C-14 dating of methane

Sampling:

Sediments are recovered in PVC liners using a gravity corer or a vibro corer. The cores are cut into 1-m long sections as soon as possible, which are either sub-sampled immediately (sediment samples containing gas) or sealed with end caps and gas tight tape for sampling gas with syringes.

1.Sediment sub-samples: The samples are taken from the core-catcher (transferred by a spoon) and from top of each 1-m section (using 60 ml cut-of syringes, 3-4 times) just after cutting the core. The samples, 200-300 ml, are transferred to a 0.5 l can as quickly as possible to minimise the loss of methane. The rim of the can has to be cleaned carefully, and the lid is then pressed firmly on the can to completely seal the sample. The cans are stored upside down in a refrigerator until gas analyses onshore.

In the lab, the cans are punctuated and adquate volumes of headspace gas is extracted for analysis of composition and radiocarbon dating.

2.Direct gas sampling with syringes: 60 ml plastic syringes penetrate the PVC liner through 4 mm holes drilled just before attaching each syringe. A spring is mounted on the plunger of the syringe in order to overcome friction and to maintain a slight vacuum inside the syringe. Gas extraction is usually completed within 15 to 60 min. In many cases a small amount of sediment (0-3 cm3) is forced into the syringe due to the overpressure in the core liner. However, the sediment usually does not block the slow flow of gas into the syringes. The gas is transferred to a serum flask and analysed on board by gas chromatography using a Microlab GC82 gas chromatograph equipped with a thermal conductivity detector (TCD). Separation of the gas components is carried out on 2 columns packed with molecular sieve 5A and Porapac C using helium (60 ml/min) as the carrier gas.

Isotopes and ¹⁴C dating:



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