Ice from Antarctica is supposed to solve climate puzzles

Wer uralte Rätsel lösen will, muss tief bohren. Für Pascal Bohleber und seine Kollegen aus zehn europäischen Ländern gilt das im Wortsinn. Jeden Tag bekommt Bohleber ein kurzes Memo über den Stand des Projekts „Beyond EPICA – Oldest Ice“. Dessen Hauptschauplatz ist ein Ort namens „Little Dome C“, mitten in der Antarktis gelegen, Jahresdurchschnittstemperatur minus 52 Grad Celsius. Der Eispanzer, der über dem Gestein des Kontinents liegt, hat dort eine Dicke von mehr als 2700 Metern. In ihn dringen die Wissenschaftler derzeit mit schwerem Gerät ein. „Die Bohrung ist jetzt bei knapp 2100 Metern Tiefe angelangt“, berichtet Bohleber. „Jetzt kommen wir hoffentlich in die Schichten, um die es uns geht.“

Das Eis knapp über dem felsigen Grund, für das sich der Glaziologe interessiert, könnte bis zu 1,5 Millionen Jahre alt sein. Aus Sicht der Forscher ist es ein natürliches Archiv, das helfen könnte, die Ursachen einer einschneidenden Klimaveränderung in prähistorischer Zeit aufzuklären – und besser zu verstehen, was in Zeiten des menschengemachten Klimawandels mit der Atmosphäre geschieht. Seine Erkenntnisse hierzu wird Bohleber, Senior-Wissenschaftler des Alfred-Wegener-Instituts in Bremerhaven, künftig mit Kollegen und Studenten der Goethe-Universität teilen. Gerade hat der 1981 geborene Physiker dort eine Kooperationsprofessur angetreten; er wird in Frankfurt Lehrveranstaltungen für angehende Geowissenschaftler anbieten und mit Forschern des Fachbereichs zusammenarbeiten.

Unter Fachleuten ist das Rätsel, dem sich das „Oldest Ice“-Projekt widmet, als Mittelpleistozän-Übergang bekannt. Bis vor etwa einer Million Jahren kam es auf der Erde ungefähr alle 40.000 Jahre zu einer Eiszeit, bei der die Pole mit einer – damals verhältnismäßig dünnen – Eiskappe überzogen wurden. In den wärmeren Zwischenphasen taute das Eis zumindest in der Arktis weitgehend weg. Dann aber verlängerte sich die Periode der Eiszeiten auf etwa 120.000 Jahre, gleichzeitig wurde es insgesamt kälter.

It is currently unclear what caused this cooling. “We know that the Earth’s orbit around the sun has not changed during this time,” explains Bohleber. “So something fundamental in the climate system itself must have changed. Volcanic eruptions are unlikely to be the cause, but rather complex feedbacks in the Earth system that lead to larger ice sheets.”

The drill cores that European researchers are currently collecting in Antarctica could reveal what feedback these were. The ice columns, which are brought to light from ever deeper layers, are sawn into pieces approximately one meter long, shipped frozen to Europe and distributed to the institutes involved in “Beyond EPICA”. The first samples are expected to arrive in the laboratories over the next year.

Bohleber's working group will search for contaminants in the ice that can shed light on the causes of prehistoric climate change. The method of choice here is mass spectrometry, which can be used to identify various chemical elements based on their mass. The method can also be used to determine the amount in which the elements are present in the sample.

Wolfgang Müller is a specialist in such analyzes at the University of Frankfurt. He will therefore work closely with Bohleber. Müller, professor of geology and paleoenvironmental research, has so far examined, among other things, ice cores from Greenland. To do this, he uses an apparatus that, according to the Goethe University, is unique in the world: a so-called cryo-laser ablation system that is combined with a mass spectrometer. It bombards the ice sample with laser beams to blast off the finest layers and achieve high resolution when analyzing the elements. “The past time is not represented linearly in the drill core,” explains the geologist. “It compresses significantly downwards: more than 14,000 years can be represented in one meter. With our methods we enable analysis down to the sub-millimeter range.”

Bohleber and his colleagues want to compare the signals in the ice before and after the Middle Pleistocene transition: “How much dust reached Antarctica and where did it come from?” The elements from the dust particles can provide clues about what happened a million years ago happened on earth. Sulfur, for example, can be an indication of volcanic eruptions, and the amount of sodium provides evidence of the expansion of sea ice, even if there is no simple connection here, as Bohleber emphasizes. Calcium, in turn, could come from mineral dust, but also from sources in the sea. The amount of this element in the samples allows conclusions to be drawn about the temperature, as Müller explains: “When it is cold, less precipitation falls. As a result, the environment is dustier and windier, and there is more calcium in the ice during cold periods.”

The evaluation of the drill cores from Antarctica is likely to provide material for a large number of scientific publications; The first publications are expected as early as next year. According to Bohleber, the overarching goal of “Beyond EPICA” is a “general understanding of how the climate system works.” This could also make it clearer what lies ahead for humanity in view of current global warming. According to Müller, a look into the past already shows how extraordinary the current changes are. The concentration of the greenhouse gas carbon dioxide was last as high as it is now 30 to 40 million years ago. And even at the time of the Paleocene-Eocene temperature maximum 56 million years ago, the peak of the warm period at that time, the change occurred ten times slower than today.

“The earth as such has no problem with such a process,” says Müller. 650 million years ago it was once a “snowball” and then thawed again. But one look at the most recent flood disasters is enough to realize: “As a complex human society, we cannot cope with even smaller changes.”