Dating the Ca CO3 fraction of sediments is often done, but it can be problematic if the origin of the carbonate in the sediment is not well understood.Carbonate in sediments can come from the natural development of carbonate nodules in the sediment (called pedogenic carbonates) or from the movement of carbonates that are dissolved out of geologic formations (like limestone, marl and other carbonate bearing minerals) which are then redeposited into the sediment.
If you want to dry your samples, heating at 90°C to 100°C for 4-24 hours is recommended.
For such sediment samples, when you date both the sediment and some plant material that is found in the sediment, the dates are usually very similar thus sometimes there is really no humic acid problem to worry about.
When sediments yield older ages than the plant (macrofossil), it is usually due to two possible reasons:(1) the plant remains were somehow intrusive (grew into the older sediment, perhaps due to erosion or long periods of low or no soil development), or(2) the sediment was getting some or all of its carbon during its formation from an older source (reworking or redeposition of already deposited sediment from upslope due to flooding, mass movements or other physical processes).
– Due to complex soil geochemistry, it may be better to date extracted macrofossils in some circumstances.
In the absence of macrofossils, radiocarbon dating can be done on sediment bulk organic fraction, humic fraction, or humin fraction.
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.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.Sending wet or frozen samples for radiocarbon dating is fine.The lab starts the analyses immediately upon arrival of the sample so moisture will not induce contamination.Many times, depending on the clay concentration and if the younger or older humic/fulvic acids are bound to clay particles, the alkali extractions may preferentially remove the original or older labile carbon and leave behind the younger clay-bound humic and fulvic acids, or vice versa.