While sieving the bulk bags of sediment allows us to constrain the particle sizes that we eventually pick for denticles, we are still left with around 260kg of sediment to work with. This is an enormous task, perhaps impossible. However, with acetic acid, we can make it feasible. Our samples are largely composed of coral and shell (calcium carbonate). These react with acetic acid to produce water, CO2, and calcium acetate, therefore eliminating the carbonate. This reaction can remove up to 99% of the sample weight, leaving behind just pieces of rock and resistant microfossils such as bone, teeth, denticles, and sponge spicules that we are interested in studying.
Our samples are so large that we’ve built a processing ‘factory’ outside the lab, where the sediment can react with acid and bubble off CO2 without ventilation issues or space limitations. Each size fraction is placed in a five gallon bucket, and acid rinses (10% acetic acid) are added daily over the course of about a week and a half. A little over 300L of 100% acetic acid is required to complete the digestion of the 65 bulk bags we collected in the Dominican Republic. That’s a lot of acid! We can also process 39 size fractions, or 13 bulk bags, simultaneously, which is 11 bulk bags more than what I was able to digest in the past using just the fume hood in the lab. This has cut the total digestion time down by months.
While this step requires a large quantity of acid to complete, it is crucial, as it turns the task of finding a needle in a haystack into the task of finding a needle in a small sprinkling of hay. Denticles are sparse in the sediment samples, but digesting away the calcium carbonate makes it possible for us to find them and reveal their secrets.