The timing and amplitude of Pleistocene sea-level lowstands and deglacials from U-Th chronology of Great Barrier Reef corals (Expedition 325)

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The study of past sea level is of great interest to researchers seeking to understand the Earth's climate. Sea level is a proxy for the amount of water locked up on the land as ice sheets. The study of sea level therefore can tell us about the past state of the climate - low sea levels reflect colder, glacial, periods in Earth history when ice sheets were larger; high sea levels reflect warmer, interglacial, intervals. Our knowledge of how modern ice sheets will respond to anthropogenic warming is not yet comprehensive. One of the most important questions surrounding ice sheet behaviour is the determination of collapse rates in response to warming, which will influence future rates of sea level rise. One way to approach this question is to examine past rates of sea level rise associated with known periods of ice sheets collapse under the non-anthropogenic conditions Quaternary glacial-interglacial transition. Fossil corals provide an indirect measurement of past sea level and therefore answering questions about how (and when) it has changed in the past. Coral only live below sea level and it is also possible to determine how far below sea level they grew. This water depth estimation is based on the types of coral we find and the other fauna and flora we find associated with them. Along with the elevation that we recovered the coral from, this water depth estimation enables us to reconstruct the height of past sea level. The time that this reconstructed sea level represents can also be measured from the coral. We use the uranium (and the product of its radioactive decay - thorium) naturally incorporated into the coral's skeleton during growth to calculate an age for the coral. This U-Th dating technique can find the age of corals from 1 to 600,000 years old. There is, however, a problem in investigating past sea levels which were much lower than present (during the most recent glacial period sea level was c. 120m below modern levels). The corals that grew when sea level was low are hard to access now sea level has risen to its modern level - they are deep underwater. To solve this problem and recover samples from deeper localities, there will be an IODP drilling expedition to the Great Barrier Reef. This Expedition will drill into fossil coral reefs that have been identified by their morphology by a previous site survey. These fossil reefs are at water depths of 44 to 123m and preserve an archive of corals from lower sea levels. It is our plan to take samples from these fossil reefs, use U-Th dating to determine their ages, and then reconstruct sea level for glacial periods while sea level was low. These estimates of low stand sea level will be used to investigate mechanisms that drive the Earth's natural climate system. The timing and rate of changes from low to high sea level are particularly important in this respect, allowing us to test any lead-lag relationship between a potential forcing (changes in the Earth's orbit around the Sun, or internal climate forcing such as atmospheric CO2 concentration) and climatic response (sea level).