Corals in the Lab
Fred Andrus
Department of Geological Sciences
University of Alabama
Analysis of deep-water corals is significantly different from analysis of other corals, not only because deep-water corals grow in unique and hard to reach environments, but also because the average size of the samples is so small. Unlike massive corals such as those found on tropical reefs, deep-sea corals of the Blake Plateau are often small solitary corals or small and delicate colonies. When the samples collected during the Estuaries to the Abyss expedition are returned to the laboratory, our analyses will include determining the elemental and isotopic chemistry of the coral skeleton. Unlike vertebrates, which generally dissolve and replace their skeleton as they grow, corals add new material on top of the old, thus preserving a record of its developmental history. This material is composed of calcium carbonate in the form of aragonite.. The chemistry within this accretionary skeleton is often a reflection of the coral's environment, and therefore an old coral contains a record of changes in the conditions around it for many years as it grew.
Most climatological and ecological reconstruction research using shallow-water corals focuses on massive colonial species that are relatively fast-growing and may live for many decades. In contrast, most deep-water corals appear to be slower growing and are often much smaller, whether they are colonial or solitary. To measure detailed chemical variation in the life history of a coral, including changes that occurred at monthly or even shorter time intervals, we must make many measurements within a relatively small area of the coral skeleton. This is difficult to do with the smaller deep-water corals. Whereas shallow corals may grow several millimeters a year, it is common for deep-water corals to grow at less than one millimeter a year. This means that the many tiny samples are required in these delicate skeletons to study time-related effects and characteristics.
We perform these analyses by isolating a small portion of each coral and measuring the chemistry within just a small region. Many samples can be measured in a line starting at the oldest part of the skeleton and moving towards the areas grown just prior to collection of the specimen. This creates a profile of change over time. This may entail the use of in situ (in place) techniques where an intact slice of the coral is placed in the path of a beam of electrons. The way these electrons interact with the sample surface indicates what major elements are present, creating a "map" of the chemistry of the coral. Very little damage is done to the coral sample during this procedure.
If we need to identify and measure the amounts of rarer chemical elements, we may have to sacrifice more of the coral sample using a different technique. A narrow laser beam can be used to ablate or burn off minute pieces of coral, creating small pits (fractions of a millimeter) in a slice of the coral. The gasses released from the ablation of the material are then analyzed to determine what elements made up that portion of the coral. Although more destructive to the specimen, this technique produces very useful data from areas that are very small, so that many small samples can be obtained from a single coral slice.
In order to measure isotopes within a coral, a larger sample is needed. Isotopes are atoms of an element that contain different numbers of neutrons. The relative amounts of different isotopes of the same element can tell us something about the environment that the coral grew in (for example, water temperature). Rather than taking the measurement in place, we need to physically remove a portion of the sample and then analyze it later by one of several techniques. Our approach has been to mill out samples using a drill, similar to that which a dentist uses, but rather than being held by hand, a computer-driven machine is used. We can use this tool to “whittle” away small samples (often less than one tenth of a milligram). Each is then measured for isotopes.
Deep-water coral analysis requires special tools and techniques, from the complex task of collecting the samples with a research submersible or Remotely Operated Vehicle to the detailed and delicate sampling required to measure the chemistry of their small and fragile skeletons. The work that is conducted on the ship during the expedition is just the beginning of a long process that can take years to complete.
