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Maine's History of Sea-Level Changes
Joseph T. Kelley
We do know that in Maine, land-level adjustments were very important during and soon after the last Ice Age, the latest Pleistocene, from about 14,000 to 10,000 years ago. During our present geologic epoch, the Holocene, which represents the past 10,000 years, global sea-level changes were the dominant effect. Nevertheless, there is some evidence that local earthquake activity and distortions of the land surface that are not accompanied by large earthquakes may be warping the coast.
Figure 2 depicts Maine's local relative sea-level curve for the past 14,000 years. We know from other studies, involving radiocarbon dating that world-wide (eustatic) sea level was some 110 meters (360 ft) below present about 14,000 years before present (BP). However, since the weight of local ice sheets over 1000 m (3300 ft) thick had bowed down the Earth's crust, the level of Maine's coast was actually below the local sea level. Eustatic sea level was low, but rising at this time. Thus, Figure 2 shows relative sea level 70 m (230 ft) above present. As the ice retreated inland, the land rebounded, but the sea was able to invade in contact with the retreating ice, creating a vast inland sea, the DeGeer Sea (named for a famous Swedish geologist). This sea deposited a distinctive bluish-gray marine mud, the Presumpscot Formation, which now is found in many parts of Maine (Figure 3). The sea ultimately reached more than 100 km (62 mi.) inland along the lowlands, as far as Millinocket and Bingham. We know the elevations shown in Figure 2 from exposed deltas and shorelines of the DeGeer Sea preserved above present sea level in interior Maine (Figure 1). We know the timing from radiocarbon dates on fossil shells and wood preserved in Presumpscot Formation sediments deposited during that time (the individual symbols of Figure 2). Ultimately, the release of the weight of the ice allowed rebound of the land (isostatic rebound) to outpace the rate of global sea-level rise. Thus, local relative sea level fell, at rates up to 43 mm/yr (1.7 in/yr), until it reached a local lowstand 55 m (180 ft) below present sea level. This caused the coastline to move well offshore of the present coast, as recorded by terraces, lowstand deltas, and patterns of erosion and deposition on the Maine inner continental shelf (Figure 4). After this lowstand, global sea-level rise rates again became dominant as the rate of isostatic rebound waned. Local relative sea-level rose at rates up to 22 mm/yr (0.9 in/yr), until the rate of rise slowed again, dramatically, about 9000 BP. We are still investigating this slower rate, but theorize that is relates to large-scale warping of the crust at a distance from the ice sheet. Finally, sea-level rise resumed, and approached its present level over the past 5000 years.
The record of sea-level changes in Maine over late Holocene (the past 5000 years) is much more detailed than the record discussed above. Indeed, Maine has one of the best-studied Holocene sea-level curves in the world. We use salt marsh peats, which grow almost precisely at mean high tide level, as markers of sea level. Salt marsh peats are extremely well preserved in many marshes in Maine, sometimes to thicknesses as great as 5 m (16 ft) (Figure 5), and even if not connected with a modern marsh, fragments of older marshes can be found in cores offshore. Radiocarbon dating supplies the time information to construct a sea-level curve from these preserved peats. Figure 6 is such a curve, for Wells, Maine. Elevation with respect to a surveying datum (NGVD) is plotted versus time, in years before present. Although there is some scatter in the data, due to the different types of peats used, we can define a clear trend of rising sea-level over the past 5000 years, that has decreased from a rate of 1.2 mm/yr (0.05 in/yr) about 4000 BP, to 0.5 mm/yr (0.02 in/yr) 2000 BP and perhaps as low as 0.2 mm/yr (0.008 in/yr)1000 BP.
Effects of Sea-Level Changes on the Maine Coast
Maine is widely renowned as the "rock-bound coast." However, there are many stretches of bluffs of soft sediment, and beautiful beaches. As sea level rises against a coast of tough, glacially smoothed rocks, erosion is very slow, and shoreline changes are primarily produced by gradual rising. On soft shorelines, however, the wave and current erosion produces shoreline retreat much more rapidly than the simple sea-level invasion.
One of the most publicized landscapes for conflict between human development and natural processes is the sandy coast. Beaches and sand dunes are dynamic areas, constantly shifting in response to waves, winds, tides, and coastal currents. A more subtle, but immensely powerful influence is sea-level change. Again, as above, the change may be too slow to be obvious in the short term, but is relatively rapid in geologic terms. Maine's Sand Dune Regulations use current floodplain elevations and take future sea-level rise into account in regulating new construction and replacement of damaged structures in these coastal settings. This approach is valuable in the face of predicted rapid climate change and uncertainties about future shoreline change.
Coastal erosion is often considered unpredictable, and an implacable foe of homeowners. Coastal erosion is in fact decidedly predictable, all we are lacking is information on precisely when the events will occur. Similarly, erosion is only a problem when we try to fix property lines or build structures in an inherently unstable environment, when the long-term change in many locations is dominated by landward movement of the shoreline.
Below ground, rising sea level can also influence our environment. Saltwater intrusion can eventually ruin wells and affect soils and ground water. Saltwater intrusion occurs as sea-level rise and causes drowning, but in actuality, human development and use of water resources is a much more rapid cause of change. This is a serious problem in many parts of the United States, and likely will become more publicized in Maine as development pressures continue.
What is the Potential for Future Sea-Level Changes?
How are we to evaluate such predictions? The only rational means is by comparing to past history and observable ongoing trends, as we have outlined above.
Last updated on November 22, 2011
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