Lab work

Panning for denticles

Like a gold miner crouched on the bank of a stream, shoveling sediment into his pan and hoping for a golden glint to catch his eye, we stood at the edge of a washing table, placing scoops of reef sediment onto our stack of sieves. However, we’re interested in something far more minuscule. Our collection of five sieves, with mesh sizes ranging from 63µm to 2mm, helps narrow the search by partitioning the sediment by particle size. Large pieces of dead coral and shell remain on the top mesh screen, while the smaller particles fall through and are caught by the finer mesh. Denticles are approximately 100µm to a little over 1mm across, so we process the 106µm-2mm size fractions to find them, saving both time and acetic acid. The very largest (>2mm) and smallest (63-106µm) fractions are stored in the lab for future use.

With two people working simultaneously, each 8-10kg sediment sample takes around two hours to sieve. After sieving, each size fraction is placed on a separate tray and deposited in our ‘drying bubble’ for a day or two until completely dry. It is then either digested with acid or stored. With one to two samples sieved per day, our mountain of bulk bags is gradually dwindling.

Phase two: return of the lab work

After several days’ wait, our precious and much-anticipated delivery of calcium carbonate cargo arrived at the lab upon clearing customs in the Panama City airport. Months of work went into preparing the agreements for these samples to pass into the country, and they were completed just in the nick of time. Now we were off and running with phase two. We also welcomed two new lab members to the team: Yamilla Samara and Henbelk Hernandez.

First, we had to unpack all of the samples from the crates and remove the plastic bags that they had previously been packaged in, allowing them to dry completely. In particular, the sediments collected underwater were still a bit soggy. Left outside or in the lab, these large, damp bulk bags could take a couple of months to dry completely without extra help. To expedite the drying process, some were moved to the ‘drying bubble’ that we had constructed in advance before leaving for the field. The drying oven that we had previously used was not nearly big enough for all of these new samples, so we decided to make our own! The ‘drying bubble’ consists of two metal bookcases, two dehumidifiers, and a fan completely enclosed by plastic, creating a hot, dry, confined space to dry samples simultaneously in large batches. It’s essentially a walk-in closet for sediment, complete with a Velcro sealed door flap. And trust me, it gets hot working in there. Now, this first batch of wet bulk bags would be dry in a week.

After each bulk bag of sediment is completely dry, it is weighed and stored in preparation for the next step: sieving. Fortunately, most of our fossil sediments were already dry, so no time was lost.