Sediment cores and chemical markers yield 1,500-year health history of Narragansett Bay
A millennium and a half ago, Narragansett Bay was a very different place. Thick forests lined the shores and extended into the watershed. Clean, clear, fresh water coursed in from numerous rivers, streams, and brooks. Perhaps the marine life was different. Perhaps, too, the bay was warmer, or colder, was stressed or tranquil.
The bay’s environmental history is expected to become clearer thanks to research starting this fall by Brown University oceanographer Jeff Salacup. The third-year graduate student in geological sciences has been awarded a three-year, $111,000 fellowship from the U.S. Environmental Protection Agency to detail the bay’s environmental legacy, including surface water temperature, the effects of changes in land use around the bay, and the role of natural phenomena such as the influence of microbial actors like phytoplankton.
The Rhode Island Sea Grant also is helping to support the Narragansett Bay work, through a two-year, $300,000 grant awarded to geological sciences professor Warren Prell.
To ferret out the bay’s past, Salacup is going beneath the waves, drilling cores at various locations to extract a sedimentary record that he hopes will go back to 500 A.D. — long before the first European settlers came ashore.
“Once we get the mud, we can reconstruct what was going during that time,” Salacup said.
The cores will yield snapshots of the bay’s condition every eight years. What that means, Salacup explained, is researchers can combine the short intervals with the overall timeline to provide the most definitive record of the bay’s health. The record, besides providing an authoritative narrative of the bay, can give scientists clues to whether Narragansett Bay has experienced the conditions of today or the kinds of changes that climate scientists forecast will affect the bay.
“If we can find a period of time that is just as warm as today, then that can help guide our thinking,” Salacup said. “But if we find that the bay hasn’t been as warm [as today], then that would cause us to think in a new way.”
Salacup, 32, is from Wakefield, Mass., north of Boston, and is an Air Force veteran. He attended the University of Massachusetts–Amherst on the GI bill, serving in the Air National Guard as he earned his degree in geology. He came to Brown because he wanted to continue his studies in geology and to stay in the area. He soon linked up with Timothy Herbert, a noted oceanographer who became his adviser, and Warren Prell, who has studied the bay extensively for decades.
While other scientists, notably Scott Nixon and John King at the University of Rhode Island, have taken sediment cores to study various indicators of the bay, Salacup’s project is unique. Perhaps most importantly, his cores will establish the longest timeline of the bay’s water surface temperatures. Knowing the temperature, and how it has fluctuated over time, helps scientists understand the air temperatures for a given period, and it is well known that surface waters interact closely with the atmosphere. The scientists will rely on a chemical called alkenones that is produced by a kind of algae that lives at the water’s surface. By examining the concentration of the chemicals in the sediment cores, the scientists can calculate the bay’s surface temperature.
Salacup also will depend on other chemical clues to glean information about nutrient cycling in the bay, the nature of organic and inorganic matter that has run off into the bay from natural disturbances (such as fires) or human practices (such as clear-cutting, farming, and manufacturing) and the availability of oxygen in the water. The chemical signatures he will study normally have been used in ocean settings; this is the first time they will be tested as proxies in an estuarine environment.
The preliminary results look promising. Salacup has found evidence in the cores of a rush of organic matter into the bay after 1700, a strong indication of soil runoff. This, he explains, aligns nicely with written records of forest clearing that started around that time.
“We’re finding we can use [the chemical proxies]. They’re as good as advertised,” he said.