231Pa/230Th and Paleoceanography
Both 231Pa and 230Th are intermediateary nulides produced, in the decay chains of 235U and 238U, on the way to stable lead. Beacuse of the long residence time of uranium in the oceans the concentration and ratio of the isotopes of uranium are aproximatly constsnt. This leads to the production of 231Pa and 230Th at a constant ratio (0.093) through time. Once produced the two nuclides are of different elements and thus have subtle diffrences in thier chemical behaviour. The principal differnce is that while the two elements are very insoluble in seawater Pa is slightly more soluble. It is this difference in solubility that allows the ratio of 231Pa/230Th to be used as a proxy for precesses that occour in the oceans.
U/Th dating of corals
The decay of 238U to 230Th (and then on to lead) provides an absolute chronometer for Quaternary science. Corals are an ideal media for the use of this chronometer because, when they grow, their skeleton incorporates uranium while thorium is excluded. This creates a disequilibrium in the decay chain which is restored, as the 238U decays to produce of 230Th, towards secular equilibrium. The rate at which equilibrium is restored is principally dependant on the half-life of the daughter 230Th (75,000 years). The presence of 234U (half-life, 245,000 years), intermediate in the decay chain between 238U to 230Th, can extent the useable range of the chronometer from 0 to 600,000 years if it is present in excess when the clock is started. For corals, which grow from seawater, this intermediate 234U has the added benefit that the excess of 234U in seawater is approximately constant over time at 1.145 times secular equilibrium. This provides a “two clock” chronometer to check the accuracy of the ages produced.
Dating fossil corals has many rewards. Their growth position with respect to sea level enables the reconstruction of local sea level history (if any relative motion of the land can be corrected for). Corals are also excellent archives of past climate. Many corals form annual layers, sometimes up to a centimetre thick. The chemical and isotopic composition of these layers has been shown to vary because of environmental conditions such as temperature, salinity and nutrient utilisation. Dating these high-resolution proxy climate records allow us snapshots into past climates. Dating corals with U/Th and radiocarbon allows the calibration of radiocarbon measurements beyond the available tree ring chronology.
U/Th dating of other carbonates
In some settings corals are not available to provide U/Th chronology. This may be because of the distribution of shallow water corals is limited to the tropics, or that the corals have undergone diagenesis and have not remained a closed system. For these reasons we are investigating whether other carbonates may provide robust U/Th chronometers. One possible candidate is microbialy precipitated carbonate (microbialite), which is abundant in deglacial reef sequences in the Pacific. The dense structure and calcitic mineralogy of microbialite may make it less susceptible to remineralisation than aragonitic corals. Microbialites do however incorporate a large amount of detrital material into their structure. This brings with it 230Th which must be accounted for and separated from the 230Th produced from the decay of 238U. If this can be done, then U/Th dating will be possible for microbialites.
My Dphil
My DPhil focused on the development of methods to measure 231Pa and 230Th in seawater and sediment samples using MC-ICP-MS. I then applied these techniques to measure samples from the Indian Ocean. Using measurements of water samples and a simple particle scavenging model I demonstrated that when the 231Pa and 230Th is transferred into the sediment from the water column the nucludes are last scavenged onto particles in the bottom few 100s of meters above the seabed. This is because of the rapid nature of the reversible scavenging. This is important to consider because the water column concentration profile of 231Pa, shows variability which may not be reflected in the sedimentary record because the 231Pa/230Th of the bottom water dominates the “signal&rdlquo;.
Down-core measurements of 231Pa230Th from the deep Indian Ocean have shown that the flow rate of water from the Southern Ocean into the Indian Ocean has remained constant over the last glacial cycle (140,000 years). This is in marked contrast to the circulation in the Atlantic Ocean, which has been shown to be highly variable on this timescale.
My Msci
My Msci research project (supervised by Tim Elliott, at the University of Bristol) was to set up the measurement of lithium isotopes from small amounts of volcanic rocks, and use those measurements to investigate recycling of surface material into the Earth’s mantle. Because of the large relative mass difference between the two isotopes of lithium (6 & 7) large fractionations can occur during physical and chemical processes. These large fractionations result in different reservoirs of lithium within the Earth having characteristic isotopic signatures. Seawater and altered oceanic crust are enriched in lithium-7 while dehydration reactions can leave the oceanic crust enriched in lithium-6. By measuring the products of mantle melting (Mid Ocean Ridge Basalts) the lithium isotopic composition can tell us which of the subducted components are recycled through the mantle.
