Deep-sea heat: now there's the rub
About 55 million years ago and over a period of less than 10,000 years, the Earth warmed up by 5 to 9 degrees C. It was so hot, in fact, that crocodiles frolicked in Canada.
But at the same time, the most severe extinction of deep-sea foraminifera recorded in the last 65 million years took place. Why? A new study published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology sheds light on the mystery.
Dr Laia Alegret and colleagues at the University of Saragossa in Spain pored over ancient marine sediments containing the remains of foraminifera, tiny creatures which build shells from calcium carbonate.
Previous studies had tentatively put the unfortunate fate of forams 55 million years ago down to ocean 'acidification': perhaps rising levels of carbon dioxide reduced the amount of calcium carbonate available to shell-builders?
But according to Alegret's experiments, foraminifera did not fall foul of a drop in the ocean's pH. Nor did they die from a lack of oxygen. What killed 'em, scientists now say, was the heat.
Indeed, the team thinks that deep-sea heat (the ocean bottom reached 15 degrees C, compared with 4 degrees C today) directly affected the metabolism of these coolth-loving creatures, impairing their ability to feed, reproduce and convert carbon into shells.
'The exact causes of the extinctions are not clear, yet', notes Dr Alegret, 'but they are likely to be related to paleoecological and paleoenvironmental instability triggered by global warming.'
Many scientists believe the past is the key to the future: could the impact which climate change had on life 55 million years ago act as a guide to the future, given that the Earth could warm by up to 4 degrees C by 2100?
Not necessarily, warns Dr William Hay, a geologist. 'The past climates of the earth cannot be used as a direct guide to what may occur in the future'.