Natural world keeps detailed records of its ecological past

The history of Maine is literally beneath our feet. Rocks alongside the road tell tales of glaciers and ancient life, and marine clay soils in flower beds as far north as Millinocket paint a picture of oceans stretching far inland.

But to understand the past, paleoecologists – those who study the history of Earth’s plant and animal communities – dig much deeper.

The sediments beneath lakes and bogs have quietly been transcribing a changing world since they were formed with the retreat of the glaciers from Maine at least 12,000 years ago. Every year, pollen and leaves fall from the trees and slowly drift to lake bottoms or cling to sphagnum moss.

“It’s like having a recorder, running from the day the lake is formed,” said George Jacobson, a paleoecologist at the Climate Change Institute at the University of Maine.

In places like Maine, where glaciers no longer exist, biological records often provide the best view into the ecological past.

“There are quite a few parts of the world that don’t have thick ice sheets to sample,” Jacobson said.

Some Maine lakes have offered sediment cores that reach as far back as 17,000 years, though 10,000- to 12,000-year cores are far more common, particularly in bogs, many of which didn’t develop until several thousand years after the ice completed its retreat. Others offer just a blink in geologic time.

“You may find that the lake just formed 1,000 years ago, and you’re out of luck,” said Ron Davis, a professor emeritus at the university and one of the founding members of the Climate Change Institute.

Perhaps 60 cores that extend all the way back to the retreat of the glaciers have been taken in Maine and fully examined by biologists. Many more samples are in storage awaiting someone with the time and funds to undertake the six solid months of research it can require to analyze a sample, Davis said.

Tests of the soil chemistry can give scientists important clues about the ancient atmosphere. Pollen grains, which have a surface particularly resistant to decay, can tell researchers which trees and plants lived in the region over time. Radiocarbon dating of organic matter in the sample can link the findings to a precise point in time.

But the organisms that allow sediment samples to be carbon dated also can mar the data. A burrowing insect or shellfish disturbs just a few inches of lake bottom, but those inches might represent 50 years of history, which become blended together, Davis explained.

Some of these less precise records remain useful, as proof of sweeping climatic change. Other, less common cores, from lakes where fewer insects or shellfish existed, can be nearly as precise as ice cores, offering a year-by-year history of the region, Davis said.

The only sediment records than can begin to challenge ice for sheer longevity are the cores of ocean sediment that can contain hundreds of thousands of years of history.

“The deep ocean just receives sediment. It just goes to the bottom [and stays there]. There’s no erosion,” said Harold “Hal” Borns, a professor emeritus at the University of Maine and the founder of the Climate Change Institute.

Elsewhere in the world, scientists also study tree rings and the similar annual development patterns of ocean corals and cave growths such as stalactites to understand a shifting climate. A wider ring could mean a milder season or an increase in precipitation, while a slight change in the chemical makeup of coral can reveal a great deal about the ancient ocean and atmosphere.

One biological record by itself means very little, but when hundreds of samples all show the same trends, scientists can begin to piece together a mosaic of ancient landscapes, Jacobson said.

“It’s really a tremendous natural history,” he said. “It allows us to piece the whole story together in a way that’s pretty incredible.”