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Darwin PhD research: North Sea subtropical and anoxic 56 million years ago
Past increases in atmospheric CO2 concentrations have led to acidification of the ocean and global warming. Petra Schoon of the Royal Netherlands Institute for Sea Research (NIOZ) found that these climate events had a large impact on the climate of Northern Europe, such as warming of 5-8˚C and removal of most of the oxygen from the water of the North Sea. 

In addition to the more acknowledged consequences of climate change, such as global warming, the current human-induced increase of anthropogenic CO2 into the atmosphere is also responsible for a change in the chemical composition of seawater. Since the start of the industrial revolution, approximately 50% of the emitted anthropogenic atmospheric CO2 is taken up by the oceans. These enhanced concentrations of aquatic CO2 is responsible for an increase of the seawater acidity leading to ocean acidification, also known as “the other CO2-problem”. The environmental consequences for the impact of the rise in CO2 are not clear yet. One way to assess this issue is to look at past time periods of geologically rapid increases in atmospheric CO2 and increase in ocean acidity.

Schoon studied several climate events, including the Paleocene-Eocene Thermal Maximum (PETM; ~56 Million years ago). The latter event is associated with a massive injection of CO2 into the atmosphere, most likely due to the release of methane hydrates. This led to large scale acidification of the ocean as evidenced by severe dissolution of chalk deposited on the ocean floor. To gain more insights into the effect on climate and environment in Northern Europe during the PETM, she studied several sediments now located in Denmark but at that time deposited in the North Sea basin.
Schoon found strong indications that right at the PETM the North Sea became completely devoid of oxygen, i.e. anoxic. This was based on the presence of isorenieratane, a molecule made by bacteria which need light to survive but cannot withstand high oxygen concentrations. Prior to the PETM there was plenty of oxygen but at the onset of global warming, most of the oxygen was removed from the North Sea due to warming and consumption by microbes. Using another set of fossil compounds she was able to reconstruct water temperatures of the North Sea which rose from ca. 24 to 30˚C at the onset of the PETM, much warmer then present day average North Sea temperatures of 11˚C. A similar warming was found for air temperatures of Northern Europe, which rose from ca. 19 to 24˚C. Based on stable isotopes of specific fossil compounds she estimated that atmospheric CO2 concentrations may have been 2 to 3 times higher than today.