Selection Criteria

Because the dating event is the last exposure to light, the most important consideration in selecting sediment samples for dating is the likelihood that the sample was exposed to sufficient light during deposition and prior to burial.  Sufficient light may be as little as a few minutes of direct sunlight.  Aeolian deposits have a high likelihood of long exposure because of airborne time.  Bleaching of fluvial deposits depends on the sediment load and the turbidity of the flow, deposits from meandering streams tending to be much better bleached than glacial outwash.  Bleaching of colluvial deposits depends on the nature of the deposition and the grain size of the sediments.  Slopewash deposits are often well-bleached but deposits from mass transport events such as landslides are not likely to be.  Buried soils have been successfully buried because of cycling of A-horizon sediments to the surface by pedoturbation.  Bleaching is affected by shade conditions and cloud cover.

                Any kind of crystalline material is potentially datable but most research has been conducted on quartz and feldspar, both ubiquitous materials.  Quartz has a more stable signal and its luminescence properties better understood, but the signal can be weak and early saturation prevents dating much beyond 100,000 years or so.  Feldspars have stronger signals and a much longer age range, but suffer from anomalous fading, an instability in the signal that can cause age underestimates if not detected and/or corrected.  Use of other material will require basic research and thus entail higher costs.  Analysis is usually done on coarse grains (90-200µm) or fine grains (4-11µm), but other sizes have been used as well.

                Where sediments down a profile are being selected, it can be useful to have one sample from the surface that represents a “zero-age” sample that can be used to test for extent of bleaching.

Sample Size

Any single sample should be a minimum of 250g, but larger samples are desirable.  If the same sample is to be used for radioactivity measurements, at least 350g is needed.  More optimal size is 500 to 1000g.   In special circumstances very small samples may be sufficient.  The vertical extent of the sample will affect resolution depending on the rate of deposition.  For example, a sample drawn from a 10cm profile section that accumulated in 1000 years will not be able to resolve at a scale less than 1000 years.  The number of samples depends on the specific problem to be addressed and financial resources, but more than one sample from any given context will certainly increase precision.

Collection Procedure

Exposure of the sample to light must be avoided at all costs to avoid an underestimation of the age.  This means the sample must be collected under conditions that minimize light and must be stored in opaque containers.  There are several solutions to the collection problem.  Perhaps most straightforward is to collect the samples at night, preferably with no moon, aided by red/orange filtered flashlights.  Red to orange light causes insignificant reduction to the luminescence signal.  Exposure to within 1 cm of the sample can be done in daylight; then under night light the overlying cm is scraped off and the sample excavated.  A common daylight solution is to drive an opaque pipe (plastic or stainless steel) into the sediment and cap both ends.  The ends, which will have been exposed, can be removed off in the laboratory.  It is important to make sure the sediment is secure, not loose.  Otherwise mixing of the exposed ends with the unexposed center will occur during transport.  If the sample does not fill the entire pipe, some kind of stopper is needed to keep the sediment from moving around.  In capping the ends or stopping, do no use a material that will oxidize or decompose when in contact with the sample (often damp) and thus contaminate it.   Aluminum foil, for example, will oxidize when in contact with wet soil for long periods of time.   In sediments that are too hard to drive a pipe in, a block of sediment can be removed as long as it keeps its integrity.  The outer surfaces are then removed in the laboratory.  Samples can also be collected by coring.  Various split-core samplers with light tight sample containers are available.  In general, the procedure is to take one core that will be exposed to light in order to define the stratigraphy and the depth of the desired sample.  Then core down to just above where the sample will be collected and attach the split core sampler to retrieve the sample.

Radioactivity
Where the sample comes from a very homogeneous deposit, the sample itself can be used to measure radioactivity.  Where the environment is more complex, considerations similar to those for heated materials must be taken into account.  In a sediment profile where movement  through time of radionuclides, either up or down, is suspected,  several 70-100g samples down the profile will be useful to
model the distribution of radioactivity.  No special precautions are needed for these “radioactivity” samples, except to avoid altering their composition.

