Rebound and Sea Level
By Ed Caryl
During the peak of the last ice age, enough ice was collected in the great ice sheets that the global sea level was reduced by more than 120 meters. The ice sheets themselves were, in places, more than two kilometers thick. The great weight of that ice depressed the earth’s crust and mantle by hundreds of meters. In some places, ground that is now a hundred meters above sea level, was pressed down below the sea level that existed before and shortly after the ice melted. Because the earth’s mantle has a high viscosity, and the earth’s crust a high bending strength, these areas are still slowly rising after 12,000 years, and will rise for another 12,000, barring another ice age to press them down again. This “isostatic rebound” or “post-glacial rebound” (PGR) complicates sea levels worldwide because it continually changes the sea bottom and coastline shapes. The University of Colorado sea level measurements add 0.3 mm/year to sea level rise to “adjust” for this. Is this adjustment reasonable?
Locally, this rebound can be measured by precision GPS. Geological studies have also determined the prehistoric amount of rebound that has taken place. I will just mention three areas that have been and will be vastly changed by rebound: the St Lawrence Seaway area in Canada and northern New England in the U. S., an island beach in Nunavit, northern Canada, and Finland in northern Europe.
Figure 1 is a world map of PGR from the Wikipedia article on that subject, here.
The present day St. Lawrence River Seaway sits at the edge of the present PGR area that marks the boundary of the great Laurentide Ice Sheet of the last ice age. North of the river, the Provence of Quebec is rising. South of the river, southern New England is rising much slower or falling. This is apparent at Lake Ontario, where the tilting has resulted in the northern shore rising faster than the southern shore, and wetlands on the north draining and drying out, while on the southern shore, beaches are drowning and wetlands are being created from formerly dry land. The whole lake is very slowly rolling southward.
At the end of the last ice age, ice had blocked the St. Lawrence valley and formed the glacial Lake Candona, covering what is now the lower three Great Lakes, Ontario, Erie, and Huron. When the ice dam failed, the water level fell 300 feet (100 meters) in a few days. At this point, Lake Ontario may have been connected to the world ocean through the Champlain Sea. The Champlain Sea covered the whole area that is now Lake Champlain and all the St. Lawrence River Valley up to Lake Ontario. Pierre’s boyhood home in Northern Vermont was under seawater, or at the shore during this period. The Champlain Sea lasted from 12,500 years ago up to 9800 years ago, when the rising land finally cut off Lake Champlain from the waters to the north. The land has continued to rise in the Lake Champlain area, and now the lake is 29 to 30 meters above mean sea level.
In the far north of Canada, where the center of the ice sheet was thickest, the land is currently rising at nearly 2 cm a year. In the past, when the last ice had melted, the rate was even higher, and the land will continue to rise into the future until the next ice age returns. The PGR uplift is constant, without fits and starts, in contrast to land in tectonically active areas like California and Japan, and other areas that are near plate boundaries where earth movements will abruptly change sea level.
Figure 2. At Bathurst Inlet, on the east side of Cockburn Island, Nunavit, northern Canada, is a wedge-shaped beach called Rebound Beach. Source.
Here at Rebound Beach are many fossil beaches, one above the other, preserved because for most of the year the ground is frozen and snow-covered, there is little rain, and very little tidal action. The beaches seen here are like a stereo tape recording (with a stream dividing the tracks) of rising and falling sea levels recorded on a steadily rising, evenly sloped land form. It appears that since the last time the tape was erased, when the ice scrubbed the slope clean over 12,000 years ago, there have been about 20 rises, falls, or hesitations in the sea level, where the rate was different from the steady PGR. A dating of each of these fossil beaches would result in a good record of sea level over the last 12,000 years.
Figure 3 (left) is a map of Finland as of 11,000 years ago. Source. The blue area was water.
Finland 11,000 years ago was mostly sea-bottom with an archipelago of islands. Many place-names in Finland reflect this history with high ground that has island or other maritime feature names. As the land rose, lakes were cut off from the sea, and the Gulf of Bothnia became smaller. The current rate of uplift here approaches 1 cm a year in the northern Gulf. On this map, at the left edge just below center is a narrowing of the Gulf of Bothnia at Kvarken. This narrowing separates Bothnian Bay from the outer Bothnian Sea further south. The water at this point is only about 25 meters deep. Bothnian Bay is already nearly fresh water due to the number and size of the rivers flowing into it. The salt content is now too low to be tasted and there are many freshwater fish species. In about 1500 years uplift will create a further narrowing, reducing the depth to about 10 meters, creating a river flowing south across the Kvarken. At that time Bothnia Bay will be a freshwater lake.
The rising of the bottom of the Bay of Bothnia and the Baltic Sea in general will reduce the volume of the Baltic and force that water into the world ocean, raising the sea level generally. Just to estimate the amount of rise, if the average PGR is 5 mm/year for the Baltic, and the Baltic is roughly 1/1000th of the total ocean area, then the world ocean will rise 0.005 mm/year.
But the Baltic is small compared to Hudson Bay, and Hudson Bay is also rising. The tide gauge at Churchill is rising (sea level falling) at 1.2 cm/year. Hudson Bay is 0.34% of the World Ocean. The PGR here will contribute 0.041 mm/year to general sea level rise. The other waters around the islands of Nunavit in northern Canada will contribute about another 0.004 mm/year, making a total for the Baltic and Canada about 0.05 mm/year.
But the PGR rising is offset by sea bottom sinking. As the earth’s mantle rises, the mantle in the surrounding area must flow down and under to compensate. As can be seen in figure 1, the North Atlantic, the bottom between Newfoundland and Greenland, and the ocean bottom north of Canada, is sinking. These areas are totally under ocean, unlike the rising areas that are mostly land. The sink rate seems to be 3 to 4 mm/year over an area much greater than the rising sea bottoms, which would appear to more than cancel any sea level rise due to PGR. The 0.3 mm/year positive “adjustment” to sea level rise by the UC sea level group does not appear to be justified.
The source for tide gauge PGR data is here.
Much of the material for this article is drawn from Wikipedia here.
The University of Colorado Sea Level website is here.