22 June, 2000
22 June 2000
The Story In The Sediments
Coring operations continue today. This morning they are doing one more 60-foot core and this afternoon they will set up an 80-foot core. With all this emphasis on coring, you may wonder what cores are for? With the help of Dr. Garry Brass, Executive Director of the United States Arctic Research Commission (USARC), I’ll discuss the answer to that question in today’s journal. First, a little bit about Garry and the role of his Commission. USARC was established by the Arctic Research Policy Act of 1984 and its seven commissioners are appointed by the President. Composed of four researchers, two businessmen, and one Alaska Native, the USARC establishes policy for arctic research for the federal government. The same act that established the USARC also established the Interagency Arctic Research Policy Committee (IARPC), an organization of twelve federal agencies and offices that have some role in arctic research. According to Garry, “We make policy and they (IARPIC) carry it out”.
Dr. Brass’s training is in Geology, with a B.S. in Earth Science from UCSD and a M.S. and PhD in Geology and Geophysics from Yale. After an early stint at Florida State, he spent 17 years at University of Miami’s Rosenstiel School of Marine and Atmospheric Sciences. The University loaned him to NSF to serve as the Ocean Drilling Program Director from 1984-86, and in 1994 Dr. Brass left Miami to work for USARC. He provided an excellent description of “what we do with a core” to help me understand how sediment samples are used. I would like to quote his summary, because it very eloquently describes the value of collecting this type of data.
“What in fact do we get out of these cores studies? Cores are the tape recorders of earth history. They are, for the most part, continuous records of what is accumulated in the ocean basins as a result of surface conditions on the planet. They record erosion rates, mountain building, volcanic activity, the evolution of marine organisms and climate. Cores are the Earth’s history book.”
However, this parcticular history book is written in several different “languages”. In order to understand the story, each “language” must be translated. This is accomplished by taking samples for three principal kinds of analysis once the core has been split and recorded (see Todd’s upcoming journal entry for this process).
One type of analysis measures the language found in the mineralogical and sedimentological composition of the sediments. In other words, what type of rock was ground up that formed these sediments and how much it was ground up before it got to its location in the sea floor. Researchers can determine grain size (how much it was ground up) with a microscope or through X-ray diffraction. Determining what was ground up requires X-ray fluorescence spectroscopy. This process involves subjecting the sediments to X-rays and seeing what X-rays they then emit. Each mineral has its own emission “fingerprint” so the scientist studying the sample can tell what is contained in each of the samples he has taken from the core. This often gives them clues of where the sediments came from in the first place. The second type of sample involves “separation and identification of the marine organisms preserved in the sediments”. Known as micro-fossils, the skeletons of the prehistoric phytoplankton and zooplankton make up another language of the story in the sediments. Different layers of these fossils represent different timelines. The mapping of these layers is known as biostratigraphy - “bio” meaning life, “strata” meaning layer, and “graphy” basically meaning picture. Scientists have determined when various types of the ancient plankton were present in the oceans and can now use them as a measuring device for the age of the layers in the sediments collected by coring.
The third language is actually contained in the shells of one of the types of microfossils - foraminiferans. These one-celled animal-like protists are a bit like amoebas with shells of calcium carbonate. When they die, their shells become part of the sediments of the sea floor. The ratio of the two oxygen isotopes 16O and 18O found in these shells is a function of the temperature of the ocean when the shells were formed. The information provided by this isotope language “is the basis for paleoclimate reconstructions which have allowed us to learn about the timing and intensity of past climate changes, especially the ice ages”. A similar language is trapped in the ice sheets of Greenland and Antarctica in the isotopes in the water that formed the ice, but their story only goes back about 150,000 years. The story held in the foramineferan sediments goes back as much as 100,000,000 years!
So, once these languages are translated, what kind of story is told about our planet’s history? A very dynamic story for what the casual observer might first take for a very static planet. Our earth is under continual change. Tomorrow, you’ll meet Dr. Larry Lawver of the University of Texas and learn how he uses a small part of the story in the sediments to fill in gaps in the dramatic story of roaming continents he studies as a plate tectonocist.
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