May 3, 03

It's not Dirt!
by Dr. Leslie Sautter, Associate Professor of Geology, College of Charleston, Charleston, SC

Along with the ROV investigations of the seafloor habitat, we are also quite engaged with sampling of the seafloor sediments. We are using an instrument called a Smith-MacIntyre sediment grab sampler (on loan by the SC-Dept. of Natural Resources) to take as many as 40 small scoops of the ocean's floor (Fig. 1) in and around the Oculina Reef. This instrument is large and bulky, and is deployed using a crane. Before sending it overboard, the two scoop-like "jaws" are set in the open position. The Smith-Mac is then lowered to the bottom and is allowed to "free fall" the last several meters, so that it hits the seafloor hard enough to release the device that holds open the jaws. In sandy sediments, the jaws can collect a sizeable amount of material - perhaps as much as 2-4 liters.

Once on deck, the analysis begins with a photograph of the sediment while it is still in the sampler
(Fig. 2), and the top 10 cm are scraped off with a specialized sediment scoop (an orange plastic beach shovel!) and placed in a container for analysis. This top10 cm will be analyzed by Dr. Kathy Scanlon of the U.S. Geological Survey in Woods Hole. Kathy was unable to join us on the cruise, but will conduct her study of the sediment composition and its "roughness." She is investigating how smooth or rough the seafloor is, for use in understanding the types of acoustic signals recovered during mapping operations. She will also analyze the grain size distribution and composition.

After this initial collection, the remaining sample is removed from the Smith-Mac. Shipboard geoscientists then describe the appearance of the sediment in terms of its color, homogeneity (i.e., is it all the same or does it have a range of components?) and consistency. We make note of relative amounts of the different "size-fractions," such as the clay + silt size fraction (more commonly known as "mud"), the sand-sized grains, and the coarse material. These sediment characteristics, along with other important site information such as the water depth, latitude/longitude, date and time of day are carefully recorded with a sample identification code in a notebook.

The sediments will also be used for other studies related to the benthic (i.e., bottom-dwelling) organisms. Sub-samples of the sediments are soaked in a solution of ethanol and Rose Bengal that stains and preserves the living protoplasm a bright magenta color. This method makes identification of the living populations of "critters" much easier. These stained samples will be studied by Dr. Murray Roberts of the Scottish Association for Marine Science to understand the benthic community of organisms associated with the different seafloor habitats associated with the Oculina Reef. Some of the reef areas have abundant dead Oculina "rubble," while other areas are rubble-free and are mostly composed of sands and broken "shell hash." A few pieces of Oculina rubble have been recovered so far. One piece was teaming with a wide variety of tiny invertebrates attached to the coral branches, as well as a couple of brittle stars (Fig. 3)!

Dr. Leslie Sautter, one of the shipboard scientists, will be studying the living populations of benthic foraminifera. Foraminifera are tiny (usually less than 0.5 mm) single-celled organisms in the Kingdom Protista. They produce an enormous array of chambered shells, or "tests," and there are thousands of species living today. Leslie will be assessing the communities of these organisms throughout the reef to determine which groups and species are indicators of certain habitat types.

When all the samples are recorded and stored in labeled containers, the next Smith-Mac sample is on-deck and ready for attention! Teamwork is very important to accomplish the sampling rocedure in an efficient manner. Once in a while there's a little time to explore the sediment's composition by washing it through a screen, or sieve, to reveal the abundance of shells and small organisms living within (Fig. 4). In other words, it's fun to play in the mud and sand (it's not dirt!*) once in a while!

After the day's sampling is complete, there may be some time to examine the material in the lab, under a microscope (Fig. 5). This is when the excitement begins, as we see the microscopic habitat and its components. There is great beauty in some of the small shells and mineral grains - beauty that is certainly difficult to appreciate when viewing it with the naked eye! Images of the fossils, living organisms and mineral grains will soon be available on the oceanica web site! We hope to learn a lot about the abundance and diversity of organisms living within these seafloor habitats, so that we can establish a "baseline" of what occurs today, for comparison in the future. These organisms will serve as some of the many indicators of environmental change over the years, as the Oculina Coral Banks are studied and monitored.

*Some of the non-geology shipboard scientists have recently learned that the sediment we are recovering is quite different from "dirt," which relates to soil and is generally a terrestrial type of sediment - often rich with organic material. It's not dirt! Much of the seafloor sediment we've recovered isn't even dirty, but instead is very clean sand and rubble!