Gravity corers

A gravity dredge was used to recover samples of the soft, dark, organic-rich muds that have accumulated thinly over the inundated clay soils of the watershed. In some locations, soft deposits such as alluvial silts could be recovered with a 2.5-inch-diameter gravity corer. 

The northern portion of Chesapeake Bay was studied. This upper section of the bay can be classified as a classic salt-wedge type estuary (15). The humic materials used in this study were extracted from several cores of sediments. Two one-meter cores interstitial water. The squeezed sediments were then dried, weighed and washed with distilled water. The sediments were then washed with 0.5N KCl to remove iron hydroxides, carbonates and exchangeable cations from the clays. Some of the fulvic acids are lost in this acid wash procedure. The humic materials were then extracted from the remaining solids by a O.IN NaOH solution. 

Sediment cores were taken with a gravity corer. They were sliced with 0.5-cm resolution and processed the same way as sediment-trap material. One of the cores (code SE-8802) was dated by Wieland et al. (41), who used the 137Cs record from bomb fallout and the Chernobyl accident. The authors determined the sediment accumulation rate at this site to be 1.84 g/m2 per day. This value agrees with the average sedimentation rate of 2.17 g/m2 per day gleaned from the sediment-trap record of the years 1984-1991 at a depth of 81 m. 

Light-weight, High-momentum Gravity Corer... 

A light-weight, high-momentum gravity corer was first described by Meischner and Ruhmohr (1974) (cf. Figure 3.13). Somewhat modified variants... 

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. 

After the usage of either tool - the gravity corer and the piston corer - the sediment core obtained is immediately dissected into pieces of 1 m in length within the tube. Usually, the one meter long tubular pieces, tightly sealed with caps, are stored at in situ temperature prior to the subsequent further processing which is to be carried out as quickly as possible. 

In principle, the box-shaped gravity corer is not dissimilar to the above mentioned gravity corer. Here, a metal box with a core length of about 10 m and a lateral dimension of about 0.1 m up to 0.3 m is used instead of a steel tube with a plastic liner. 

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... 

In order to analyze the quantitative element profiles, consisting of approximately 250 samples derived from one single core which was extracted with the gravity corer, several weeks are required for drying, mortaring, homogenization, weighing,... 

The element profiles of a sediment core from the Cape Basin (Wien et al. 2005a), which is shown in Figure 3.29, were obtained with a trans-portable XRF on board of a research vessel, the results were obtained within a period of 24 honrs after core extraction with the gravity corer. 

Meischner, D. and Rumohr, J., 1974. A Light-weight, High-momentum Gravity Corer for Subaqueous Sediments. Senckenbergiana marit., 6 105-117. 

Fig. 8.18 Determination of sulfate reduction rates in sediment cored by multicorer (A and B) and by gravity corer (C) from 1176 m water depth in the sulfidic part of the western Black Sea. A) Sulfate reduction rates (SRR) measured experimentally using either directly in situ on the sea floor using a benthic lander or shipboard in the...

The typical maximum sample recovery is 1,6.4, and 43.8 m, respectively. Figure 5.2 shows a simple cable-controlled gravity corer assembly that is designed around a weight stand, a pipe, a core cutter, and a core catcher. At the upper end of the tube is a one-way valve that provides an exit for water. The correct length of the free-fall portion of the piston cable is very important. Partial penetration combined with delayed stopping of the downward movement of the piston results in the greatest number of misinterpretations. 

Vibratory corers were developed to recover cores of granular material that could not be recovered by gravity corers (De Ruiter, 1975). The tool consists of a steel pipe that is driven... 

Samples of sediment were taken with a gravity corer for analysis, four samples next to the wreck and two reference samples at a distance of one kilometre from the wreck. The sediment samples were analysed chemically for the presence of mustard agent by the National Defence Research Establishment. Mustard agent has a very low water solubility. Especially viscous mustard has a very long life in water and can remain on the seabed for a very long time after that the ammunition has corroded. 

---