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IODP (International Ocean Discovery Program) at MARUM

Acquisition

In addition to pore water squeezers, pore-water acquisition with the new generation of “Rhizone pore water samplers” are used on MSP expeditions in the less indurated parts of the records. They have been proven to be an excellent technique that saves core sample material.

Rhizon samplers (Rhizosphere Research Products, Netherlands) are narrow elongated cylindrical microporous filters (type CSS, 0.2 µm pore size; 2.5 mm in diameter, 5 cm long) with a stiff plastic core (Seeberg-Elverfeldt et al., 2005; Dickens et al., 2007; Miller et al., 2017).

They can be inserted into the sediment from the cut core surface of core segments, or alternatively through small holes drilled into the liner. The samples can be collected in either 10 mL vacuum tubes designed for medical use or disposable with syringes requiring a minimum of experience and maintenance.

Rhizon pore water samplers

If time and the type of sediment allows, pore water samplers called rhizones will be used (Seeberg et al., 2005). The rhizons in use consist of a microporous tube, with a glass fiber wire as strengthener, connected to PVC tubing, terminated with a Luer connector, so it can be easily attached to needles or syringes. Rhizones are simply pushed into the wet sediment and vaccuum is attached by connecting a vacuum tube or a syringe, applying an under pressure manually. The microporous tube has a pore width of 0.2 µm such that additional filtering is not necessary.

Preparation

Quick: Connect a rhizon to a disposable syringe. Immerse the rhizon in a beaker of purified water and force some uL of water in and out by slowly pulling and pushing the plunger.

Slow: Immerse the microporous (white) tube end in a beaker of purified water for 30 min.

Once the microporous tube is wet, it is not permeable for air anymore.

Placing rhizons in the sediment

Loose wet sediment: Put the sediment in a beaker. If necessary use a 2 mm plastic wire (or HPLC PEEK tubing) to preform a channel and push the rhizon gently into the sediment from above by firmly holding the wire in the clear PVC tubing rear part of the sampler.
The tip of the wire is glued to the tip of the microporous tube. This way the sensitive microporous tube is pulled in from the tip rather than pushed in. For best performance the microporous tube should be covered with wet sediment completely. Yet, it still works if only part of it has contact with pore water.

Sediment core: Carefully drill a 3.8 mm hole in the liner at the sample depth of interest. Make sure to avoid contaminating the sediment with the tip of the drill. If necessary preform a channel with a plastic wire. Gently push the rhizon in all the way into the core material until the connector between microporous tube and PVC tubing seals the 3.8 mm hole of the liner.

Pore water aquisition

Disconnect the blue protective cap from the rhizon. The rhizons have very low dead volume. Discard the first 0.2 mL of sample.

Connect a rhizon to a disposable syringe. Slowly pull the plunger – if the microporous tube is wettened correctly it will keep the vacuum - and fix it with a wooden spacer. Discard the first 0.2 mL and reassemble. Wait. If necessary, repeat.

Vacuum is applied after the rhizon is placed in the sediment and kept by a wooden spacer.

Sampling with vacutubes: Connect a Luer needle to the rhizon sampler and push the needle through the rubber stopper of a plain vacutube. Use a separate vacutube to collect the first 0.2 mL . Attach the sample vacutube for sampling. One vacutube will ideally sample up to 8 mL of water. If necessary attach additional vacutubes.

Cleaning

All syringes, vacutubes and rhizons are meant to be used one way. Yet, if necessary the rhizons may be cleaned and re-used. Gently clean the rhizons mechanically from the outside and wash with purified water. Inspect the microporous tube for damages. Force some 10 mL diluted nitric acid through the microporous tube from the inside using a syringe. Afterwards force some purified water in and out for three times using a syringe.

Change water between steps. If vacuum is attached while the rhizon is wet but NOT in contact with water, it should keep the vacuum. If not it is likely to be damaged.

References