

Our study was based on samples from 11 (Figure 1) 1000-foot core holes drilled in the Gulf of Mexico by four oil companies Humble, Chevron, Gulf, and Mobil. All cores are from the present continental slope within three morphological areas the Upper Continental Slope off Texas and Louisiana, the Upper Continental Slope off west Florida, and the upper reaches of the Mississippi Cone—a mass of sediment derived from drainage of the Mississippi River which has locally buried the continental-slope morphology. 

In the upper part of the sediment column, total particulate cadmium content is approximately 10 mg/kg, whereas in the deeper anoxic zone 20 mg Cd/kg have been measured. The results of the sequential extractions of the core sediment samples separated at 2-cm levels (Figure 5-8) indicate, that in the anoxic zone cadmium is associated by 60-80% to the sulfidic/organic fraction. In the oxic and transition zone, sulfidic and organic fractions decrease to approximately 30-40%, whereas carbo-natic and exchangeable fractions simultaneously increase up to 40% of total cadmium concentrations. Thus, it is notable that high proportions of mobile cadmium forms correlate with the marked reduction in total cadmium contents. 

Direct evidence for the formation of authigenic metal sulfide comes from x-ray microprobe and light-microscope examination of the salt-marsh sediments. In the presence of sulfur, iron monosulfides react to form pyrite, FeS2, which is known to occur as distinctive, characteristic aggregates of octahedral microcrystals of FeSj (framboids Berner, 1970 Sweeney and Kaplan, 1973). In the Farm River samples, framboidal FeSj was found to be common within at least the upper 14 cm of core examined, either as discrete framboids of 10- im diam. (Fig. 7) or as ordered clusters of framboids. Inspection of polished thin sections reveals a frequent association with the organic matrix, which appears to act as a template for their formation (see Fig. 8). 

At least the upper 10 to 30 cm of the core length obtained with either tool is usually adulterated in that it is not appropriate for pore water analysis. The multicorer, high-momentum gravity corer, or at least the box corer should be employed in a parallel procedure to ensure that this layer will also be included as part of the sample. It should not be overlooked that, especially in the deep sea, sampling with two different tools at the same site might imply a distance of several 100 m on the ocean floor. From this deviation considerable differences in pore water composition, and in some of the biogeochemical reactions close to sediment surface, are likely to result. Hence the specification as to same site must be acknowledged with caution. 

Fig. 9. Depth and age plots of Fe, IRM, Zn, Pb, and As all normalized to the Ti abundance at the same depth. The vertical lines are average (mean) values for surface soil samples in the vicinity of HinkeFDeegan Reservoirs. Note the similarity between soil and reservoir data in the upper portion of the core (about 1984 and later) for normalized Fe and IRM values. In contrast, reservoir sediments post-1984 are lower than nearby soils and Zn is greater.

Figure 3. Oat / iV/ge National Laboratory unsaturatedflow facility. The intact cores are contained in large polyvinyl chloride pipes, and a filter membrane was c ed to the sediments within a fabricated acrylic endcap at the lower boundary. Tension was maintained through the large white vacuum chambers, which also housed a fraction collector for samples of column effluent. Influent was delivered by a multi-channel pump at the upper boundary. Tensiometers, visible in the side of...

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