for Sediment Samples Background
The protocols described here apply to dating of samples by the
liquid scintillation counting method. Sample sizes and treatment
steps are described for each of the types of material that are commonly
dated using this method. The analysis involves three major steps:
- pretreatment (to remove contaminants)
- chemical preparation (to
convert the sample's carbon to a datable form)
- and the actual
To determine how old a sample is.
Datable materials include:
The sample sizes listed below are the minimum needed to produce
3 ml of benzene equivalent to 2.41 gm of carbon), which is the
minimum volume of benzene required by the counter. Samples yielding
less than 3 ml of benzene can still be counted, but must be diluted
first with dead benzene to bring the total volume up to 3 ml,
which tends to increase the magnitude of the counting deviation
(the ± figure in the date).
Please note: Except in the case of charcoal,
these are minimum sizes. A larger quantity of material may be
required if the carbon content of the material is unusually
Charcoal & Wood
- Charcoal: 5-6 gm of clean charcoal (actual pieces of charcoal,
not counting any included dirt).
- Wood: 6-7 gm of hardwood or 10 gm of softwood.
- Peat: 15-20 gm if silt-free, or proportionally more if there is
a significant silt content.
- Other organic materials: Varies depending on the proportion of
carbon to other constituents.
- Shell: 20-100 gm of clean shell, depending on the degree of weathering.
In the case of badly weathered shell as much as 90% of the outer
part of the shell may have to be discarded.
- Other carbonates (including mortar): 20-30 gm if silt-free, or
proportionally more if there is a significant silt content.
- Bone: 1 kg of clean bone, or proportionally more if the bone is
badly weathered. As with shell, as much as 90% of the original
sample may have to be discarded if the bone is badly weathered.
It is desirable to date both the apatite and collagen fractions,
if possible, although it frequently is not possible to extract
enough collagen to obtain meaningful results.
- Soil: Two one-gallon ziplock bags (or the equivalent) of soil
if only a total humate date is required; twice that amount if
humin or humic acid dates also are to be run.
- Ground water: To date ground water, carbon suspended in the water
must be precipitated out as strontium carbonate by a separate
chemical process, which the Radiocarbon Lab is not equipped to
perform. 60-70 gm of clean, dry precipitate are required.
- All adhering dirt and other foreign matter is removed by washing
the sample with hot water through a USGS screen of appropriate
mesh. For charcoal samples, any large chunks are broken up with
a knife (preferred size is ca. 1 cm.); wood samples are cut into
- The dirt-free sample is placed in a 1-liter beaker, immersed
in 2% HCl, and boiled for 10-60 minutes, depending on the
size of the sample. This step removes any CaCO3 in the sample.
The sample is left in the cold HCl solution to digest overnight.
It is then washed repeatedly with distilled H2O until the
pH is 5.5 (near neutral).
- The sample is washed into a clean beaker. 2% NaOH is added
and the sample is boiled to remove possible humic acid contaminants.
The sample is again left standing overnight and then is washed
repeatedly with distilled H2O until the pH is 5.5 (near neutral).
The NaOH is decanted, and saved if the humic acid fraction
is to be dated. Otherwise, it is discarded.
- Because NaOH tends to imbibe modern and ultra-modern CO2
from the atmosphere during the removal of the humic acid fraction,
to form Na2CO3, the sample is again immersed in cold 2% HCl
to ensure the removal of atmospheric CO2. When the sample
is again freed of HCl by washing with distilled H2O, it is
dried in an oven overnight at approximately 100°C. The
dried sample is then visually inspected under low magnification
to remove any rootlets and other contaminating foreign matter.
Normally, no pretreatment is required except to break the sample
into small pieces, and remove any obvious concentrations of silt.
Soil Total Humate
However, pretreatment may be required if there is evidence of
significant carbonate deposits
- About one gallon of the sample is placed in a 5-gallon stainless
steel pot. Deionized water is added, the sample is stirred
well, and it is allowed to stand overnight to disaggregate.
- The following day, the contents of the pot are stirred well
a second time and any floating particles are skimmed off with
a 60-mesh or more appropriate size USGS screen. The stirring
and skimming is repeated until no floating material remains.
- The material remaining in the pot is passed through a 230-mesh
sieve into a second 5-gallon pot, stirring the original pot
and adding deionized water as needed to keep fine material
in suspension. Two full pots will result from this transferring
and sieving. The water and sediment remaining in the bottom
of the original pot is discarded.
- If any floating material is observed in the new pots, it
is skimmed off with a 230-mesh or finer sieve. The sample
is allowed to stand until settling has occurred, or centrifuged
in several 1-liter batches to separate out the solids.
- The pots are decanted, and the sediment is flushed into
3-liter beakers using distilled water.
- Steps 1 through 5 are repeated for each gallon of the sample
until all have been processed.
- A small amount (1-2 ml) of concentrated HCl is added to
one of the beakers to check for carbonates. If the material
is not reactive, it is acidified to a pH of about 5. If there
is a reaction, HCl is added about 5 ml at a time, alternating
with distilled water, until the reaction appears to have stopped.
The acidified sample is allowed to stand overnight to ensure
neutralization of any dolomite that may be present.
- The sample is stirred, transferred to wide-mouth, 1-liter
centrifuge bottles, centrifuged, and decanted, discarding
the supernatant. Distilled water is added, and the sample
is agitated and then centrifuged; this is repeated several
times to rinse the sediment. During this process the sample
is kept at a pH of about 5 by the addition of a few drops
of HCl as needed.
- The sediment is transferred into clean beakers, and allowed
to stand until all solids have settled out. The supernatant
is siphoned off, and the solid residue is dried in an oven
at approximately 100°C.
- The solid residue is pulverized in a mill. The resulting
powder is the total humate fraction, which can be dated as
is, or given additional processing to separate out the humic
acid and humin fractions.