By Brian Fitzgerald
Britt Argow has long been interested in the evolution of our planet's shores, particularly their vulnerable salt marshes. So naturally she is alarmed that rising oceans, triggered in part by global warming, may cause the loss of two-thirds of U.S. coastal wetlands by the end of the century, as many scientists believe.
Argow (GRS'06), a doctoral student in the CAS department of earth sciences, is studying the fate of salt marshes in a world with rising sea levels, which she says are caused by both natural and human factors. “The sea level has been rising for the past 18,000 years, since the end of the last ice age, due to natural changes in the Earth's climate,” she says. However, she points out, the global greenhouse effect is melting the continental ice sheets and accelerating the thermal expansion of the ocean at a disturbing rate. Both processes are rapidly increasing the volume of the world's seawater, and a projected rise in the height of the ocean by up to one meter by 2100 may spell doom for many salt marshes — especially, because of their makeup, those in New England.
Scientists call these coastal wetlands “nurseries of the sea” because they provide breeding, spawning, nesting, and feeding grounds for many animals; a large number of fish species depend on salt marshes for food and shelter for some part of their lives. Animals can also hide from predators in the marsh grass.
“Marsh grass is really amazing,” says Argow, who is working on a doctorate in coastal sedimentology. “It's plunged into salt water twice daily with the high tides and is able to survive, which is an impressive characteristic, and rare in the plant kingdom.” Although resilient, these plants rely on a delicate balance of other natural factors, such as water turbidity, the duration of tidal inundation, and changes in regional climate. “If a marsh floods more frequently, many species of grasses won't be able to handle it,” she says. “They will die out, and the marsh has the potential to fail.”
Carbon dioxide, mostly from the burning of fossil fuels, is seen as largely responsible for the greenhouse effect — the gas traps heat that would otherwise radiate into space. Argow says this may be causing the recent observed acceleration in the rate that the sea level is rising. “Eventually, salt marshes may be flooded and destroyed,” she says. “With their loss, coastal areas become increasingly vulnerable to wave attack and inundation during storms, which in turn increases the potential for coastal erosion and property damage.”
Fighting back naturally
Still, salt marshes have their own natural way of fighting back. “For thousands of years, salt marshes have been maintaining their elevation relative to rising sea levels by means of vertical accretion — inorganic and organic sediment accumulation,” says Argow. But how long will they be able to keep up? Argow has been studying salt marshes in New England, which have been building up at an average rate of 2.21 millimeters a year, compared to the rate of 2.5 to 2.7 millimeters that the sea level has been rising. Most marshes still appear healthy, but Argow believes there may be early signs of deterioration in many coastal areas. She is especially interested in the effects of colder climates on salt marsh survival. She says that sediment that accumulates in ice in the winter is eventually deposited in the marsh when the ice melts, in a process known as ice rafting. This helps to build up a marsh. But in another process, called ice loading, she says, snow and ice on the surface “may compress peat, which leads to shallow compaction and a net loss of elevation in some areas of the marsh.”
At present, Argow is investigating vertical accretion in a salt marsh in Wells, Maine, as well as in the Essex River salt marsh in Ipswich, Mass., and in Nauset Marsh in Orleans, Mass. “Preliminary data indicate that ice rafting contributes a significant volume of sediment to the marsh surface,” Argow says. “But results are inconclusive regarding the role of ice loading.” This spring, she will take sediment readings at all three marshes to further study this balance.
“Salt marshes in this country have the capability to grow vertically and may be able to keep up with the rising sea level for a while,” she says. “The catch is that most of the high rates of vertical accretion occur in southern marshes, especially in marshes located near large rivers, where there is a strong fluvial influx — meaning that the rivers down there are bringing in a lot of extra inorganic sediment. This helps build up the surface of the marsh much more rapidly than in the Northeast. Over the next century, New England marshes have the potential to break down much more quickly than their southern counterparts.”
At stake is not only a portion of New England's biodiversity. Salt marshes also help stabilize the shoreline. “It's important to remember that the marsh grasses trap sediments, filter out pollutants, and help control coastal flooding,” says Argow. “The consequences of the loss of these wetlands would be enormous.”
On March 27, Argow will present her research in a session at the annual joint meeting of the Northeastern and Southeastern Sections of the Geological Society of America, in McLean, Va.