Appendix A Recovery of Ejecta Material from Confirmed, Probable, or Possible Distal Ejecta Layers A.1 Introduction In this appendix we discuss the methods that we have used to recover and study ejecta found in various types of sediment and rock. The processes used to recover ejecta material vary with the degree of lithification. We thus discuss sample processing for unconsolidated, semiconsolidated, and consolidated material separately. The type of sediment or rock is also important as, for example, carbonate sediment or rock is processed differently from siliciclastic sediment or rock. The methods used to take and process samples will also vary according to the objectives of the study and the background of the investigator. We summarize below the methods that we have found useful in our studies of distal impact ejecta layers for those who are just beginning such studies. One of the authors (BPG) was trained as a marine geologist and the other (BMS) as a hard rock geologist. Our approaches to processing and studying impact ejecta differ accordingly. The methods used to recover ejecta from unconsolidated sediments have been successfully employed by BPG for more than 40 years. A.2 Taking and Handling Samples A.2.1 Introduction The size, number, and type of samples will depend on the objective of the study and nature of the sediment/rock, but there a few guidelines that should be followed regardless of the objective or rock type. All outcrops, especially those near industrialized areas or transportation routes (e.g., highways, train tracks) need to be cleaned off (i.e., the surface layer removed) prior to sampling. The researcher should remove any objects (e.g., rings, necklaces, watches) that contain gold or B. P. Glass and B. M. Simonson, Distal Impact Ejecta Layers, Impact Studies, 625 DOI: 10.1007/978-3-540-88262-6, Ó Springer-Verlag Berlin Heidelberg 2013 626 Appendix A: Recovery of Ejecta Material from Confirmed, Probable platinum if there is any chance that the samples might be used for PGE studies as this can cause a contamination problem. A.2.2 Sample Interval and Size If the researcher is doing a reconnaissance study to determine if a particular layer has an impact origin and the sampling site is easily accessible, then one large sample (at least 100 g) from the suspected ejecta layer and at least one similar sized sample from below and one from above the layer should be taken. If it is already known that the layer is a distal ejecta layer, then it would be best to take a series of small samples (the size will depend on the thickness of the layer) continuously through the layer as well as a large sample (*1 kg) from the layer. Furthermore, a series of samples should be taken above and below the layer over a distance of at least a meter in addition to one or more large samples from and adjacent to (above and below) the layer. At a minimum, the lower, middle, and upper part of the layer should be sampled separately. If the sampling location is difficult to access, it is best to take more and larger samples than you think you will need. We have never found that we took too many or too large samples, but we have often wished that we had taken more and/or larger samples. If the layer was previously unknown or not well studied, other researchers will often request samples so they can make additional studies of the layer/ejecta. If the samples are from a core, then, prior to processing, the outer surface of the core samples should be thoroughly cleaned or trimmed off. Trimming off of the outer layer (a few millimeters) is especially important if the core is unconsolidated sediment. If the sample is dry and coherent enough, a band saw can be used to trim off the outer layer. If the sample is still damp and clay rich, a knife with a serrated blade can be used. A.3 Processing the Samples After cleaning off the samples or trimming off the outer surface of the core samples, the samples should be thoroughly dried and weighed. A.3.1 Unconsolidated Samples Each sample should be put into a beaker filled with water and allowed to break up. We usually use a 250 ml beaker for a 10 g sized sample. Distilled water is best and it should be filtered prior to putting it in the beaker in order to avoid contamination. If the sample does not readily breakup, then it might be best to let it sit in water overnight. Gentle stirring with a stirring rod (polyethylene, Appendix A: Recovery of Ejecta Material from Confirmed, Probable 627 Teflon, or glass) may help to break up the sample. After it has broken up as much as it is going to, we then use ultrasonics to disaggregate the clay fraction. A.3.1.1 Sieving Prior to sieving we put the sieves in water in an ultrasonic bath to clean them. We then dry them and examine them using a binocular microscope to make sure that there are no grains caught in the mesh. If grains are caught in the mesh we try to remove them gently with a stiff brush. If that is not successful, we try to push them out with a sharp pick pushing from the underside of the sieve; but this must be done with great care so as not to damage the mesh. Once the sieves are cleaned, we then begin the sieving process. We have always used 300 (*7.5 cm) diameter sieves. Larger sieves would take less time, but are harder to keep clean and more grains are lost due to being caught in the mesh. We usually use brass sieves (occasionally stainless steel), but we experimented using plastic sieves with replaceable nylon mesh. The mesh for the plastic sieves can be removed and discarded after sieving and replaced with a new mesh. This saves time in cleaning the mesh and avoids cross contamination. However, the design of these sieves results in numerous grains being trapped between the mesh and the walls of the sieves, resulting in more grains being lost than when using the brass sieves. Furthermore, electrostatic charges can build up when trying to brush the grains out of the plastic sieves and make it difficult to get the grains out of the sieve, especially the finer grains (\125 lm). We generally use a 63 lm and a 125 lm sieve, but if the layer is known to be very coarse grained, it might be useful to use a 250 lm and a 500 lm, and maybe even a 1 mm mesh sieve as well. However, the 63 and 125 lm sieves are generally sufficient for the more distal portions of distal ejecta layers. The beaker containing the sample in water is put into an ultrasonic bath. We only leave the sample in the ultrasonic bath long enough to make the water as cloudy as it will get. For some samples this may take only *30 s. We stir the sample briefly while it is in the ultrasonic bath. We take the beaker out of the ultrasonic bath and allow the coarse fraction to settle and then pour off the cloudy water through the nested sieves making sure that the 63 lm sieve is beneath the 125 lm sieve. If there is a lot of clay- and silt-sized material, the 63 lm sieve may clog and begin to overflow. If this happens, the 125 lm sieve can be removed and, using a spray of filtered water from a squeeze bottle, clear a small area on the 63 lm sieve, which will allow the water to go through. This may happen repeatedly with some samples. We then add more water to the beaker, ultrasonify for another 30 s or so, and then pour the cloudy water through the sieve set. We continue this process until the water does not get cloudy during ultrasonification. At that point we wash the coarse material out of the beaker into the sieve set using filtered water from a squeeze bottle. We then gently wash (with a squeeze bottle of filtered water) the grains in the top sieve to make sure the fine material has been washed through. We take the top sieve off and set it under a heat lamp to dry. We then gently wash the grains in the lower sieve and put this sieve under a heat lamp to dry. If there is even a remote possibility that the finer 628 Appendix A: Recovery of Ejecta Material from Confirmed, Probable fraction will be searched, a beaker can be put under the sieve set, before beginning to sieve, to collect the \63 lm size material. We generally use a 500 ml or 1 l sized beaker for a 10-g size sample. For larger sample sizes, several large beakers may be required to collect all the water that goes through the sieves, particularly if the sample requires repeated ultrasonifications to break up all of the clay material. Do not put the heat lamp too close to the sieves when drying them. If you burn your fingers when you try to pick up the sieve, the lamp is too close to the sieve. Alternatively, the sieves could be put into an oven, set at about 40 °C, to dry or, even better, let them air dry. Air drying can take several hours or even overnight, depending on the humidity in the room. Unless several sieve sets are being used, air drying may take too long.