Marine bivalves (shelled animals such as mussels or clams) provide a fascinating insight into the marine realm over the most recent centuries and millennia. Their annual growth bands, like tree rings, can be used to accurately work out the age of the shell and when it lived. Chemistry of the shells, through the oxygen and carbon isotopes, can give us climate relevant information about the ocean. We use the bivalves as a proxy for temperature and salinity, and to tell us about the water masses at a site and how they have changed over time.
The longest-lived animal known to science is the marine bivalve Arctica islandica, one of which lived for 507 years. Individual bivalves can live for hundreds of years, but the records from different shells that overlap can be combined to create even longer, annually resolved and exactly dated records of marine change. Researchers from University of Cardiff (led by David Reynolds and Ian Hall) and the University of Exeter (led by Professor James Scourse), together with other colleagues globally, have generated multi-centennial records bordering the North Atlantic, the longest of which is over 1000 years of annual continuous oxygen and carbon isotope measurements (North Iceland, https://www.nature.com/articles/ncomms13502).
Climate scientists at the University of Exeter with expertise in physical and biogeochemical oceanography and climate modelling are now helping to interpret these records (Dr Paul Halloran and Dr Freya Garry), with support from the Met Office Hadley Centre. The climate of Europe is dependent on the North Atlantic ocean state, particularly for decadal timescales, and so understanding change in the marine North Atlantic is key for accurately forecasting future climate. The climate and oceans are currently warming but there are natural variations or ‘wiggles’ in the state of the ocean which also need to be understood for climate prediction.
Before the influence of anthropogenic warming, changes in the North Atlantic may have been driven by solar forcing changes, volcanic aerosols, cryosphere-ocean, atmosphere-ocean forcing, internal ocean variability or (most likely) some combination of these factors. Small changes in the total radiation reaching Earth over centennial timescales will change energy injected into the climate system, and in particular the changes to the UV part of the spectrum influence North Atlantic climate via the UV interaction with ozone in the tropics. Temperature anomalies generated in the tropical upper atmosphere change the pole-to-equator temperature gradient and the winds, resulting in surface atmosphere changes, which may drive ocean change over the following years. Volcanic aerosols cool the atmosphere, changes in the amount of ice formed results in different freshwater fluxes in the North Atlantic which can change circulation, whilst chaotic atmospheric forcing can adjust the ocean state. Combining the high resolution data from bivalves together with state-of-the-art modelling helps us understand which of these mechanisms have changed Atlantic ocean climate over the past millennium.
Dr Freya Garry is Post-Doctoral Research Fellow at the University of Exeter, UK. You can contact Freya at firstname.lastname@example.org or follow her on twitter @freyagarry.