A Geologic History of the Gulf of Maine

Contributed by Abigail Manahan
Bigelow Laboratory for Ocean Sciences
West Boothbay Harbor, ME 04575

Geography of the Gulf of Maine

The Gulf of Maine, sometimes referred to as a “sea within a sea,” extends 7,500 miles from Cape Cod to Cape Sable, Nova Scotia. The region’s boundaries are divided between three New England states: Massachusetts, New Hampshire and Maine; and two Maritime provinces: New Brunswick and Nova Scotia (Fig.1). Regarded by some as one of the most striking topographic features of the east US coast, this rectangular depression covers approximately 36,000 square miles and has an average depth of 150 meters (1). The Gulf of Maine has its own oceanographic characteristics, distinct from those of the open Atlantic (1).

Geologic timeline for the Cenozoic Era

The Gulf of Maine has a rich history. We can trace the formation of the modern day Gulf of Maine by beginning in the Cenozoic Era (Fig. 2). The Cenozoic, our present era, is divided into two geologic periods: Tertiary and Quaternary. The Gulf of Maine shifted landward and seaward over the course of the Tertiary period, but bore no resemblance to the contemporary Gulf of Maine. The Quaternary period, which is divided into two epochs: the Pleistocene and the Recent (or Holocene), is the period when the Gulf of Maine was constructed. This construction was marked by a series of advances and retreats of an enormous ice sheet that originated in the Hudson Bay area of Canada.

The Gulf of Maine owes its existence to the Laurentide Ice Sheet. This was the last major ice sheet that advanced from northern Canada and reached its maximum in southern New England about 25,000 years ago (2). The advance of this ice sheet scoured the bedrock surface of Maine, removing most of the loose material from the surface and shaping the underlying bedrock (3). At the time of glacial maximum, the only unglaciated portions were offshore islands and peninsulas that are now part of the broad continental shelf, including Georges Bank, Stellwagen Bank, and Sable Island (4). Such exposed areas served as refuge for many hardy species of plants and animals. Fishermen trawling Georges Bank have pulled up specimens of prehistoric mammoth and mastodon teeth (5,6). The vegetation of glacial Georges Bank probably consisted of a mixture of tundra, which would have attracted mammoths, and coniferous trees and black spruce, which attracted mastodon (4).

History of sea level change in Maine

A significant amount of water is required as continental ice sheets–such as the Laurentide–form and recede, thus causing changes in global sea level. Maine’s sea-level history over the past 15,000 years (Figure 2) has been especially complex. It involves not only worldwide (eustatic) changes in the level of the ocean generated by melting ice sheets, but also more local effects. These include land-level (isostatic) changes produced by depression of the Earth’s crust by local ice sheets, and the subsequent rebound of the land (7,8). Isostatic rebound of the crust occurred simultaneously with thinning and retreat of the glacier margin. Rapid rebound caused a fall in local relative sea level to 60 m below present sea level approximately 10,000 years ago, when a short-term equilibrium was reached between eustatic sea level rise and rate of isostatic rebound.

Fossils and geomorphic features recorded evidence of this sea level variation and changing environmental conditions. This record offers proof that there was a marine intrusion in areas that are today, terrestrial. For example, southern and eastern areas of Maine, including far up the Kennebec and Penobscot valleys, flooded with seawater (2). Sand and mud from the glaciers blanketed this temporary sea floor, and fossils preserved in this sediment are usually 11-12,000 years old. Most of the fossils preserved in this marine clay are shells of marine invertebrates (such as bivalves, gastropods, and brachiopods), although fossil wood and some vertebrates (including mammoth, walrus, and seal remains) have also been found (9). Archeological evidence shows that the Gulf of Maine had Arctic pinniped (true seals, sea lions, fur seals, and walrus) species, once the glacier retreated. Walrus remains are of particular interest as they can be indicators of sea level and temperature (10). Walrus prefer to feed in shallow water using their tusks to dig mollusks. Only five (reported) walrus specimens have been recovered from Georges Bank, but many more have been recovered on the continental shelf further west (5,10). The presence of fossil records of walrus and bearded seals, suggest that sea conditions were far colder and shallower than they are today (11). Other indicators of sea level change in this offshore area include the recovery of intertidal salt marsh peat and shells from the Arctic wedge clam (Mesodesma arctatum) a shallow, cold-water mollusk species (6, 10). As time marched on, sea level varied and the rebound of the crust greatly influenced the depth and boundaries of the coastal ocean, thus affecting ocean circulation. A low point of sea level briefly exposed Georges Bank, Grand Banks, and other shallow areas, creating a relatively warm, inland sea, partially isolated from the deeper, colder Atlantic Ocean (12, 13). Such environmental changes would likely have forced cold-water pinnipeds to relocate further north. As sea level rose, exposed areas once again submerged, and the Gulf of Maine came under the influence of more contemporary ocean circulation patterns and tidal mixing of the Northern Atlantic (11).

