Sea level from fossil corals
Knowledge of the level of the oceans in the past is of particular interest to a wide range of fields with Earth Sciences. Vertical rates of land movement in tectonically active areas or the subsidence of ocean islands, such as Greece or Hawaii, are important constraints for our understanding of lithospheric deformation. Where the motion of the land can be constrained, measurements of past sea level markers allow local sea level changes to be determined. Determining changes in sea level, their magnitude and timing is important for understanding the relationship between climate and continental ice volume and for discriminating between potential drivers of past climate change.
Corals provide a useful sea level marker because they are exclusively marine organisms, so provide an absolute constraint that sea level must not have been bellow the position of the coral when it was alive. Additionally many corals have a limited water depth habitat, and therefore provide some constraint as to how much higher sea level could have been above the coral when it grew. In reality extra information is used to narrow the possible water depth the coral lived in and hence reduce the uncertainty in the sea level elevation reconstruction. This evidence could include: The sedimentary facies the coral was found; the assemblage of other organisms such as benthic foraminifera and coralline algae; evidence for photic zone dependant bioerosion; and the geomprphology of the deposit the coral was recovered.
To place these markers in the past, corals also prove particularly well suited. Their calcium carbonate skeleton offers up the potential for carbon dating (up to 30-40,000 years), because it is 12% carbon. Higher precision, and longer ranging, chronology can, however, be determined through uranium-thorium dating. When the coral precipitates its skeleton the calcium carbonate incorporates small quantities of natural uranium from the surrounding seawater (typically 3 parts per million). This uranium is locked into the skeleton and as the fossil coral gets older the uranium decays to thorium. Using trace metal free sample preparation, chemically isolating the uranium and thorium, measuring the thorium/uranium ratio by mass spectrometry allows the age of the fossil to be determined.
Integrated Ocean Drilling Program (IODP) coral drilling expeditions to Tahiti and the Great Barrier Reef
To recover corals that grew at times when sea level was lower then at present it is necessary to drill into the sea floor at depths of up to 140 meters below sea level. Two IODP expeditions to Tahiti, French Polynesia, and the Great Barrier Reef, Australia recovered such material. In both expeditions holes were drilled into the sea bed at a range of depths to recover corals that grew during the rise of sea level from the last glacial maximum (20-26,000 years ago) when sea level was approximately 120 m below present.
In addition to recovering corals that recorded the sea level rise of the last deglaciation, corals were recovered form earlier periods of lower sea level from reefs that were preserved underneath the postglacial sequence. At Tahiti these older corals have proved invaluable in determining the timing of the penultimate deglaciation, and in providing some insight into the longer term geological history of the island and constraining the subsidence rate.