The University Record, September 3, 1997
This rock is covered with a dark, 4-inch-thick crust of iron-manganese deposits, which have been building up for 50-60 million years. Photo by James R. Hein, U.S. Geological Survey
U-M geologists have decoded a precise record of long-term changes in Earth's ancient climate and ocean currents hidden inside dark, crusty deposits coating rocks on the Pacific ocean floor.
According to John N. Christensen, assistant research scientist in geological sciences, these deposits-called ferromanganese crusts-constitute an important new data source for geochemists who currently rely on ice cores and shells of microscopic ocean animals, among other sources, for data on Earth's paleoclimate.
"Lead isotopic records in ferromanganese crusts can provide a detailed record of changes in ocean circulation over time," says Alexander N. Halliday, professor of geological sciences.
The crusts form when dissolved elements such as iron, manganese and lead precipitate out of seawater to form solids in oxide layers, which build up over time growing at a rate of a few millimeters every million years. As these crusts grow, they record subtle changes in the lead isotopic composition of the seawater from which they came.
An article analyzing lead isotopes from two central Pacific deep water crust sites by Christensen, Halliday and colleagues was published in the Aug. 15 issue of Science. Crusts at both sites started growing about 55 to 60 million years ago, and were much closer to the equator than they are today.
As water and wind continually erode the world's continents, lead is washed into the ocean in river water and blown in with dust. "Once deposited in the ocean, lead dissolves, but it stays in solution only for short periods of time--less than 400 years-- before it precipitates and is incorporated into solids," Halliday says.
The relative amounts of different lead isotopes vary depending on the mineral content and the age of the rock from which they came. "The isotopic signature is distinctive. It's like adding different-colored dyes to the ocean and tracking them as they swirl around," Christensen says.
"In both of our mid-Pacific deep water sampling sites, we detected lead isotopic fluctuations at specific times over the past 50 million years which matched the timing of fluctuations in other known indicators of major paleoceanographic and paleoclimatic changes," Halliday says.
"The two most significant changes we found in lead isotopic composition occurred about 30 million years ago and about 15 million years ago-both times of changes in erosion and continental weathering," Halliday adds. "Did a change in global climate produce more erosion or were different types of rocks exposed to weathering by wind and rain? More data from more sampling points will be required before we can know the answer."
Crusts analyzed in the U-M study were from samples collected by the U.S. Geological Survey that were organized and catalogued by James R. Hein. "USGS has one of the largest collections of ferromanganese crusts in the world," Halliday says. "Our study would not have been possible without this collection."
Christensen and Halliday used a new technique called laser ablation, multiple-collector, inductively-coupled plasma mass spectrometry to ablate or remove tiny amounts of crust and precisely measure the different lead isotopes in each sample. "ICPMS technology allowed us to produce an extremely detailed set of data in just two to three months. It would have taken years of work with thermal ionization mass spectrometers," Halliday says.
The study was funded by the U.S. Department of Energy. Co-researchers include Linda V. Godfrey of Cornell University, James R. Hein of the U.S. Geological Survey and David K. Rea, U-M professor of geological sciences.