Joanne Ballard and André Bijkerk (USA)
In this article, we will argue that the extinction of megafauna on the mammoth steppes of the Northern Hemisphere may ultimately have been caused by the release of massive quantities of methane in the North Atlantic Ocean at the Amazon Fan near the Brazilian coast and also from the Ormen Lange gas field off the coast of Norway. We will suggest that these events caused significant changes in the flow of water at the surface of the ocean that, in turn, led to very rapid changes in the levels of rainfall.
Scientists have already recognized that increased precipitation gave rise to changes to ecosystems (or, more precisely, to biotopes) that destroyed the mammoth steppe. However, much of the evidence we will use in this article to support our argument has been used to support other sorts of explanation for the extinction. Therefore, this primary evidence now appears to be in need of revision.
About 11,000 years ago, all of the remaining herds of mammoths suddenly disappeared. During the Pleistocene, these mammoths once thrived on a vast, megafauna steppe stretching across the Northern Hemisphere. It may have resembled the African steppes of today with lions, hyenas and several species of large grazers being present. However, the debate about the cause(s) of the extinction continues. In North America, things appear to be simple – the appearance of early humans on that continent seems to coincide with the downfall of the megafauna. However, there are also profound discrepancies in the details.
In Siberia, where the woolly mammoth was one of the predominant species, very few humans were around, yet the extinction was just as rapid as in North America. Moreover, there are clear signs of rapid and drastic climate changes in Siberia. A sudden shift in the climate changed the dry, treeless, mammoth steppe into marshes, swamps and forests probably covered with thick snow in winter – a habitat completely unsuitable for mammoths. However, the big question is, “What caused these sudden habitat changes?”
Ice core observations
To answer this question, a thorough review of the literature on paleoclimate is required, particularly articles concerning the time around the end of the Pleistocene. This takes us first to the ice cores of Greenland, where we see rapid changes. Figure 1 shows how the heavy oxygen isotope ratios (δ18O) of the snow appear to follow a roller coaster ride. (These isotope ratios are referred to as “proxies” that are measurements that can be used to represent something else, in this case, paleoclimate change.) We also see that, when the isotopes go up at the beginning of the Bølling Allerød interstadial (about 14,500 years ago) as well as the Preboreal, which is the start of the current epoch referred to as the Holocene (about 11,700 years ago), the snow accumulation and the methane concentration do as well.
These graphs have become rather familiar in climate change studies and the prevailing viewpoint is that rapid changes in global temperature caused the changes in rainfall. In turn, this caused swamps to proliferate, which then increased the methane in the atmosphere. However, there are some problems with this view. We know that the Siberian mammoths appear to have thrived in the alleged cold areas existing at the times of the Last Glacial Maximum (about 18,000 to 22,000 years ago) and during the Younger Dryas (about 12,750 to 11,7oo years ago).
And, of course, woolly mammoths do not have that name for nothing! They must surely have been capable of enduring extremely cold winters. Nevertheless, the grass of the steppes needs a certain minimum temperature to grow and to provide 150 to 300 kg of fodder per mammoth each day, all year round (on the assumption that their metabolism was comparable to closely-related modern elephants). This fact alone assures us that the summers during these periods were not that cold.