Aquatic soft sediment sampling methods: freeze coring and grab/hand coring

To obtain reliable cores of shallow aquatic sediments requires a pragmatic marriage of the substrate type that you will be coring to the research goals that you wish to achieve. Aquatic sediment substrates vary from extremely "soupy" to well compacted. The goals can vary from climate histories to geotechnical to geochemical, each of which may require specific sampling techniques. The resolution required in the study commonly dictates the type of substrate and the type of coring method required. The highest resolution lake coring, with also the best preservation of geotechnical properties, is freeze coring. When properly deployed this technique preserves sediment stratigraphy in hard to very soft substrates with sediment disturbance reduced to virtually nil either during or post sampling. Freeze cores can be transported back to the lab for detailed stratigraphic study, with resolution at millimetre scales possible through microtoming and X-Ray examination. Freeze coring is logistically difficult, requiring transport of dry ice into the field, and the maintenance of frozen conditions subsequent to sampling. The amount of sediment can also be relatively small in freeze cores, especially for the sediment-water interface. Freeze coring of marine substrates is the same as for fresh water. Other soupy sediment techniques include gravity coring (Sam Alpay) and hand coring. Hand cores can be used delicately and precisely, and hence can be effective coring devices in the softest of substrates. Hard substrates can cause as many problems as soft substrates, but are advantageous in that they are resistant to disturbance and thus can be relatively easily transported back to the lab. Many gravity corers are unsuited to these kinds of substrates due to a lack of penetration, no matter how good the operator or how heavy the weight used. Hand coring is a good option in these substrates, and can yield long and well preserved cores, although depth penetration can be limited by the amount of weight one can use to push the corer into the substrate (a sharp-ended coring nose is preferable). Short (<30cm) cores of hard substrates can also be obtained by coring the sediment retrieved in a box corer or large grab sampler. This technique provides large sediment volumes with easily correlated core depths. Lake or marine cores >1-2 metres generally require vibracoring or piston coring techniques, which are logistically difficult unless significant lifting capabilities are available. Small piston and vibracorers can be deployed off small vessels but the depth penetration of these systems is often limited by substrate layers or lift capacity. These techniques can disturb the uppermost 0.5-1 m and so need to be used in conjunction with gravity or other coring techniques that better capture the sediment-water interface (if that is of importance). Stratigraphy from different coring techniques can generally be spliced using radiometric dating and/or proxy data measurements.

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