Comparison of albedo changes due to shelf exposure and ice during the Late Ordovician glacioeustatic regression

Abstract

In a glacioeustatic regression the amount of exposure of the continental shelves and the size of the ice cap are intimately related. We use a numerical model to compare the effects on planetary albedo of the formation of ice caps and the exposure of continental shelves due to an eustatic regression such as was the known case in the Late Ordovician. To compare the two effects in a meaningful way, we link them by relating the size of the ice sheet to the change in volume of the ocean and thus to the exposure of the shelves. For the conditions of the Late Ordovician (Caradocian and Ashgillian) the change in albedo from the exposure of continental shelves and epeiric seafloor is many times greater than that due to the formation of ice (up to Δαs = 0.0233 versus Δαs = 0.0009, 25% versus 1% of the global mean surface albedo; Δαp = 0.0079 versus Δαp = 0.0001, 2.6% versus less than 0.1%). The change in planetary albedo is enough to lower the earth's effective temperature by up to three quarters of a degree Kelvin. We extend the hypothesis that the Ashgillian ice age was intensified and possibly even triggered by the exposure of extensive areas of epeiric floor during the Caradocian. The effect is more pronounced for the Ordovician than more recent times due to the differences in continental positions, today's lack of epeiric seas, differences in atmospheric composition, and the absence of land plants.