Shell & Carbonates
Shell & Carbonates
Shell and Pure Carbonates
Preferred submission size: >40 mg
Minimum submission size: 20 mg
Carbonate samples undergo a chemical pretreatment protocol to remove secondary calcite on the surface or between nacreous layers that may alter the apparent age. Standard leaching removes 25-30% of the sample mass to ensure that only carbonate produced by the organism remains.
Sample Selection & Submission Recommendations
Choosing the most appropriate sample for submission is vital to the success of analysis.
- Isolate carbonates from sediment.
- Rinse and/or scrape away adhered sand or soil.
- Dry completely.
- Select the best-preserved specimen or fragments.
- If portioning prior to submission is required, sample from the thickest part of the shell: the columella (inner spiral) in gastropod, the umbo and area near the hinge in bivalves.
- If the submission contains more than one shell, specify preference for dating a single shell or a fragment from multiple individuals.
- Package and ship samples according to our general recommendations.
Pretreated Shell, Carbonates, & Foraminifera ("Forams")
Preferred submission size: >20 mg
Minimum submission size: 12 mg
"Pretreated" shell has been abraded or chemically leached prior to submission to DirectAMS, and can be submitted as whole, individual specimens, multiple fragments, or crushed powder. Please provide a brief summary of the protocol applied in the Comments section on the Submission Form.
For some samples, pretreatment is either too risky or unnecessary. These are considered "pretreated" as is, and priced accordingly. Examples include small carbonates, such as forams and fish otoliths, calcium carbonate sediments, such as caliche and speleothems, and CO2 adsorbents, such as Sodasorb® and hydroxides.
Submit samples according to our general recommendations.
Dating and Interpretation
The shell carbonate of terrestrial mollusks derives from respired carbon dioxide and will reflect isotopic ratios of contemporaneous plants. Marine and aquatic mollusks, however, also incorporate dissolved inorganic carbon (DIC) through fluid exchange with the environment. The ocean is a reservoir of organic and inorganic carbon, with regional variability derived from currents, upwelling events, terrestrial inputs, volcanic activity, etc. 14C dates on marine shell and terrestrial organic matter of the same age can differ by hundreds of years, as can 14C dates of shell from different oceans. Differences can vary over time as current patterns change. Reservoir effects have been well-characterized for many regions, allowing for calibration of radiocarbon dates; shells from less-well-studied areas will yield dates with less certainty. Information about reservoir effects at specific sites can be found here.
Interpreting the age of fresh water shell can be problematic because DIC contribution is difficult to estimate. Even when the geology of the local water catchment is well understood (e.g. relative abundance of limestone), rivers and streams may bring DIC from hundreds of kilometers away, and these contributions will vary continuously with long-term changes in climate. If a site yields both freshwater shell and organic matter, the organic matter will provide a more reliable date.
Some shell-derived artifacts (e.g. jewelry) are retained for decades before being discarded. If such a determination is possible, consider the longevity of the associated artifact when selecting material to date.