Understanding glacial behaviour
With his new method, Gowan has now reconciled sea level and glacier mass: according to his calculations, the sea level at the time was ca. 116 metres lower than it is today. Based on his approach, there is no discrepancy in terms of glacier mass. Unlike the previous global model, Gowan closely examined the geological conditions in the glaciated regions: how steep was the ice surface? Where did glaciers flow? How much did the rocks and sediment at the base of the ice resist ice flow? His model considers all of these aspects. It also takes into account to what extent the ice sheet pressed down on the Earth’s crust in the respective areas. “That depends on how viscous the underlying mantle was,” Gowan explains. “We base our calculations on different mantle viscosities, and therefore arrive at different ice masses.” The resulting ice masses can now be reconciled with the sea level without any discrepancy.
The established model is flawed
The recent article by Gowan and his team critically re-examines the long-established scientific method used to estimate glacier masses: the oxygen isotope method. Isotopes are atoms of the same element that have different numbers of neutrons and therefore different masses. Oxygen, for example, has a lighter 16O isotope, and a heavier 18O isotope. According to conventional theory, the lighter 16O evaporates from the oceans, while the heavier 18O remains in the water. Accordingly, during glacials, when large inland glaciers form and the volume of water in the oceans decreases, the 18O concentration in the oceans should increase. However, as has been shown, this established model produces discrepancies when it comes to reconciling sea-level height and glacier masses for the period 20,000 years ago and earlier. “For many years, the isotope model has been frequently used to determine the ice volume of glaciers up to several million years ago. Our study calls into question the reliability of this method,” says Gowan. His aim is to now use his new method to improve the traditional oxygen isotope method.