Slicing, Washing, Chemically Analyzing, Hundreds of Times

Yair Rosenthal and Sindia Sosdian based their work on the study of the fossilized calcareous shells of foraminifera – tiny marine animals that live on and in the sea floor, and whose shells are buried in the bottom sediment. The deeper in the mud the shells are buried, the older they are. Because the shells' content of magnesium and calcium varies with temperature, scientists can trace the temperature changes over time by analyzing their chemical composition.

Their samples were taken in July 1983 by scientists on a ship, the Glomar Explorer, belonging to what is now the Integrated Ocean Drilling Program, an international scientific effort that yearly sends specialized drilling ships to collect sediment samples from the ocean bottom.

"On the coring cruises, when they collect a sediment core, it is recovered in something like a pipe," Sosdian said. "Once they have it aboard, they cut it in half and archive one half and then keep the other half for working projects. A core is normally cut into 1.5-meter sections, sealed, labeled, and then brought into the core laboratory for processing. When they sample it they have to remove the plastic wrap covering the exposed sediment."

Sediment samples are stored in one of several core repositories, or libraries, one of which was located until recently at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York. The samples Rosenthal and Sosdian used were taken from the Atlantic Ocean bottom, about 130 miles northwest of Corvo, the northernmost island in the Azores, where the water is 3,427 meters deep. "I put in a sample request with the core repository, and they mailed me the mud," Sosdian said. "The mud" came in small cakes, 5 centimeters thick.

"You slice it every 5 centimeters," Rosenthal said. "You wash the mud away and you're left with a sample that includes the fossils. You work with a microscope and a tiny paintbrush. Then, when you have the samples, you put them through some chemical preparation, measure them with the mass spectrometer, and you get your results."

"I first washed the mud, meaning I put the sediment in a sieve, and washed it with water and collected the microfossils," Sosdian said.

With the mud washed away, the tiny shells looked a bit like sand or salt to the naked eye. Under a microscope, however, they reveal themselves as shells. Sosdian put those fossils in a mass spectrometer, which helped her determine the chemical composition of each sample – the amount of magnesium and calcium. She did this hundreds of times in Rosenthal's New Brunswick lab.

"To speed the process up and, since Rutgers was so close, I went to the Lamont-Doherty core depository myself," she said. "I would go into this huge refrigerated storage area where they stored Atlantic cores from all different cruises. I would select which core and section I wanted to sample and then get to sampling."