Other information
For context considerations, treatments, moisture contents, data for cosmic ray dose, drawings, and age estimations, the same procedures as for heated materials should be followed.  Moisture contents are somewhat more critical for sediments since they have a larger influence on the dose rate than for pottery or lithics.  It is often useful to obtain a measure of current moisture contents.  All that is needed is to collect a 70-100 g sample of sediment from the back of the sampling hole (sediment close to the profile surface will have dried) and put it in an airtight container.  Triple bagged zip-lock plastic bags are usually sufficient.

on Criteria

Because the dating event is the last exposure to light, the most important consideration in selecting sediment samples for dating is the likelihood that the sample was exposed to sufficient light during deposition and prior to burial.  Sufficient light may be as little as a few minutes of direct sunlight.  Aeolian deposits have a high likelihood of long exposure because of airborne time.  Bleaching of fluvial deposits depends on the sediment load and the turbidity of the flow, deposits from meandering streams tending to be much better bleached than glacial outwash.  Bleaching of colluvial deposits depends on the nature of the deposition and the grain size of the sediments.  Slopewash deposits are often well-bleached but deposits from mass transport events such as landslides are not likely to be.  Buried soils have been successfully buried because of cycling of A-horizon sediments to the surface by pedoturbation.  Bleaching is affected by shade conditions and cloud cover.

                Any kind of crystalline material is potentially datable but most research has been conducted on quartz and feldspar, both ubiquitous materials.  Quartz has a more stable signal and its luminescence properties better understood, but the signal can be weak and early saturation prevents dating much beyond 100,000 years or so.  Feldspars have stronger signals and a much longer age range, but suffer from anomalous fading, an instability in the signal that can cause age underestimates if not detected and/or corrected.  Use of other material will require basic research and thus entail higher costs.  Analysis is usually done on coarse grains (90-200µm) or fine grains (4-11µm), but other sizes have been used as well.

                Where sediments down a profile are being selected, it can be useful to have one sample from the surface that represents a “zero-age” sample that can be used to test for extent of bleaching.

Sample Size

Any single sample should be a minimum of 250g, but larger samples are desirable.  If the same sample is to be used for radioactivity measurements, at least 350g is needed.  More optimal size is 500 to 1000g.   In special circumstances very small samples may be sufficient.  The vertical extent of the sample will affect resolution depending on the rate of deposition.  For example, a sample drawn from a 10cm profile section that accumulated in 1000 years will not be able to resolve at a scale less than 1000 years.  The number of samples depends on the specific problem to be addressed and financial resources, but more than one sample from any given context will certainly increase precision.

Collection Procedure

Exposure of the sample to light must be avoided at all costs to avoid an underestimation of the age.  This means the sample must be collected under conditions that minimize light and must be stored in opaque containers.  There are several solutions to the collection problem.  Perhaps most straightforward is to collect the samples at night, preferably with no moon, aided by red/orange filtered flashlights.  Red to orange light causes insignificant reduction to the luminescence signal.  Exposure to within 1 cm of the sample can be done in daylight; then under night light the overlying cm is scraped off and the sample excavated.  A common daylight solution is to drive an opaque pipe (plastic or stainless steel) into the sediment and cap both ends.  The ends, which will have been exposed, can be removed off in the laboratory.  It is important to make sure the sediment is secure, not loose.  Otherwise mixing of the exposed ends with the unexposed center will occur during transport.  If the sample does not fill the entire pipe, some kind of stopper is needed to keep the sediment from moving around.  In capping the ends or stopping, do no use a material that will oxidize or decompose when in contact with the sample (often damp) and thus contaminate it.   Aluminum foil, for example, will oxidize when in contact with wet soil for long periods of time.   In sediments that are too hard to drive a pipe in, a block of sediment can be removed as long as it keeps its integrity.  The outer surfaces are then removed in the laboratory.  Samples can also be collected by coring.  Various split-core samplers with light tight sample containers are available.  In general, the procedure is to take one core that will be exposed to light in order to define the stratigraphy and the depth of the desired sample.  Then core down to just above where the sample will be collected and attach the split core sampler to retrieve the sample.

Radioactivity
Where the sample comes from a very homogeneous deposit, the sample itself can be used to measure radioactivity.  Where the environment is more complex, considerations similar to those for heated materials must be taken into account.  In a sediment profile where movement  through time of radionuclides, either up or down, is suspected,  several 70-100g samples down the profile will be useful to
model the distribution of radioactivity.  No special precautions are needed for these “radioactivity” samples, except to avoid altering their composition.

Other information
For context considerations, treatments, moisture contents, data for cosmic ray dose, drawings, and age estimations, the same procedures as for heated materials should be followed.  Moisture contents are somewhat more critical for sediments since they have a larger influence on the dose rate than for pottery or lithics.  It is often useful to obtain a measure of current moisture contents.  All that is needed is to collect a 70-100 g sample of sediment from the back of the sampling hole (sediment close to the profile surface will have dried) and put it in an airtight container.  Triple bagged zip-lock plastic bags are usually sufficient.