Sampling Protocol for Pb-210 Analyses
Skidaway Institute Sedimentological Laboratory
October 20, 1999
1.) In the field, I usually use a 15-cm diameter x 50-cm long PVC barrel to subsample a box core or salt marsh. For a kasten core, the dimensions of the barrel are 300 cm long x 12.7 cm x 12.7 cm square, and you subsample directly out of the barrel. Extrude (box core) or section (kasten core) at 1-cm intervals, unless your specific circumstances call for a larger (i.e., high rate of accumulation) or smaller (i.e., low rate or high detail needed) interval. You can subsample these sampled intervals in the lab later for the Pb-210 analysis and you should have plenty of sediment left for grain size, clay mineralogy or geochemical analyses. I exclude discrete pieces of organic matter (e.g., chunks of wood, marsh grass stems) from the sample containers when sampling. Samples can be put into a whirlpak bag or whatever you choose for long-term storage, and don't need to be stored any special way unless you want to do organic carbon, trace metals, C-14, or organic pollutant studies. If that is the case, they need to be stored in a container suitable to the planned analyses and frozen ASAP.
2.) After sectioning the cores in the field, subsample the bags for Pb-210 in the lab. I usually homogenize the sediment in the bag by kneading it for several minutes before subsampling. When sampling kasten cores in the field and several subsamples for different analyses will be collected from each interval, you can homogenize each 1-cm interval as it is collected in a small plastic dish, and split the sample up at that time so as to preclude non-homogenous sampling. I need to have ~30 gms of dry sediment for each Pb-210 sample. Sediments need to be dried such that porosity can be determined later (necessary for correcting Pb-210 profile for compaction). To allow me to calculate porosity, take the following measurements: Record the initial beaker weight (dry and empty), the beaker + wet sediment weight, and after drying for 4-5 days (or until it smells dry), beaker + dry sediment weight. I will need an estimate of the pore water salinity as well. Samples must be cooled in a dessicator for a few hours and left there until it is time to weight them, or the mud will absorb a great deal of moisture from the air. After weighing, the samples should be ground to a powder (there will usually be some submm-size mud lumps left, but that's not important).
3.) Grind samples and seal in sample jars. Grinding is done in a porcelain mortar and pestle (large Coors-type, #60322-mortar and #60323-pestle). It's best to use two to avoid contamination of the samples. Grind samples from the bottom of the core upward so that low activity material can't be contaminated with high activity material. I grind every other interval as I go up using one mortar/pestle and use the second mortar/pestle to do the remaining samples. As each sample is ground, it is put into a sample container (Nalgene straight-side, screw-top polypropylene jar; holds 30 mls. Nalgene #2118-0001). I buy them from VWR, but you can get them from Fisher (Fisher Catalog #11-815-10A, 72/$68.70 list price). Make sure that the core ID, sample interval, date sealed and sediment weight contained within the jar are all written on the jar body and lid with a Sharpie. It is easiest to get the sediment weight in the jar by taring the jar before you fill it. Basically, each jar should be filled up to between 3-4 mm from the rim of the jar. It is important to maintain a consistent geometry between samples as the detectors are calibrated on geometry, not mass. As the jar is filled, periodically tap it gently on the bench to compact the sediment in the jar. When done, clean the threads on the outside of the jar and screw the cap on tightly to inhibit migration of Rn-222 (a gaseous daughter product of Ra-226) out of the container. Then wipe down the outside of the jar. Any sediment on the outside of the container can fall off onto the detector face, contaminating it. I will need you to provide me with a datasheet that compiles all the sediment weights from drying (see #2 above), and the sample details that are written on the jars.
I can send you an example sample jar if you wish. The sample jar has a black line inside showing how full to make the containers. We always orient the jars the same way, with the uppermost thread end toward you. When oriented correctly, the black line in the example jar I send you will be facing you. |