Climatic warming caused the Laurentide ice sheet to start receding soon after it reached its terminal position at the present day location of Long Island, New York (14). About 15,000 years ago, the seaward edge of the ice sheet roughly paralleled the present Gulf of Maine shoreline. It began its retreat from the continental shelf in an irregular manner. During periods of steady withdrawal, no deposition took place along the ice-margin. At other times, the ice sheet caught on highlands or grounded in valleys. During these periods, the ice retreat either slowed or halted altogether, depositing sediment at the ice margin to form end moraines. In addition fans and deltas were deposited by stream discharge from melting ice (15). Such deposits can be used to provide an historical reconstruction of the orientation and relative position of the ice margin. The changes in sea level and the redistribution of glacial sediments by tides, currents, and waves have contributed to the sculpture of the Gulf of Maine into the complex topography we find today (1).

Graph showing the relative temperature change from the Medieval Warm Period to the Little Ice Age.
Figure 3. Graph showing the relative temperature change
from the Medieval Warm Period to the Little Ice Age.
From http://calspace.ucsd.edu/virtualmuseum/

Small-scale climatic changes continued to occur after the disappearance of the Laurentide Ice Sheet, as shown in Figure 3. These changes have been linked to a number of biological transformations, or shifts, in biota and communities. Between the 9th and 14th century (~1400 years ago) there was a “Medieval Warm Period”, when the average temperature in the Northern Hemisphere reached its highest point in the past 4,000 years, which was only about 1°C higher than at present (16). It has been documented that during this period, American oysters (Crassostrea virginica) and bay scallops (Aequipecten irradiens) formed populations as far north as Sable Island (13). Neither of these species exists there today. Radiocarbon dating of relict oyster and bay scallop shells compare reasonably well with the dates of the post-glacial warm period (13). From the 16th to the 19th century there was a “Little Ice Age”, when the average temperature of the Northern Hemisphere was a degree or two cooler than now (16). It is during this time that salmon are hypothesized to have relocated to the New England area. Salmon may have migrated from Europe after the end of the Pleistocene, across the Atlantic. Immediately prior to the Little Ice Age, the Medieval Warm Period diminished sea pack ice around Iceland and Greenland (18). It is possible that salmon migrated during this time to Davis Strait between Labrador and Greenland (Fig. 4), an area that today is still an important feeding ground for both European and American salmon populations. This allowed them to reach North American shores. As the Medieval Warming Period came to a close and the Little Ice Age set in, cooler conditions south of the Labrador coast initiated salmon range expansion into the New England region (18).

Coastal wasters of Northwest Atlantic

So how does the geologic history correlate with marine life in the Gulf of Maine? The topography of an area has many biological influences. The dynamic physical evolution of the Gulf of Maine created a unique body of water that is characterized by its productivity and diversity. Non-biological elements such as geology, climate, and water chemistry also greatly influence the plant and animal communities found there. The glacial activity left in its wake a mosaic of seafloor types. For example, Table 1 describes the type of bottom habitat that makes up the floor of the western Gulf of Maine. Each of these substrate types forms physical habitats that support life in this marine region (8). While biodiversity comprises an almost countless variety of adaptations to biological, chemical, and physical surroundings, it is also inseparable from the formation of the landscape and the types of habitats, which occur upon it.

