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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.
In most cases, humic acids travel downward and make the underlying sediment appear to be younger (sometimes by a small amount, sometimes by a large amount of bias).
– 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.
This is mostly true for sediments that are organic rich (dark black or brown in coloration) as well as for sediments that are not well drained and where water ponds (like swamps, peat bogs, etc).
In areas where there is not much rain fall or the sediment is well drained or is low in carbon (light tan or gray sediments), humic acids may not be much of a problem.
– Sediments are complex systems containing carbon of multiple forms, sizes ranges and sources.
Maniatis, Yannis Facorellis, Yorgos Soter, Steven Katsonopoulou, Dora and Kromer, Bernd 1995. To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies.
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).
Radiocarbon Analyses from Cincinnati, Ohio, and Their Implications for Glacial Stratigraphic Interpretations.
Controls on the transport and carbon isotopic composition of dissolved organic carbon in a shallow groundwater system, Central Ontario, Canada.
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.