Researchers at the Swiss Federal Institute of Technology (ETH) in Zurich as well as the Universities of Zurich and Milan examined the sediments from the Cretaceous period in the “Cismon Apticore” in Northern Italy. They reflect a time in the Earth’s history when the CO₂ levels in the atmosphere were high and world’s oceans had a low content of oxygen. The scientists looked into the causes of this as part of their study.

The world’s seas were also oxygen-deficient, like the Black Sea is today, in certain phases of the earth’s history: sediment deposits such as black shale, which is rich in organic material and is usually a parent rock of crude oil, indicate so-called anoxic events of this kind. The carbon dioxide content of the atmosphere, which was four to six time higher than it is today may have contributed to the low oxygen levels in the world’s seas, and it altered flows of deep water in the oceans.

To date it has not been conclusively determined what mechanisms trigger greenhouse pulses and related anoxic events. Such greenhouse episodes have episodically caused the extinction of numerous species in the earth’s history; such as the event around 251 million years ago, at the transition between the Permian and Triassic period, in which 90 percent of sea life and 70 percent of all life on land was made extinct. It is known that environmental conditions of this kind were accompanied by a rapid change to warm climate.

Volcanic activity or gas hydrates

A possible cause for greenhouse conditions of this kind and the associated anoxic events is the emission of CO₂ from gigantic magma eruptions from the core of the earth, which occur on the surface of the earth nowadays in the form of flood basalts or volcanic provinces. Examples of this are the so-called Deccan Traps in India and the Siberian Traps in Russia, which are up to 3000 metres thick and cover an area the size of Europe.

Another possible mechanism for the temperature increase is the emission of enormous amounts of methane into the atmosphere. According to this hypothesis the methane comes from gas hydrates, clathrate compounds of methane and water molecules, which are stable and similar to ice at low temperatures and when under high pressure. They are present in large quantities on the ocean floor – the estimated energy potential of gas hydrate is many times that of oil, gas and coal. Earthquakes and landslides on the seabed can cause them to become unstable and allow methane to be released.

CO2 and life

The research group of both ETH professors Helmut Weissert and Stefano Bernasconi from the Department of Geosciences examined how the biosphere has reacted to extreme disturbances to the global climate in the history of the earth. “Here the question of why some climate changes triggered large-scale extinction of species and others didn't, is of particular interest to us”, says Weissert. To date there have been no convincing explanations for this. They therefore compare the times of major extinctions with the times when life was better able to react to extreme greenhouse conditions.

The black shale from the “Cismon Apticore” was formed in the Cretaceous period, around 120 million years ago, in what was then the Tethys Ocean. It belongs to one of the two large “Oceanic Anoxic Events” in the Cretaceous period. Conditions favouring black shale deposition lasted about one million years and climate was extremely warm compared to today’s conditions. Carbon isotope graphs generated from the sediment analyses from this period record major changes in the global carbon cycle.

The researchers have now reconstructed the evolution of the carbon cycle at the very beginning of the global ocean anoxia in detail. They measured the carbon isotopic signatures of the inorganic carbon in carbonates, the isotopic signature of the organic carbon as well as the isotopic composition of biomarkers. Biomarkers are fossil molecules, which were synthesized by organisms of marine or terrestrial origin. “The analysis of these biomarkers shows us how the terrestrial and marine biosphere reacted to the increase in CO₂”, explains Bernasconi.

No sign of a methane explosion

The high-resolution graphs covering a time of a few tenthousand years records major changes in both the isotopic record of marine carbonate and of marine and terrestrial biomarkers. It can be split into five intervals, of which the first three are particularly important for scientists. They interpret the first interval, in which the isotopic signatures of the inorganic carbon decrease slightly, and while those of the organic carbon decrease severely, as an indication of a massive input of CO₂ of approximately 3200 billion tonnes as a result of volcanic activity lasting several thousand years. The second interval shows, according to the researchers, a global change in oceanic and terrestrial carbon content as well as continuing volcanic activity and CO₂ emissions.

The decreasing isotope values in the third interval coincided, according to researchers, with the beginning of black shale deposits. “At this time the atmospheric CO₂ concentrations may have reached values of up to 1000ppm”, says Weissert. This can also be deduced from the decreased number of calcareous algae in the sediment. The reduced pH value of the water, caused by high carbon dioxide concentrations, reduced the living space of the calcareous shell-forming organisms and caused some of them to die out.

The scientists interpreted their results to the effect that the primary cause of the extraordinarily rapid rise in CO₂ 120 million years ago was volcanic eruptions, which created the so-called Ontong Java Plateau to the north of the Salomon Isles in the Pacific. “Methane explosions would have left a distinct carbon isotopic signal in both the biomarkers and the marine carbonate. This covariant change was lacking in the geological archives we examined”, states Weissert.

A further study by an international research team published in the same issue of the scientific journal “Geology” underpins the study of the researchers, as they also discovered a direct connection between the formation of the Ontong Java Plateau and the anoxic event in their publication using osmium isotopic measurements.

Literature reference

Méhay S, Keller CE, Bernasconi SM, Weissert H, Erba E, Cinzia Bottini C & Hochuli PA: A volcanic CO2 pulse triggered the Cretaceous Oceanic Anoxic Event 1a and a biocalcification crisis,  Geology (2009), 37, 819-822, doi:10.1130/G30100A.1