Don't panic! We can correct for organics.

My lagoon sediments are rife with living creatures.  Bacteria, protists, algae, and sea grass live on top of, or down in, the sediments.  At the bottom of my carbonate sediment cores, the lagoon muds turn into organic rich peat left over from a vegetated area that flooded with seawater and drown.  If left over tens of millions of years, compressed and gently heated, this peat layer would turn into a coal seam.

At present, they are only about 5,000 years old and could cause some problems in interpreting the uranium/thorium ages I have spent the summer working on.  Carbon-rich sediments act like a sponge - they are good at absorbing uranium out of seawater and holding on to it.  If they begin with excess uranium, they will appear to have less thorium which occurs after uranium decays.  The result is an artificially young age.

Problems like this arise all the time in earth science because nature is typically not a closed system.  Different cycles and processes interact with one another and result in a signal that is smeared.  It is for this reason many experiments and data sets require long periods of observation or replicates (or triplicates, etc.) with the idea that more data will give a consistent average value which is the closest "true" value of a process.

Anyway, I digress.  In the case of my muds, I need to know how much organic content exists so I can correct for any absorption of excess uranium they might cause.  To do that, I essentially set them on fire.  First I way out a few dozen milligrams of my carbonate material and place them in glass wells.  I also weigh some standards (material we know the organic content of very well and compare it to the unknown samples).

My sample wells.

A close up of samples.  The dark samples are organic-rich carbonate mud.  The orange is a plankton standard.

I add hydrochloric acid to each of the wells to dissolve away the carbonate (the top left plate is already done in the image above).  Carbonate has carbon (it's in the name) in the form of CaCO3.  If I were to measure the pure sediments, I would get an overwhelming signal of carbonate.  By adding acid, the carbonate reacts to produce water and carbon dioxide gas which fizzes out of my sample.  This is good because now the carbonate carbon has left in the form of a gas, and the residual material is mostly unaffected by the acid.  I can dry the sample, collect my organic carbon residue, and stuff it into little tin capsules.  I compress these into little discs, and they are ready for measurement.

Tin capsules, a holding cell, and a stylus I use to compress them.

Once compressed, I can open the hold cell to get the disc.

Here is a before and after of a compressed tin capsule.

Now the samples are held in a container until it is time to ignite them.

By incinerating the tin capsule and measuring the carbon coming off of it, I can determine how much organic carbon was there compared the original mass of my mud.  This will give me a percentage (say, 2 or 3%).  I would use this value to then correct any oddities I see in my uranium data to help reconstruct the age of my sediment cores.  This extra step gives me more confidence that my ages are correct, and then I can move on to reconstructing the lagoonal environments that existed at those points in time in the past.