Analyzing Coral in the Lab: A Delicate, Detailed Process

Fred Andrus
Department of Geological Sciences
University of Alabama

The analysis of deep-water corals differs significantly from the analysis of other corals. This is true not only because deep-water corals grow in unique and hard-to-reach environments, but also because the average sample size is so small.  Unlike massive corals, such as those found on tropical reefs, the deep-sea corals of the Blake Plateau are often small solitary corals or small and delicate colonies. We will transport the samples we collect during the Estuary to the Abyss Expedition to the lab, where 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 (grown by organic enlargement) 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.

Profile of change over time
Climatological and ecological reconstruction research using shallow-water corals usually 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 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, deep-water corals commonly grow at less than one millimeter a year. As a result, we must make many tiny samples 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 skeleton's oldest part and moving toward the areas grown just prior to collection. This creates a profile of change over time. The process 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. The procedure causes very little damage to the coral.

Laser precision
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 (burn off) minute pieces of coral, creating tiny pits (fractions of a millimeter) in a slice of the coral. The gasses released from the ablation are then analyzed to determine what elements made up that portion of the coral. Although more destructive to the specimen, this technique produces useful data from miniscule areas, so that many small samples can be obtained from a single coral slice.

Measuring isotopes
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 (e.g., water temperature) in which the coral grew. In order to measure isotopes within a coral, we need a larger sample. Rather than taking the measurement in place, we need physically to 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. Rather than hand held, however, the drill is a computer-driven machine. We can use this tool to “whittle” away small samples (often less than one tenth of a milligram). Then we measure each sample 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 we are conducting on the ship during this expedition is just the beginning of a long process that can take years to complete.