Dec. 4, 2000 BLACKSBURG, Va. — Much about the Earth is unknown—including such things as what influenced the environment or oceans in times before historical records. Rebuilding a lost record of the Earth takes a process much like fitting puzzle pieces together, and one of those pieces is determining the age of things.
Monica Carroll, a master's student in geological sciences at Virginia Tech, and colleagues at Virginia Tech and other universities have fit one more piece into the puzzle. They have expanded the dating of marine animals beyond mollusks to brachiopods, and the method has been shown to work back to the time of Aristotle.
Carroll, along with Michal Kowalewski of Virginia Tech, Marcello G. Simoes of the Institute of Biosciences, UNESP, in Botucatu, Brazil, and Glenn A. Goodfriend of George Washington University, have provided the first quantitative estimates of time averaging for present-day brachiopods. Brachiopods are marine invertebrates about the size of a dime that are superficially similar to clams and mussels in that they have two valves and filter food, but there the resemblance ends. Because they are not palatable to most animals, including humans, their relevance to humans was unrecognized.
The shells of brachiopods are made of calcite, and, within the calcite, amino acids are preserved. From the way the amino acids degrade through time, Carroll can figure out how old they are. The process is simple and works when calibrated with C-14 (radio-carbon) dating that has long been used to determine the ages of things from the past but is prohibitively expensive for dating large data numbers of samples. Carroll uses amino acids with C-14 dating to come up with a calibration curve.
Using amino-acid racemization/epimerization rates, or the way amino acid breaks down after an organism dies, Carroll and her associates analyzed the ratio of L-isoleucine, which is present in living brachiopods, with the amount of D-alloisoleucine, the material to which L-isoleucine breaks down after the death of the brachiopod. This determines the age of the shell.
Their preliminary results showed that time-averaging patterns in brachiopod shell accumulations were very similar to those derived previously for mollusks. The similarities included the fact that, while most shells are younger than a few hundred years, the accumulations included shells from a period of thousands of years. The researchers also determined that the shell's taphonomy, or the degree to which it appears to be preserved, is not a reliable indication of its age.
"Once we come up with a date, we can do other geochemical analyses," Carroll said. For example, they can look at trace elements to get an environmental signature over the past 2000-3000 years. "We can detect strontium, which is related to erosion rates, and, in Brazil, to deforestation," Carroll said. "For the period preceding historical records, we can use shells such as these to see what the human effects have been on the environment over time."
Although mollusks such as clams have been used for amino-acid dating, using brachiopods is new. Most previously studied brachiopod assemblages have been found in cold waters off the coasts of South Africa and New Zealand. Carroll and her colleagues collected the brachiopods in their study off the coast of Sao Paulo, Brazil. "This is noteworthy because it's a tropical area, and that makes it more comparable to the Paleozoic environments when brachiopods were more common," she said.
Such dating helps provide an environmental history of a region. "If we can look at the age structure in modern deposits and find out what time scale they cover, we can look back on old deposits of brachiopods and better understand what they represent."
The brachiopods provide individual records, "time slices of what has been going on in the oceans over time," Carroll said. This can help in determining the recent history of the environment. Carroll presented the group's findings at the Geological Society of American meeting in Reno in November.
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