The evolution of Chalk Sea ecosystems: biodiversity, resilience and ecological function in a warming world

The amount of carbon dioxide in our atmosphere is already at a level last recorded millions of years ago, and is steadily rising. As our planet continues to warm, scientists are increasingly turning to the fossil record to help understand what marine ecosystems look like and how they were able to function under extreme climate change. One past warming event occurred in the Late Cretaceous about 94 million years ago and is recorded in the British Chalk. Known as the Cenomanian-Turonian Boundary Event, it led to global extinctions and the highest sea levels of the past 250 million years. We will study how marine ecosystems responded to, and were shaped by, this event in unprecedented detail, using the vast and untapped chalk fossil collections of the Natural History Museum, coupled with new fieldwork and a novel method of measuring past temperatures.

From the white cliffs of Dover and the rolling downs of southern England, west to Devon and north to Yorkshire, the chalk is an iconic and important part of the British landscape. The rock we see today is made up of the microscopic skeletons of fossil plankton (nannofossils) that lived in the Late Cretaceous Chalk Sea. When they died, their tiny skeletons drifted down to the seafloor forming an ooze that gradually accumulated over time and turned into rock. The skeletons and burrows of other Chalk Sea species were also fossilised, providing a unique record of the entire ecosystem; from the tiniest plankton to the largest apex predators such as sharks and marine reptiles. Owing to its use in building and other industries, numerous chalk pits and quarries were excavated across the UK. These provide a dense network of study sites, enabling us to see how Chalk Sea ecosystems changed in space and time in far more detail than for any other past warming event.

Fossils from these sites have been collected for over 200 years and most are housed in the Natural History Museum. One reason why this 'whole ecosystem' archive has not been studied before is that most specimens collected in the 1800s lack details of exactly which part of the chalk they came from, and whether they were alive before, during or after the warming event. We have shown recently, however, that it is possible to extract dust-sized nannofossils from the chalk rock that still adheres to the larger fossils, and to use these plankton to date the specimens. This opens up the fossil collections for study in a way that has not been possible before.

We will also undertake new field studies of key sites around the UK to study detailed bed-by-bed changes; counting and identifying all the fossils present to help us understand the whole ecosystem. Usually, such studies only focus on one fossil group, such as ammonites or foraminifera, but we will collect information on everything so we can show how the entire Chalk Sea ecosystem changed through the warming event.

Determining the temperature of the Chalk Sea is our final challenge. Traditional techniques require assumptions about the chemical composition of past seawater - something that cannot be known for certain. Instead, we will apply a recently developed chemical technique, called clumped isotope palaeothermometry, to measure the bonds between different, rare, heavy isotopes within the well-preserved shells of fossil animals. These isotopes tend to 'clump' together as temperature falls, and so the bonds between them provide a direct measurement of temperature at the time the shell was formed. By analysing individual growth bands within the shells we will reconstruct seasonal changes across several years, showing how local winter and summer temperatures change with global warming.

As well as having the first, detailed study of how Chalk Sea ecosystems changed in response to past warming, we will also compare our findings to projections of how current marine ecosystems might change in response to present-day warming; using the past to test predictions of future change.