by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
This spring on a Q&A feature of www.Nantucket.net a question was submitted by a teacher named Vienna. She wrote: “Is Nantucket sinking? My class is doing a state report and my student needs to know if Nantucket is sinking. Google is not helping.” Dear Vienna: The short answer is: No, Nantucket is not sinking. The longer answer is, although Nantucket is not sinking, sea level is rising, and we’ll eventually be under water either way.
I then went on to elaborate with a shorter version of the explanation below. Google was a little more helpful to me, and I have several books that explain the processes involved with land sinking or rising. Hydrologists call the occurrence of land sinking subsidence. Subsidence is the motion of a surface (usually, the Earth’s surface) as it shifts downward relative to a datum such as sea-level. In order to measure land sinking or sea level rising, you must have a stable constant known height of land to measure the change in elevation. Determining that point when everything is moving, even extremely slowly and almost imperceptibly, is not as easy as you think. Scientists and the government are just now releasing much more accurate estimates of mean sea level (height between low and high tide averaging out tides and storms) this year.
Ground subsidence is of concern to geologists, geotechnical engineers and surveyors, and to anyone living within 20 feet of sea level or the unlucky few who have cars that have fallen into sinkholes. Common causes of land subsidence from human activity are pumping water; oil, and gas from underground reservoirs; dissolution of limestone aquifers (sinkholes); collapse of underground mines; drainage of organic soils; and initial wetting of dry soils (hydrocompaction). Land subsidence occurs in nearly every state of the United States. When you look at the map provided by this link you’ll see that Massachusetts is not considered to be impacted state wide by subsidence. http://geochange.er.usgs.gov/sw/changes/anthropogenic/subside/. The news is not good for parts of Louisiana which has some of the highest subsidence rates in the world (read more about this state’s problem at http://www.nola.com/coastal/index.ssf/2008/12/part_1_because_of_subsidence_a.html).
The opposite of subsidence is uplift, which results in an increase in elevation. In fact, most of New England at one time was undergoing isostasy which is one of my favorite scientific terms. Sounds fun doesn’t it? This is the opposite of subsidence and causes uplift of the land, which results in an increase in elevation. Glacial isostasy is the process of lithospheric depression beneath the weight of an ice sheet and subsequent rebound when the ice mass is reduced or removed. When we were covered in ice, it pushed the land down with its weight and when that ice retreated, the land started bouncing back, slowly rising over thousands of years.
Glacial eustasy, on the other hand, refers to worldwide changes in sea level as a consequence of changing volume of glacier ice on land. Both glacial isostasy and eustasy are related to the volume—thickness and areal coverage— of ice sheets. You may remember from school that our continents float around on top of a plastic fluid asthenosphere just like ice cubes in a drink or logs in a river. The asthenosphere is a portion of the upper layer just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. If mass is added to a local area of the crust (e.g. through deposition), the crust subsides to compensate and maintain isostatic balance.
The Cape and the Islands were formed during the last glaciation in the area when one of the lobes of the Laurentide Ice Sheet pushed down this far during the recent Wisconsin Glaciation. In fact, the ice sheet moved back and forth a few times, but essentially it stopped right along the area on Nantucket where we have a few hills on the northern side. The southern side is mainly outwash plain, or bits of sediment that melted out and stayed there as the ice retreated north as the earth warmed up. The islands became islands as the sea level rose around 5,000-6,000 years ago. The wind and waves started moving the sand around, redistributing it in storms and depositing it just like a pile of sand in a bucket with some water would do if it were swirled around. Although the state as a whole and Nantucket itself is not sinking, there are some areas of New England’s shoreline that are sinking due to subsidence. Scientists have determined that the area around Boston (which contains lots of fill) may settle as much as 6 inches in 100 years; which is quite a bit. The Boston area has filled in close to two-thirds of its wetlands, which would have protected it as sea level rose.
Of course you all may recall that something is happening all over the world which makes it appear as if subsidence is occurring, and that is sea level rise. Worldwide, the conservative midrange estimate predicts oceans will rise 18 inches by 2100. Louisiana faces a far more alarming forecast. Here, those same models predict that, relative to land, water levels will rise 2 to 6 feet, with the highest rates in the southeastern coast surrounding New Orleans. This is occurring because Louisiana has the highest rates of subsidence, and its salt marshes have been deprived of sediments from rerouting of the Mississippi. Only the Atchafalaya region (where I did some research as a graduate student) is building up sediments naturally to keep pace with sea level rise.
As you may have noticed, Nantucket is not very high; with the highest points (Altar Rock-second highest and Folger Hill, highest) approximately 108-109 or so feet about sea level. And that is where the concern would be, because sea level is rising all over the globe and in the New England area at a rate of almost a foot over the last 100 years. The New England area has experienced accelerated sea level rise over the last 500 years from 1 mm per year to 2.5 mm/year (in 1990) to close to 3.0 mm/year now. The most accurate recent estimates for Nantucket can be found at the National Oceanic and Atmospheric Administration (NOAA) sea level online site at: http://tidesandcurrents. noaa.gov/sltrends/sltrends.shtml. “The mean sea level trend is 2.95 mm/year with a 95% confidence interval of +/- 0.46 mm/year based on monthly mean sea level data from 1965 to 2006 which is equivalent to a change of 0.97 feet in 100 years.”
The fact that our sand moves around helps us some: the island is more nimble and able to adapt to change. But most likely within 400-600 years, much of the island will be flooded and only a few high points and higher elevations will still be sticking up out of the ocean. I give a talk about three or four times a year on the effect of climate change on Nantucket. One of the tools I use in the talk can be found online at http://flood.firetree.net/. It is a pretty sobering wakeup call when you input a 1 meter rise in sea level and see most of Coatue, Madaket, and parts of downtown, Brant Point, and the marsh
edges at the field station under water. So far, most of our marshes have been relatively stable and build up to keep pace with the rising sea level, but higher astronomical tides, more intensive storms which accelerate erosion, and projects which divert the sediment flowing to and along our shores can hurt the ability of the island to adapt to sea level rise.
To read more about our natural world, I highly recommend Peter Brace’s new book coming out in a week or two entitled Nantucket: A Natural History. Go to Mitchell’s or Bookworks to get your copy. I’ll be giving a talk on climate change and how it affects Nantucket this July at the UMass Boston Nantucket Field Station. Stay tuned for details.