When dinosaurs roamed Antarctica
Antarctica - icy, empty, desolate, cold - these are words you may use to describe it, but it hasn’t always been that way.
There was once a time when the great southern landmass was covered in forests and dinosaurs roamed free. How could such an icy wilderness once have been so warm that it could support Earth’s most gigantic creatures?
To understand this we have to go back in geological time. Antarctica was ice free during the Cretaceous Period, lasting from 145 to 66 million years ago. That long ago may seem unfamiliar but we know it because it was the last age of the dinosaurs before an asteroid hit the earth and ended their time on this planet.
要回答这个问题，我们必须追溯到远古的地质年代（Geological Time，指地质学或考古学用来划分地球历史的时间单位）。南极洲在白垩纪时期（Cretaceous Period，地质时代中的中生代最后一个纪，开始于1.45亿年前，结束于6600万年前）是无冰的。我们可能并不了解这个离我们如此久远的时期，但我们熟知白垩纪是因为它是小行星撞击地球并结束恐龙在地球上存活的最后一个时代。
During this time period there were forests at both poles. Fossils of trees and cold-blooded reptiles have allowed scientists to build up a picture of what the climate was like. Cold-blooded reptiles need the warmth of the sun to survive; today we see them basking in the sun to warm up during the day. At the poles where the sun disappears during the winter months it must have been warm enough for them to survive through the darkness.
Scientists also use the shells of fossil organisms that lived in the ocean called foraminifera to understand past climate. By analysing the chemistry of their shells and knowing the age intervals when different species lived they can get an estimate of ocean water temperature during that time.
Dr Brian Huber from the Smithsonian Museum of Natural History investigates the Cretaceous with a particular focus on deep-sea sites around Antarctica. He explains; “foraminifera provide some of the best records because you've got both bottom dwelling ones living in the sediments and recording bottom ocean temperatures and then you've got the planktonic ones that live in the top fifty meters of the ocean recording atmospheric temperatures. When you couple those records through time and analyse the shells from different parts of the ocean all over the world, you get a really good idea of the evolution of climate.”
史密森尼自然历史博物馆（Smithsonian Museum of Natural History）的胡贝尔（Brian Huber）博士调查白垩纪时期时，特别关注南极洲周围的深海遗址。他解释说：“有孔虫提供了一些很有价值的记录，因为通过它们你可以得到居住在沉积物中的底栖生物以及海底温度的变化记录，然后你还会得到生活在海洋顶部五十米的浮游生物，后者记录着当时的大气温度。当你把这些记录与时间结合起来并分析来自世界各地海洋不同部分的贝壳时，你就会对气候的演变有一个很好的了解。”
Huber elaborates that what they found in the Southern Ocean around Antarctica was hard to believe at first because it just seemed too warm; “we found temperatures of 30C at 58 degrees south,” close to the Antarctic Circle.
These high temperatures occurred during the middle of the Cretaceous known as the ‘Cretaceous Hothouse’ - a hot greenhouse effect caused by increased carbon dioxide in the atmosphere. But what happened in the Cretaceous to create a world where there were trees and dinosaurs roaming Antarctica unlike the barren ice fields of today?
Huber explains; “what we know about the mid-Cretaceous in particular is that we had much faster rates of sea floor spreading and so more volcanic sources of CO2.” Huber and colleagues are still investigating whether the ‘hothouse’ occurred as a result of a major amount of volcanism erupting CO2 and creating a greenhouse blanket that warmed the earth.
We all know the climate changes, it has in the past, it is changing now and it will in the future, but what is different about what we are doing now compared to what happened in the Cretaceous? Could Antarctica be ice-free again soon?
“It's really an unprecedented rate and magnitude of change compared to geologic events that we know of from the past. We're releasing hundreds of billions of tons of CO2 into the atmosphere in just a matter of decades. Volcanoes can't produce that amount of CO2 in such a short time span even if they are huge volcanoes;” says Huber.
In terms of the future, Huber suggests; “I think what we'll see possibly in decades, maybe centuries is what are called ice streams that start flowing faster and it could be that West Antarctica in particular starts to deglaciate. Given the rate at which ice flows, we won't see [the whole of] Antarctica deglaciate in a matter of decades. Glaciologists predict that once you start raising sea level you start getting a positive feedback where that allows ice to flow faster and sea level rises faster, so then it just sort of keeps going. So yes I think the signs are there already.”
We may not have dinosaurs roaming Antarctica again, but we can’t rule out it being ice free in the future, and we have no way of knowing what that would be like for humans as we have never lived on Earth when there wasn’t ice at the poles.