Accurate climate reconstruction from fossil corals: The impact of seawater carbonate chemistry on the Sr/Ca, d18O and d11B of coral aragonite

The chemistry of tropical coral skeletons reflects the seawater temperature and chemistry at the time the coral lived and the analysis of fossil coral skeletons offers an excellent route to reconstruct high resolution (~monthly) climate records. These high resolution records are essential for understanding past changes in global climate e.g. for studying the frequency and severity of El Nino in the past, and for testing global climate models for predicting 21st century climate change.

However, despite this potential, it is clear that other factors also affect the chemistry of coral skeletons and these influences need to be resolved if the climate information recorded in fossil skeletons is to be accurately interpreted.

In this research we will investigate if past changes in seawater pH affect the chemistry of coral skeletons. Seawater pH was probably higher in the recent past (over the last 500,000 years), reflecting lower atmospheric CO2 levels. We will culture corals over a range of seawater pH and determine how pH affects skeletal chemistry. We will develop a methodology to correct for past variations in seawater pH, allowing us to reconstruct accurately past seawater temperatures from the chemistry of fossil skeletons.

This research will benefit a wide section of the scientific community studying the causes of past climate variations and predicting future climate change.

Coral skeletons can produce high resolution (~monthly) records of past climates. In particular, coral-based climate records have been used to estimate past seawater temperatures and the frequency and severity of ENSO and interdecadal climate events. However the interpretation of coral records is currently hampered by the observation that many specimens yield SSTs that are significantly cooler than other climate proxies, a phenomenon termed "cold bias". Resolving the cause of cold bias would significantly increase confidence in the interpretation of coral-based climate reconstructions.

Our research will determine how seawater pH affects coral geochemistry and thereby potentially explain cold bias, since oceans in the past ~650 ky had higher pH. This will improve the accuracy of coral-based climate estimates, potentially by several degrees C. This will benefit researchers using corals to produce estimates of past climate. It will also benefit climate modelers. Coral-based records of past climate can potentially be used to produce and test climate simulations which investigate the factors driving climate change. Records of past ENSO can be used to study the response of the coupled tropical Pacific ocean-atmosphere system to particular forcings e.g. Milankovitch, solar insolation and volcanic and aerosol events, while accurate estimates of past seawater temperatures in the tropics are essential to test the sensitivity of the climate system to natural (solar and volcanic) radiative forcing and to predict the potential effect of anthropogenic change on future climate.

Our development of a stringently pCO2 controlled culture system also has a multitude of applications in palaeoceanography (in determining accurately the effects of past seawater pH changes on the chemistry of marine proxies) and in ocean acidification (in determining accurately the effects of future ocean acidification on marine organisms).

To disseminate our results to the academic beneficiaries, we will produce a number of high-profile publications and will present at two major international conferences in years 2 and 3 of the project.

We will also actively disseminate results of our research to a wide public audience. Our research group has a good record of press coverage in the UK and US media and articles on our research have been carried in regional and national newspapers and on the BBC website. We will identify key research highlights and use the University Press Office to release short descriptive pieces. We will identify key images from our research that would provide photogenic and engaging accompaniments to text. We contribute to presentations to the general public as part of Science week, supervise schoolchildren visiting the University of St. Andrews as part of an outreach programme and present science demonstrations in local primary schools.