By Sally Pobojewski
News and Information Services
Tiny stones called otoliths that form within the inner ears of fish are actually paleothermometers preserving a record of seasonal temperature variations over the past 20 million years, according to a U-M graduate student.
“By analyzing tiny bands of otolith material deposited on a daily basis, just like growth rings in a tree, we can determine changes in water temperature over the life of the fish, whether it died last week or a million years ago,” says William P. Patterson, graduate student in geological sciences.
Patterson will discuss his research during a presentation at the annual meeting of the Geological Society of America in Cincinnati this week.
Otoliths, or “ear stones,” form from built-up layers of aragonite—a mineral synthesized by fish every day, which includes oxygen from lake water. Different species form different sizes and shapes of otoliths, ranging from microscopic to more than one centimeter (0.39 inch) in width.
“Because we can reconstruct average weekly temperatures with this technique, we can accurately track seasonality—the difference between winter low and summer high temperatures,” Patterson says. “Seasonality is the most significant factor in determining which plants and animals can survive in a particular area.”
Using otoliths to study past climates will help scientists make more accurate predictions of future climate change, Patterson says, especially in mid-continental regions, which are particularly sensitive to seasonal climate changes. Paleoclimatic research in continental areas has been difficult until now, because the usual source of information on past climates—fossils of one-celled organisms called foraminifera—can only be found in ocean sediment.
“Otoliths give us a new source of physical evidence to determine the rate and geographic pattern of ancient climate change,” Patterson adds.
Early Native Americans in the Great Lakes area collected and saved large otoliths, and they are often found in archaeological excavations of Native American sites. “This gives us a good control on the age of these otoliths, which have been dated as far back as 11,000 years ago,” Patterson says.
Otoliths up to 155 million years old have been found in layers of sediment, Patterson says. Otoliths are preserved in museum collections giving scientists a wide selection of dated specimens to study.
Patterson said instrumentation in the Department of Geological Sciences’ Stable Isotope Laboratory was crucial to the success of the research effort. K.C Lohmann, associate professor of geology and mineralogy, directs the lab and assisted with the research.
“This is the only laboratory in the United States capable of measuring different ratios of oxygen isotopes and able to drill the microscopic samples—up to 100 per millimeter—needed for the analysis,” Lohmann says. “It’s like cutting a grain of salt into 10 pieces and using each piece as a sample.”
Using instruments in the Stable Isotope Laboratory, Patterson was able to remove samples of otolith growth bands drilled at precise intervals using a computerized program developed at the University. Mass spectrometers then analyzed the ratios of two isotopes of oxygen, O-16 and O-18, found within each band. O-18 is slightly heavier, because it contains two extra neutrons in its atomic structure.
When he examined modern fish from the Great Lakes, Patterson discovered that the ratio of oxygen isotopes deposited each day in the otolith growth band was identical to the ratio of isotopes found in the lake water on that same day.
“When the water and the fish are cold, the fish absorbs more of the heavier isotopes,” Patterson says. “When the water is warmer, more of the lighter isotopes are absorbed and incorporated in the otolith. The ratio of the two isotopes varies directly with the temperature of the water. If we know the isotopic composition of the water as stored in the otolith, we can infer past water temperatures.”
Patterson says the next phase in the research effort will be an analysis of Great Lakes climate for the past 11,000 years using otoliths from various museum collections. The second phase will track climate change over the past 10 million years.
In addition to Lohmann, others assisting with the otolith research include Gerald R. Smith, professor of zoology; David Dettman, graduate student in geological sciences, and Aimee Dolin-Laughlin and James W. Burdett, technicians in geological sciences.
The project is funded by the National Science Foundation and the U-M’s Scott Turner Fund in Geological Sciences. Otolith specimens were donated by the Museum of Zoology and the Cleveland Museum of Natural History.