$279 Standard, $349 Expedited
Bulk carbon: Preferred: 20-50 g, Minimum: 2 g, depending on organic content
$349 Standard, $436 Expedited
Humin OR humate: Preferred: 20-50 g, Minimum: 2 g, depending on organic content
Sediments vary wildly in composition, consisting of inorganic particles and carbonaceous material in differing proportions. Datable carbon from sediment falls into five categories:
Macrofossils: Preserved organic remains (e.g. charcoal, plant, shell), visible without a microscope. Macrofossils undergo the chemical pretreatment protocol appropriate to the extracted material.
Humates: Colloidal organic acids, such as tannic or fulvic acid, adsorbed to surfaces in the soil matrix. Humates are soluble in basic solutions and insoluble in neutral and acid solutions. Pretreatment involves rinsing them from the soil matrix with a weak base solution, the precipitating them by addition of acid.
Humins: High-molecular-weight organic compounds formed over time by polymerization of organic compounds in the soil. Humins are insoluble in basic and acid solutions; pretreatment involves an acid-base-acid procedure to remove carbonates as well as adsorbed organics (humates).
Bulk Carbon: The total organic content of a sediment, including humates, humins, and fine particles cleaved from macrofossils (if present). Bulk Sediments undergo an acid pretreatment to remove potentially contaminating carbonates.
Carbonates: Calcium carbonate, usually inorganic. Dating carbonates is useful in very limited contexts.
Fraction Selection Recommendation
Macrofossils provide the most accurate radiocarbon date if they were entrained in the sediment as it formed. However, it can be difficult to distinguish an original deposition from intrusive materials (rootlets, insect parts, seed caches) that are younger than the given horizon. Of the myriad macrofossil types, charcoal is preferred because it is not likely to be confused for another, potentially younger material. Moreover, charcoal is largely ignored by the biota, and is therefore less likely to be affected by insect bioturbation.
In the absence of macrofossils, Bulk Carbon dating is the most common approach. Bulk Carbon dating is most appropriate for well-buried, capped, undisturbed sediments (e.g., peaty sediment), and sediments with lower carbon content. A Bulk Carbon date will be the weighted average of the ages of the many different carbonaceous materials in the sediment (except carbonates). This can complicate interpretation of the age.
Dating the Humin or Humate fraction may be preferred in some contexts, but site-specific variables make it difficult to offer general guidelines. Consult available literature for the given site, context, or application to determine if humin and/or humate dating is appropriate.
Sample Selection & Submission Recommendations
Choosing the most appropriate fraction for submission is vital to the success of analysis.
- Isolate macrofossils from sediment. Follow the guidelines for the appropriate macrofossil type and submit for Organic or Carbonate Service.
- If no macrofossils are present, or they cannot be extracted, dry the sediment.
- Select an adequate representative portion of sediment. Dark clays, loams, and silts have abundant organic carbon, requiring a smaller submission. Light-colored or sandy sediment contains less organic carbon, requiring a larger submission.
- Package sediment samples in individually labelled, sealed plastic bags (e.t. Ziploc or Whirl-Pak), or conical centrifuge tube.
- Remove as much air as possible from the inner bag or seal the tube tightly, and place in a secondary sealable bag to avoid cross-contamination of samples.
- Refer to general packaging and shipping instructions.