Gravity-core sampling

Sulfide and pH measures were not always made on the same gravity cores used for Fe and Mn pore-water analyses, but the general values of these parameters are relatively predictable deep in the sediment at each station based on other cores. These are listed in Appendix B of Part I along with pore-water analyses. Sulfide levels are highest at FOAM (Gold-haber et al., 1977) and lowest at DEEP where they are below the detection limit of a sulfide electrode, pS s 16, in the gravity-core samples (but not in the box cores). 

Analyses were made on a number of samples of seawater and marine sediments which appear essentially uncontaminated by any artificial radionuclides and which can indicate the total analytical blank found in these measurements. The sediment samples were deep sections of gravity cores from the deep ocean (sections well below the level of the least detectable fallout nuclides), and the water samples were from deep in the Southern Atlantic Ocean. These data may be compared with the analytical data in Figure 1 and Tables V and VI to demonstrate that the analytical blanks are extremely low compared with the nuclide concen-... 

The sediments were obtained using short gravity coring devices (Niemistd-corer, 1988 Multi- corer, 2005). From each of the cores, the uppermost slices (1988 0-3 cm 2005 0-2 cm) were separated for analysis, freeze dried, and homogenized. In total, 308 (1988) and 56 (2005) surface samples were investigated. 

Gravity cores were processed in the same way, with two differences. The core was sectioned into 10-cm intervals in a miter box using a hacksaw. When sections were squeezed, water taken from each was either kept separate and assigned to different 5-cm intervals or combined into a single sample. 

Most bottom samples were obtained by Scuba divers using specially constructed plexiglas box cores (Fig. 13 of Part I). The method of obtaining cores is described in Part I, Gravity cores were also taken at each station on one or more occasions as previously described. Box cores were taken in summer, fall, and winter-spring periods from 1974 to 1976 sampling was carried out over 2-year periods at FOAM and NWC and for 1 year at DEEP (Table III of Part I, this volume, p. 253). 

MnCOs (rhodochrosite) or mixed Mn, Ca, Mg carbonates are believed to be forming in many marine environments and to be acting as important controls on pore-water Mn " concentrations (Li et al., 1969 Calvert and Price, 1970,1972,1977 Holdren eta/., 1975 Suess, 1979). IAP calculation indicates that at FOAM the pore-water samples taken from box cores are very near saturation with MnCOs below —4 cm during the summer and winter but are undersaturated during the fall (Fig. 16). The same pattern is found at DEEP (Fig. 16). NWC box cores are undersaturated (—5-10 x) with respect to rhodochrosite. Gravity cores indicate that deeper pore waters are slightly (—5 x) undersaturated at NWC and DEEP and are... 

A comparison of wet bulk densities derived from gamma ray attenuation with those measured on discrete samples is shown in Figure 2.4a for two gravity cores from the Arctic (PS 1725-2) and Antarctic Ocean (PS 1821-6). Wet bulk densities,... 

Fig. 2.4 Comparison of wet bulk densities determined on discrete samples by weight and volume measurements and calculated from gamma ray attenuation, (a) Cross plot of wet bulk densities of gravity cores PS 1821-6 from the Antarctic and PS1725-2 from the Arctic Ocean. The dashed lines indicate a difference of 5% between both data sets, (b) Wet bulk density logs derived from gamma ray attenuation for two 1 m long core sections of gravity core PS1725-2. Superimposed are density values measured on discrete samples. Modified after Gerland and Villinger (1995).

Fig. 2.5 Influence of an iterative mass attenuation coefficient determination on the precision of wet bulk densities. The gamma ray attenuation log of gravity core PS 1725-2 was used as test data set. (a) Wet bulk densities calculated with a constant mass attenuation coefficient ( processing porosity =50%) are displayed versus the data resulting from die iteration. A pore fluid density of 1.024 g cm and a constant grain density of 2.7 g cm were used, and the iteration was stopped if densities of two successive steps differed by less than 0.1%o (b) Cross plot of wet bulk densities measured on discrete samples versus wet bulk densities calculated from gamma ray attenuation with a constant mass attenuation coefficient (O) and with the iterative scheme (+). (c) Influence of grain density on iteration. Three grain densities of 2.65, 2.75 and 2.1 g cm were used to calculate wet bulk densities. Modified after Gerland (1993).

Fig. 2.7 Formation factor versus porosity for six gravity cores retrieved from different sedimentation provinces in the South Atlantic. Porosities were determined on discrete samples by wet and dry weights and volumes, formation factors by resistivity measurements. The dashed lines indicate Archie s law for a = 1 and cementation exponents (m) between 1 and 5. For a description of the sedimentation provinces, core numbers, coring locations, sediment compositions, water depths and constants (a) and (m) derived from linear least square fits please refer to Table 2.1. Unpublished data from M. Richter, University Bremen, Germany.

Fig. 2.8 Porosity logs determined by resistivity measurements on three gravity cores from the South Atlantic (see also Fig. 2.7 and Table 2.1). Gray curve Boyce s (1968) values were used for the constants (a) and (m). Black curve (a) and (m) were derived from the slope and intercept of a linear least square fit. These values are given at the top of each log. Superimposed are porosities determined on discrete samples by weight and volume measurements (unpublished data from P. Muller, University Bremen, Germany).

Fig. 8.6 Pore-water concentration profiles from gravity core GeoB 3714-9 from the Benguela upwelling area (2060 m water depth), South Atlantic. The shaded bar marks the sulfate-methane transition zone. The methane sample labeled C.C. was taken from the core catcher immediately after core recovery. From Niewohner et al. (1998).

Fig. 16. Rock-Eval 6 calibrations (a) calibration curve relating Rock-Eval 6 Y factor to oil API gravity and (b) predicted API for core samples from Rock-Eval 6 Y as a function of extract hydrocarbon/nonhydrocarbon ratio, (c) Plot of predicted API for core samples compared with API gravity measured from core extracts.

During ODP Leg 157, samphng of organic turbidites was done immediately after splitting the cores as they came on deck, after an equilibration period of few hours. The samples were immediately stored frozen. During the Meteor cruise 37/1, similar sampling procedures were maintained and aU samples were kept frozen until processed for analysis. Multicores, box cores, and gravity cores were taken (Wefer et al., 1997). 

Emphasis is given to the proper compaction of the asphalt layers. In core sampling or nuclear determination, the average of five density determinations should be equal to or greater than 96% of the average density of laboratory-prepared specimens, and no individual specimen should be lower than 94%. When compacted density is compared to the theoretical maximum specific gravity, the above percentage values become 92% and 90%, respectively. 

The preliminary assay provides general data on the oil and is based on simple tests such as distillation range, water content, specific gravity, and sulfur content that enable desirable or undesirable features to be noted. This form of assay requires only a small quantity of sample and is therefore particularly useful for the characterization of oil field samples produced from cores, drill stem tests, or seepages. 

Fig. 3.13 Application of a high-momentum gravity corer (Meischner and Rumohr 1974) to obtain samples from marine sediments. The device can also be stationed on smaller vessels and is suited to extract almost unperturbed cores measuring 1 m in length to be applied in pore water analysis.

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. 

As for both substances, sulfide and methane, concentrations found in the core are similar to that of sulfate. For the purpose of sampling, small windows (2x3 cm) were cut into the plastic liners with a saw, immediately after the meter-long segments from the gravity corer were available. In each of these windows, 2-3 ml samples of fresh sediment were punched out with a syringe. For the... 

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