Paradise Harbour Paradise Harbour Chinstrap penguins Paradise Harbour Neko Harbour <strong>22</strong> FEB- 2 March <strong>2020</strong> All photos © Sandra Petrowltz Volume 2, Issue <strong>14</strong>
When and How the Earth Got Cold David Macdonald, Lecturer (Geology) & Expedition Guide <strong>The</strong> Earth’s climate has two end-member states: greenhouse and icehouse. In a greenhouse climate, there are no polar icecaps (although there may be valley glaciers in high mountain areas) – the climate of the Cretaceous Period (<strong>14</strong>4-65 million years ago) is a typical greenhouse. We are currently in an icehouse climate, since there are icecaps at or near both poles. Although life on Earth goes back 3.5 billion years, the main expansion in numbers of species and hence of easily found fossils occurred 540 million years ago. During the time from then until now, greenhouse climates have dominated, with three periods of icehouse climate, lasting a total of about 100 million years. Our current icehouse period began abruptly 35 million years ago, with formation of an icecap in <strong>Antarctic</strong>a. Why did it happen then, and why did it have such an abrupt beginning? Water temperature (°C) <strong>Antarctic</strong> convergence October 2019 8 6 4 2 0 54 56 58 60 62 64 Latitude (°S) 18 Figure 2: Temperatures in the Drake Passage from Friday 25 October to Sunday 27 October 2019 as Ocean Atlantic sailed south across the <strong>Antarctic</strong> Convergence, where the sea temperature falls below 4°C. It was the severing of the link between the <strong>Antarctic</strong> <strong>Peninsula</strong> and Tierra del Fuego that allowed deep cold water to circulate around the planet at 50-60°S and thermally isolate <strong>Antarctic</strong>a from the rest of the world. This situation continues today (Figure 1) <strong>The</strong> key area for this was the Drake Passage, which is the western end of the Scotia Sea (Figure 3). Geological and geophysical studies of the sea floor show that the <strong>Antarctic</strong>a-<strong>South</strong> America link was severed by the growth of ocean crust, beginning 35 million years ago. Opening of this deep-water gateway cooled the planet and turned <strong>Antarctic</strong>a into the white continent. Figure 1: <strong>The</strong>rmal structure of the <strong>South</strong>ern Ocean showing the position of the <strong>Antarctic</strong> Convergence (Polar Front) at the junction of the dark blue and mid blue shading. This is the line of the 4°C isotherm, where <strong>Antarctic</strong> surface water plunges below cold temperate water. <strong>The</strong> first, and most important factor was that we had a polar continent. <strong>Antarctic</strong>a was in roughly its present position over the <strong>South</strong> Pole, so would have had strongly differentiated winters and summers. However, although the former supercontinent of Gondwana had largely broken up by then, there was still a land bridge to <strong>South</strong> America and <strong>Antarctic</strong>a was still forested, probably with a migratory fauna. Warm currents bathed <strong>Antarctic</strong>a’s shores and, 35 million years ago, the temperature of the <strong>South</strong>ern Ocean was a relatively mild 6°C. <strong>22</strong> FEB- 2 March <strong>2020</strong> Figure 3: <strong>The</strong> Drake Passage and the Scotia Sea formed from 50 million years ago, when there was slow extension between <strong>South</strong> America and the <strong>Antarctic</strong> <strong>Peninsula</strong> which stretched the crust and allowed surface waters to circulate through this former land bridge. <strong>The</strong> temperature of the southern Ocean fell from 12°C to 6°C between 50-35 million years ago, then abruptly fell to 0°C when the deep water gateway of the scotia Sea opened, sundering the link between <strong>Antarctic</strong>a and <strong>South</strong> America and allowing continuous circulation of deep water, thermally isolating <strong>Antarctic</strong>a. Volume 2, Issue <strong>14</strong>
Change language