Table 1. Geology of Maine’s inner continental shelf. Adapted from The Seafloor Revealed, (8).
Substrate type Percent coverage
Bedrock-dominant in water depths less than 50m 41%
Muddy basin-dominant below 50m 39%
Gravel plains-most common in the 10-30m depth range 12%
Sandy areas-rare but present at all depths to 100m 8%

For more information:

http://www.state.me.us/doc/nrimc/mgs/mgs.htm The official site of the Maine Geological Survey offers a fantastic resource covering the history of Maine’s geology through fact sheets and topic links. Appropriate for secondary/college students and general audiences.

http://www.bigelow.org/virtual Offers a valuable resource to students and educators. Find activities, discussions, and links to Earth Science resources on the Internet including specific information on the Gulf of Maine. Appropriate for students, grade level 7-12 and general audiences.

http://calspace.ucsd.edu/virtualmuseum/climatechange2/04_3.shtml Hosted by the California Space Institute, this site provides a comprehensive explanation of global climate change in the past 1000 years. Appropriate for secondary/college students and general audiences.

http://museum.gov.ns.ca/mnh/nature/nhns/about.htm This electronic version of the book The Natural History of Nova Scotia provides a great overview in an easy to use on-line format. Appropriate for secondary/college students, educators, scientists, tour guides and anyone with a penchant for natural history.

http://www.bofep.org/ The Bay of Fundy Ecosystem Partnership website offers information to a variety of audiences concerned with the health and environmental issues facing the Bay. Appropriate for general audiences. http://stellwagen.nos.noaa.gov This site offers background information on the geologic origins of Stellwagen Bank. Appropriate for general audiences.

http://pubs.usgs.gov/gip/capecod/sea.html Cape Cod and the Sea provides an overview of the geologic history of Cape Cod, Massachusetts. Appropriate for general audiences.

http://www.priweb.org/ The Paleontological Research Institution provides teacher friendly resources for teaching the geology of the Northeastern United States. Available on-line through PDF files or in hard copy bound format by contacting the Institution. Appropriate for students grade level 7-12 and general audiences.

References:

  1. Apollonio, S. (1979) The Gulf of Maine. Courier of Maine Books. Rockland. pp 7-24.
  2. Marvinney, R.G., Thompson, W. (2000) A Geologic History of Maine. Maine Geological Survey Factsheet. http://www.state.me.us/doc/nrimc/pubedinf/factsht/bedrock/megeol.htm
  3. Daly, J. (2003) The Glacial History of Maine. Geological Society of Maine Conference Abstract.
  4. Pielou, E. C. (1991) After the Ice Age: The Return of Life to Glaciated North America. University of Chicago Press, Chicago.
  5. Uchupi, E. (1964) Oceanus, 10, p. 20-22.
  6. Whitmore, F.C., et al. (1967) Science, 156, 1477-1481.
  7. Kelley, J. T., Dickson, S. M., Belknap, D. (1996) Maine’s History of Sea Level Changes. Maine Geological Survey Fact sheet. http://www.state.me.us/doc/nrimc/pubedinf/factsht/marine/sealevel.htm
  8. Kelley, J.T., et al. (1998), The Seafloor Revealed: The Geology of the Northwestern Gulf of Maine Inner Continental Shelf. Maine Geological Survey, Augusta.
  9. Maine Geology Survey Fact sheet. Maine Fossils http://www.state.me.us/doc/nrimc/pubedinf/factsht/paleo/fossil.htm
  10. Backus, R. (1987) Georges Bank. MIT Press. Cambridge.
  11. Harris, D. Anecdotal reference. 2004.
  12. Emery, K.O. et al. Limnology and Oceanography, 10, suppl. R97 (1965).
  13. Clarke, A.H., et al., (1967) Nature 215, 1146-1149.
  14. Sirkin, L., (1986), New York State Museum, Bulletin 455, p. 6-21.
  15. Ashley, G. M., et al. (1991) Geological Society of America, Special Paper 261.
  16. Bay of Fundy Ecosystem Partnership. Issues Fact Sheet #18. http://www.bofep.org/climate.htm
  17. Lamb, H.H., (1979) Quaternary Research 11, no. 1,1-20.
  18. Carlson, C. (1996) Common Ground. Vol. 8(3/4). Find entire article at http://www.cr.nps.gov/aad/cg/fd_vol8_num3-4/salmon.htm
  19. Virtual Vacationland, http://www.bigelow.org