Geologists say mud shows bay warming Fossils in sediment include organisms found in warm water

November 05, 1998|By Heather Dewar | Heather Dewar,SUN STAFF

Tiny one-celled fossils dug up from centuries-old mud at the bottom of the Chesapeake Bay are leading some geologists to a startling conclusion: The bay has been getting steadily warmer for about 300 years, they say.

In the beginning, the warming was due entirely to natural causes, say researchers at the U.S. Geological Survey in Reston, Va. But in the past 100 years, they've found, the rate of warming has increased.

The researchers are trying to extend knowledge about bay conditions to the era before colonists came here from Europe. Their results are preliminary and need to be confirmed by other studies, said USGS researcher Thomas Cronin.

The finding -- which has wide implications for bay life -- has raised some eyebrows among skeptical scientists. But the geologists' data from the past century roughly match other research suggesting that the bay has warmed since the beginning of the Industrial Age, in a trend many scientists link to rising levels of greenhouse gases from human activities.

There's no easy way to measure water temperatures from past eras, and no way to pin a number on the degree of warming that might have taken place.

But USGS geologist Stacey Verardo estimated the changes using a new technique: detailed counting and dating of the fossils left behind by a warm-water-loving, single-celled aquatic organism that is common from North Carolina to Florida. Over the past 1,000 years, the tiny creature's abundance in bay sediments has nearly doubled, Verardo said.

The method has not been tried in the Chesapeake before. Bay ecologists warned that the USGS results should be approached with caution until they've been reviewed by other scientists.

"I'm from Missouri. Show me," said Donald Boesch, director of the University of Maryland's Center for Environmental Science. "I'd like to see the evidence."

If the trend is confirmed, it foretells major changes for the bay, Boesch and Cronin agreed. Some shellfish and aquatic grasses that need colder water could disappear, and warm-water varieties might replace them. Because warm water expands, the sea level would continue to rise, covering more of the coastline.

Warm water holds less oxygen, so oxygen-deprived "dead zones" within the bay would probably expand. And because higher temperatures usually produce more rainfall, the sediment and nutrients washing off the land could increase, making it tougher to restore the bay's health.

"I don't think anybody has really looked at the water temperature records. It should be done," Boesch said. If the USGS results hold up, "it would be a nice example of something that you'd expect to happen here, given global trends," he said. "And it would portend some interesting implications for the future."

The research is part of a three-year USGS effort to learn more about conditions on the Chesapeake over the past 1,000 years. Scientists want to understand how the bay and its creatures respond to subtle fluctuations in rainfall and temperature, and how humans have altered the cycles of bay life.

Geologists have techniques for spotting changes that happen over millions of years. Ecologists can use maps and measurements to sort out the last hundred years' worth of changes. But in the era from the birth of Christ to the advent of modern science, the life cycle of the bay is uncharted and mostly unknown.

To remedy that, the USGS researchers took samples of the sediment on the bay bottom near the Patuxent and Potomac rivers. The sediment cores range from 10 to 40 feet long; geologists estimate they cover the past 1,000 to 9,000 years.

"Anything you can imagine that lived in the bay is preserved in those sediments," Cronin said. "We've got a history book, and we're using different tools to read the history."

One of those tools is Verardo's analysis of the fossils left behind by dinoflagellates, single-celled organisms that straddle the boundary between plant and animal life.

In one life stage, dinoflagellates form a hard cyst that remains in ,, the sediment, Verardo said. After samples from the sediment core go through a 97-step preparation process, she said, slides of the cysts end up in her laboratory, where she uses a sensitive microscope to identify and count them.

About nine different dinoflagellates are common in the sediment cores, Verardo said, including one called Gonyaulax spinifera. This small creature hasn't been studied much. Unlike the toxic organism Pfiesteria piscicida, which has not turned up so far in Verardo's sediment tests, it doesn't do anything dramatic. But scientists know it is a warm-water species.

In samples from 1,000 years ago, its fossils, called spiniferites, made up about 45 percent of dinoflagellate remains in bay sediments, Verardo said. In the 17th century they began increasing in abundance, and in most recent times they make up 85 percent of the sample. The increase is steady and isn't affected by other factors that can also be detected from the sediment core, Verardo said.

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