Outcrops are commonly surface exposures of bedrock, although they can include superficial deposits (i.e. unlithified Quaternary sediments). They may be covered by soil and vegetation. If contamination is likely to be a problem, as far as possible control samples should be taken.
For rock samples this may involve simply keeping some empty containers, whereas for fluids it may be necessary to take subsample neighbouring substrate (e.g. soil). Subsequent geochemical analyses on these control samples will enable the potential influence of contamination to be identified.
The location (GPS or map-coordinates plus any local name) is crucial information for the purposes of sampling and later verification that the correct units have been sampled. It is also essential to record accurately the position of samples within an outcrop face. Height above a datum needs recording – preferably a well recognized datum such as a formation bottom/top, if recognizable (which can even be refined to distance relative to an O-isotope stage for more accurate dating if sufficient information exists). A photograph or sketch and a description of the sampling site and exposure are basic requirements.
If the stratigraphy is known from previous geological mapping, then identifying the Formation or Group being sampled should be reasonably straightforward for a field geologist with experience of the area. Otherwise, samples for biostratigraphy may also be useful.
The simplest method for rocky outcrop sampling is to use hammers and chisels. Powered mechanical or hydraulic hammers can also be used, but there are associated logistical problems of transport and site access.
For geochemical sampling the depth of weathering needs to be assessed in order that only the freshest, unaffected underlying material is recovered, and the outer weathered layer discarded. The thickness of the weathered layer depends upon the rock properties, and can be several mm thick. The weathered layer will typically have an altered signature resulting from oxidation and water-washing, and so does not accurately represent the true geochemical characteristics of the unit being sampled.
Consideration should be given to frequency of sampling in order to reflect adequately variations within an interval. However, sampling can be limited by how easy excavation may be, how much can be carried out of a remote location and the time available. It may be necessary to visit different outcrops of the same stratigraphic unit in order to cover the full time range represented by a unit and to examine geographical variation.
Geochemical sampling may not be the sole consideration. If any paleomagnetic study is to be done (e.g. to aid dating), then orientation of the sample should be marked on the surface in a suitable, unambiguous way (e.g. arrow pointing to magnetic N) and compass dip readings will be needed. Sufficient material should be recovered for all the planned analyses, ideally with surplus for archiving, to permit future different types of analysis or re-analysis following ambiguous results.
Where detailed micropalaeontological studies are required, a large volume of rock may be needed if target organisms are relatively sparse. In this respect, some foreknowledge of the section from other sites is useful (if available). For geochemical purposes a minimum of 50−100 g of any outcrop should be collected.
Samples should be stored in containers to minimize contamination and which facilitate adequate labelling in order to permit unambiguous identification with regard to location of outcrop and position within outcrop. For large rock fragments, contamination is unlikely to be a problem, but for more friable material, which readily breaks up into small grains, containment and contamination are important considerations. Such sample can be collected in jars (Isotech Iso-Jars), tin cans, aluminium foil (dull side inwards, to avoid rolling oils on the shiny side) or plastic bags (some surficial contamination by plasticizers may occur).
Recent sediments are most commonly sampled as part of surface geochemistry studies, and further details of sampling methods are discussed on that page.
Surface seeps typically occur as tar-pits or tar/asphalt-like layers on soil or rock surfaces. Samples should be recovered as free of extraneous material (particularly soil) as possible by the use of a suitably clean metal implement (to avoid cross-contamination). Samples may be stored in jars, cans or aluminium foil (dull side to sample); further details can be found in the surface geochemistry section.
Tar mats will have experienced weathering in their outer layers, but discarding external layers can be extremely difficult during sampling and may be best handled by sub sampling from the interior back in the laboratory.
Sub-sampling in the laboratory
Depending upon the degree of surface contamination or weathering of samples, it may be necessary to employ a sub-sampling strategy in the laboratory to minimize the effects. Where it is important that identical subsamples are used for various analyses, care should be taken to mill sufficient rock material for all projected analyses, and ideally a proprietary sample-splitter should be used to avoid any potential inhomogeneity in sub-samples arising from grain-size/density differences.