Shark teeth give clues to history of North Sea
An analysis of fossilised shark teeth reveals that the North Sea was once cut off from the rest of the world's oceans. For a period of 2 to 4 million years, salinity fell until the sea was less salty than many lakes, resulting in a dramatic loss of biodiversity. The findings, by a team of scientists in Germany, Oman and the UK, are published in the Journal of the Geological Society. The aim of the study was to reconstruct the climate of the North Sea during the Palaeogene period, between 40 and 60 million years ago. Just before the start of the Palaeogene, 65% of all species (including the dinosaurs) were wiped out, probably by a massive meteor slamming into the Earth 65 million years ago. The Palaeogene therefore marks an important period of diversification for the mammals. The climate at this time was relatively warm; yet although scientists have discovered much about the climate on land during this time, less is known about what was going on in marginal seas like the North Sea. The scientists turned to sharks for their research. Sharks shed their teeth throughout their lives, continually growing new ones to replace those that are lost. As a result, shark teeth are fairly common in the fossil record, and examples dating back 450 million years have been found. Depending on the species and the age of the individual shark, the time taken to grow a new tooth ranges from several days to several months. Crucially, the chemical makeup of new teeth reflects the chemical composition of the seas where the shark lives. In this case, the researchers were particularly interested in the oxygen isotopes found in the ancient teeth. Warmer seas contain more of the heavier oxygen 18 isotope (18O) because the lighter oxygen 16 isotope (16O) evaporates more easily. By extension, sharks living in warmer seas have a greater proportion of 18O in their teeth than sharks living in colder seas. The oxygen isotopes also provide information on the salinity of the seas - water becomes saltier as more water evaporates. In this study, the team studied shark teeth found at sites in Belgium, Denmark, the Netherlands, Sweden and the UK. The teeth ranged in age from 65 to 32 million years. The teeth came from both surface and bottom-dwelling sharks, although most came from the sand tiger shark, which today lives in tropical to warm temperate waters. Analyses of the oxygen isotope ratios revealed that around 55 million years ago, the 18O to 16O ratio was much lower than normal for sea waters of that period, indicating that the salinity of the ancient North Sea had dropped significantly. This phenomenon coincides with a brief episode of global warming called the Palaeocene-Eocene Thermal Maximum, or PETM, when temperatures rose by around 6°C in 20,000 years. At the same time, sea levels fell and tectonic uplift caused western Scotland to rise up by two to three kilometres, creating a land bridge between the Faroe-Shetland and Rockall basins. As a result, the flow of water between the North Sea and the Atlantic was restricted. Meanwhile, freshwater was being pumped into the North Sea from surrounding rivers, leading to a drop in salinity of the now-isolated sea. As a consequence, the diversity of life in the North Sea dropped. The researchers estimate that the period of isolation lasted between 2 and 4 million years.
Countries
Germany, United Kingdom