Temperature, pH
Density and Porosity
Diffusion coefficients
C-14 dating of methane

Temperature, pH

Temperature of sediment is measured in the central part of cores, immediately upon retrieval, using a thermistor probe (3 mm diameter and 15 cm long). Gravity cores are cut into 1-m sections and the temperature is measured immediately in the exposed ends of sections.

pH is measured by a pH combination electrode which was inserted into the sediment immediately upon retrieval of cores.

Density and Porosity

15 ml of sediment is sampled in syringes from which the luer ends have been removed. Seal with Parafilm and tape. Can be stored frozen.

The water content is determined from the weight loss.

Diffusion coefficients

Experimental determination of tracer diffusion coefficients of sulfate using ³⁵SO₄^2-. Sediment cores of min. 10 cm depth are taken in 26 mm ID tubes. Care must be taken to obtain intact cores without animal burrows, voids or bubbles.

Core is stoppered at the bottom (radioactive pore water!). Overlying seawater (if surface sediment) is carefully removed with large syringe with plastic tubing. A well-fitting filter (e.g. GFC filter) pre-soaked with seawater is placed flat on the sediment surface so that no air is trapped below (would otherwise be a diffusion barrier). The core is stoppered on top.

Place core in thermostated water bath or in thermostated room. Temperature must be very stable! Preincubate for 1 day.

Prepare 35SO42- tracer solution by adding stock solution to seawater diluted 1:1 with dist. water (lower density than pore water to avoid convective transport down into the sediment core). Final radioactivity ca 50 kBq per ml. Keep in water bath for temperature equilibration. Place 0.5 ml tracer solution on filter and ensure even distribution over surface.

Remove filter and supernatant water. Insert a non-leaking piston into coring tube from above providing a close contact with the sediment surface (use syringe needle to let out air). Turn around and remove lower stopper. Push up core to leave 6-7 cm of core in tube and cut off the extruded sediment. Mark tube on outside with 0.5 cm intervals. Section remaining core in 0.5 cm slices by pushing piston and consecutively cutting off sediment with knife. Transfer each section to a centrifuge tube with 25 or 50 ml tap water. Centrifuge and take out 5 ml for liquid scintillation counting. The dilution factor is not very critical but the sections should be as even as possible.

Plot log 35SO42- radioactivity (use raw data from scintillation counter) against depth squared (x2) and determine linear correlation.

Where x is depth (cm), C is radioactivity at depth x (counts), A is a constant, D is molecular diffusion constant (cm²s^-1) , and t is time (s).







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