British and German scientists have used fossilised shark teeth to determine that the North Sea became fresher and less biodiverse during the changing climate some 4o to 60 million years ago.

The team used the fossils to reconstruct the climate of the North Sea during the Palaeogene period, and concluded that North Sea conditions changed for a brief period during that time when it became isolated from surrounding oceans.

Greenhouse conditions prevailed during the Palaeogene, and mammals were diversifying in the wake of the mass extinction of the dinosaurs and 65 per cent of all other species at the end of the Cretaceous 65 million years ago. The Palaeogene also experienced a brief episode of global warming known as the Palaeocene-Eocene thermal maximum (PETM), which was a rapid climatic disturbance that saw temperatures rise by around six degrees in a 20,000-year period.

Whilst scientists already have a lot of information about the climate on land and in open water during the period, they previously knew very little about the climate of marginal seas like the North Sea. The British and German research team used oxygen isotope data obtained from shark teeth recovered from the London and Hampshire basins, as well as from sites in Denmark, Belgium, the Netherlands and Sweden. Those fossils cover a 33-million-year period during the Palaeocene-Eocene epochs, representing both shallow and deeper water depths.

Shark teeth are continually shed throughout their lives, so they are relatively abundant in the fossil record. Often they are the only part of a shark to be fossilised, with some specimens dating from as far back as 450 million years ago.

When the teeth grow, their oxygen isotope ratio mirrors that of the sea water they are in, so fossilised teeth can be used as a record of palaeo-temperatures. Warmer seas contain more of the heavier isotope, ¹⁸O, as it is easier for the lighter ¹⁶O to be vaporised. Isotope ratios can also be used to determine the salinity of water, as water becomes more salty with increased water evaporation.

The research team’s findings indicate the North Sea was once far less salty than it is today. For a period of between 2 and 4 million years, the ratio of ¹⁸O to ¹⁶ was a substantially lower than the average for sea waters of that period, with the value lower even than some contemporary freshwater lakes.

The period of surface water freshening began close to the PETM. At this time, around 55 million years ago, relative sea-level fall, tectonic uplift and volcanic activity meant that the North Sea was temporarily isolated from surrounding oceans. Tectonic uplift raised Western Scotland by around 2 to 3 kilometres, causing a land bridge to form between the Faeroe-Shetland and Roackall basins.

With circulation between the North Sea and the North Atlantic Ocean restricted, the waters of the North Sea became significantly fresher. This freshening was increased as rivers and precipitation flowed into the isolated North Sea. The result was a significant drop in the diversity of life in the North Sea, with the microorganism foraminifera being particularly affected.

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