Gwaii Haanas National Park Reserve and Haida Heritage Site

Sea Level Changes

"The rate of sea level change in the Hecate Strait is one of the fastest observed to date, worldwide," says Heiner Josenhans, a researcher from the Pacific Geoscience Centre. He's collecting information about the depth, geology and underwater features of the ocean floor in the shallow strait to learn more about past changes in sea level. Using sophisticated electronic scanning devices (side-scan sonar and sub-bottom profiles), scientists have found islands, river beds, lake basins and former beach terraces that now lie under 150 meters of sea water. These recent findings give strong support to the concept that extensive areas of the continental shelf were exposed during the time when people were moving along the coast. The evidence shows that more than half of the width of Hecate Strait was dry land up to 13,000 years ago.

Sixteen thousand years ago, a great glacial ice field covered half of North America. At that time sea levels on the inner continental shelf, including Hecate Strait, were at least 100 meters below present levels. At the same time, the areas at the upper ends of mainland fjords were submerged by up to 200 meters. The tremendous weight of ice sheets on the mainland pressed the crust down, resulting in the displacement and further lifting of the Queen Charlotte/Haida Gwaii archipelago. Evidence for this east-west tilt was obtained from the studies in Hecate Strait. In an ancient lake basin that existed for several hundred years (now under Hecate Strait, east of Lyle Island) the sediments indicate that sea levels at this location fell to at least 170 meters below present levels. At a former coastal beach terrace 50 km east of the lake, deposits indicate that sea levels during the same time period were 110 meters lower than at present. A minimum east-west tilt of 60 meters, over a horizontal distance of 50 km is indicated by these findings.

Sediments taken from underwater features have been used to plot landscapes now submerged. Core samples taken from a lake basin show typical lake sediment deposits and microfauna, switching abruptly to marine sediments about 10,000 years ago. From an analysis of pollen, seeds and twigs in the sediment, it has been determined that prior to about 12,000 years ago, the lake was surrounded by tundra-like landscape covered with grasses, sedges and dwarf willow. The source of the pollen within these sediments would have been from the surrounding vegetation species at the time, since pollen from these species does not travel great distances.

Sonar data has identified features in Hecate Strait that show a step-wise retreat of the glaciers south of Gwaii Haanas. Similar ice retreat features have been identified in all of the major troughs that indent the continental shelf. Seismic profiles were undertaken in a number of locations to determine the maximum seaward extent of glaciation, and samples were taken to obtain dateable material that would define the age and rate of ice retreat from the continental shelf. Offshore evidence of fluctuations in sea levels has correlated well with information obtained from upland sites in Gwaii Haanas.