Radiometric and radiocarbon dating
Most macrofossils can be treated with (1) acid to remove carbonates, and (2) alkali to remove humic acids that might be in the sediment. Sediment and rain water or ground water can move these humic acids up or down through the sedimentary profile bringing carbon that is either younger or older into a sediment layer.
The source of this carbon can be from more recent humic and fulvic acids or ancient labile carbon that is alkali-soluble.Please remove excess water, wrap the samples with plastic (e.g.Saran Wrap) to limit air contact, place them in a Ziplock bag, and ship to us.In general the plant dates are usually more reliable as they typically represent a more unique event in time.The plants were relatively short-lived as compared to the time it may have taken for the sediment to form.If macrofossils are found, the lab will inform the researchers and they can decide if they want to date the macrofossils or sediment.
In some cases, it is wise to date both separately to see if the macrofossils or sediment is yielding the best (most reliable) date.
This can be useful information if a large core or sequence of cores are being dated.
Sediments that are treated with alkali to extract humic acids (and then dated on the humic acid or humin fraction) can sometimes yield problematic results depending on the localized soil geochemistry.
For sediment samples, the lab performs flotation in water then progressive sieving through 225-micron, 180-micron, and then 125-micron sieves to see if there are any macrofossils that can be extracted.
Examples of these macrofossils include charcoal, wood, plant, bone, shell, and seeds.
– Due to complex soil geochemistry, it may be better to date extracted macrofossils in some circumstances.