Doctoral candidate Beth Caissie tracks climate patterns from millennia past
Beth Caissie, a doctoral candidate in Geosciences, spent most of her summer on the Bering Sea between Alaska and Siberia as member of an international team of scientists researching historical weather patterns. A sedimentologist, she is one of eight on the expedition analyzing core samples from deep beneath the ocean. The samples are made up of layer upon layer of material accreted during succeeding geological epochs.
Caissie and her colleagues set off July 5 for a two-month mission on the drill ship JOIDES Resolution (the acronym stands for “Joint Oceanographic Institutions for Deep Earth Sampling”). In a July 17 blog entry, Caissie noted the ship’s arrival at the Umnak Plateau, one of several passes between the Aleutian Islands and long the focus of her research. She said that she and her fellow sedimentologists were braced for an exhausting few weeks: “We’ve practiced all the analyses that we will be performing on every core…. [T]his is the reason that we’re all here—to read the history of ocean currents and climate in the cores that will soon rise from the sea floor to fill the lab.”
After receiving the samples as long cylinders extracted from the cores of the drills that dug them, the scientists analyze each sediment layer for various indicators that enable them to build a climate timeline going back hundreds of thousands of years. These indicators include oxygen, carbon, calcium carbonate concentrations in the shells of species such as foraminifera, and Caissie’s specialty: diatoms, tiny algae that after dying sink to the sea floor.
Different diatom species dominate in a given sediment layer depending on whether the climate was warmer, with more open water, or colder, with more sea ice. For example, if 40 percent of all the diatoms in a sediment layer are sea-ice diatoms, it suggests the area from which the sample was taken was covered with ice for as much as eight months of the year. If only five percent of a layer’s diatoms were of the open-water type, open water likely dominated the period when they sank.
“I describe what comes up in the sediment cores and make slides for future study,” Caissie explains. “I’m particularly interested in a couple of different times over the last 400,000 years when the climate shifted from glacial to inter-glacial—that is, it warmed. I’d like to document the whole process from before each event until after each one.”