In Tufts microbe count, clues to future sea levels
Researchers take a regional focus
MEDFORD — Cutting-edge research to understand fluctuations in sea levels caused by climate change looks like this: In a spartan, fluorescent-lighted laboratory, Tufts University researchers peer through microscopes to count microorganisms that resemble tiny snails.
These simple marsh-dwelling creatures called foraminifera, or forams, are choosy about how much time they spend underwater, so they turn out to be surprisingly precise indicators of ancient sea levels.
The unsophisticated foram census at Tufts shows that the quest to understand the impact of global-scale climate change can sometimes depend on fairly mundane clues.
Unlike many climate predictions, which describe average changes over the entire globe, tallying up forams to understand past sea level changes can provide clues to help predict the future on a smaller scale — narrowing the focus to specific regions and shorter time frames.
“The problem we have with predictions is our obsession with the year 2100 — that’s of little to no use for someone planning for the city of Boston,” said Andrew Kemp, assistant professor of geology at Tufts who has counted up tens of thousands of forams over the years.
The studies being conducted by Kemp have taken him from North Carolina to Long Island Sound, and now he would like to extend that work to Massachusetts, to build a fine-grained portrait of how sea levels have changed over the last several thousand years in order to make more informed predictions.
The scientists wade into salt marshes, take cores of squishy sediment they transfer to PVC pipes for storage, and then start counting forams. A single, centimeter-thick layer can contain anywhere from a handful to a few thousand remains of the microscopic creatures.
Forams are very particular about their living conditions — some species are characteristically found in high marshes that flood less frequently while others prefer low marshes that spend more time under water. The type of forams present in the sediment can indicate what the ancient marsh was like — which in turn can be used to infer sea level.
Scientists know from studying modern-day forams what conditions species live in, and researchers look for those same patterns in sediment samples deposited decades and centuries ago.
Kemp works in the field of paleoclimate, studying the ancient climate, and is using lessons written in the sediment to discern historical patterns that could help refine models of climate change and sea levels. Generally, local sea level rise is calculated by taking the overall changes predicted by climate models and then factoring in the local conditions. But those are complex and aren’t all understood — a knowledge gap that research like Kemp’s could help fill.
Already, his painstaking studies of 47,000 forams from New Jersey, twice that number in Connecticut, and about the same number in North Carolina have revealed that even on the east coast of North America, “sea level rise” is not the same everywhere.
Sea levels rose in New Jersey from about 500 to 900 A.D., but a similar rise didn’t happen until about 500 years later in North Carolina. Meanwhile, Florida’s sea level was basically unaffected until it began experiencing accelerated rise in the 20th century.
The complex regional patterns suggest that ocean currents play an important role in how sea level rise affects different places.
Sea level rise in one place doesn’t necessarily mean it will occur to the same extent or even in the same century at another nearby spot. That is why Kemp is trying to get a grant to count forams in and around Boston, including a site in Winthrop and a marsh near Cohasset.
The work shows the importance in science of borrowing tools and insights from other fields. While biologists might be interested in forams and the ecosystems in which they live for their own right, geologists can use the different species of forams they find and their distribution in the sediment to extrapolate the conditions of the ancient marsh.
To figure out what era the forams are telling them about, the scientists also become historians of industrial waste.
“Pollution,” Kemp said, “works very well for us.”
Deposits of the radioactive element cesium-137, produced during nuclear test explosions in the 1950s, for example, have become a key way of identifying that decade in the sediment.
Reading lead levels in the sediment can be used to date layers, as the levels dropped off after leaded gasoline began to be phased out. Changes in pollen in the sediment may be used to time the arrival of Europeans, when forested areas were cut down and there was an increase in the number of weeds.
And some references are even more local: Spikes in mercury pollution from the hat-making industry, once strong in Connecticut, help researchers date sediment layers from Long Island Sound.
The entire field of paleoclimate has been energized over the past decade because it may present a way to test and refine the many models being created to make climate predictions. If researchers can understand in detail how the climate evolved over the past 1,000 years, they can calibrate their models to make better future predictions.
“Weather forecasts have gotten better over the past 30 to 40 years, but they have the advantage that every day they make a forecast and at the end of the day they can see if they got it right or wrong,” said Jeremy Shakun, a geologist at Boston College who studies stalagmites to discern past climate and rainfall patterns. “Obviously, climate is different — we make a forecast for 80 years from now and you can’t just wait around and see.”
Correction: An earlier version of the online version of this story contained a photo caption that incorrectly characterized foraminifera.