Direct dating of fossil bone using Lu-Hf and Sm-Nd geochronometry

Around 250 million years ago 80-95% of all species on Earth became extinct in the largest mass extinction in over 600 million years. This worldwide event affected life on land and in the sea, but we do not yet understand how it occurred. To determine the causes of mass extinctions, it is important to establish their rates and temporal relationships to possible causal events such as global climate change, asteroid impacts and volcanism. Establishing absolute ages of rocks is a fundamental challenge faced by all Earth scientists. The best dating methods are radiometric, where the age of formation of a mineral grain or crystal is determined from the decay of radioactive isotopes within the crystal and growth of their radiogenic daughter isotopes. These isotopic clocks are reset when new minerals form, incorporating the parent isotope. Sediments and sedimentary rocks are largely made from grains that formed elsewhere, so dating sedimentary grains cannot give the age of deposition. Hence, most sediments are not dated directly, but their relative age is assessed by comparing any contained fossils to those from other areas. Unfortunately it is difficult to correlate marine and terrestrial rocks from different parts of the world, as they do not have fossils in common. Consequently the age of many continental sedimentary rocks is poorly known (+/-10 million years at best), and we cannot tell whether Permian/Triassic (P-T) extinctions occurred across the world at the same time or over long periods. We also cannot establish unequivocally the relationship between extinctions and events such as volcanism. If an accurate and precise method for dating continental sedimentary rocks could be developed, robust chronologies could be constructed to study the P-T extinction and other key events in Earth History. Recently, evidence has emerged showing that trace elements, including the naturally radioactive isotopes 147Sm and 176Lu, are incorporated rapidly into bone post-mortem. Bone recrystallises after death on timescales of <100ka, and once recrystallised, behaves as a chemically closed system, allowing no further exchange of elements. The Sm-Nd and Lu-Hf isotopic clocks in fossil bone may thus record the time since recrystallisation. As recrystallisation is rapid with respect to the age of many geological sequences, the age of recrystallisation serves as a proxy for the depositional age of the host sediment. We aim to exploit this new knowledge to develop a geochronometer based on the Lu-Hf and Sm-Nd systematics of fossil bone. Bone is a complex tissue, however, and different regions of the same bone may behave differently after death. Hence, it is important to characterise the rate and nature of trace element uptake, and the chemical stability of each bone before deriving a date. We have identified criteria by which the uptake history of trace elements in bone can be assessed. A pilot study has shown these criteria correctly identify individual bones suitable for dating, and initial attempts to date these bones using Lu-Hf and Sm-Nd methods recovered an age that agreed with the known age to within 1Ma. We will extend this pilot study to determine the conditions under which bone may be used as a substrate for radiometric dating, to test the efficacy of our screening protocols, and ultimately to to establish absolute chronologies for sedimentary sequences. We will assess the accuracy and resolution of our technique by determining ages in bones from sedimentary sequences including P-T sequences whose age has been constrained independently. We will then apply our methods to determine ages for important P-T sequences with poorly resolved dates. Successful development of this technique would provide a method to establish absolute ages of bone-bearing sedimentary sequences, and will provide a chronological framework within which the rates and nature of the most profound crisis experienced by life on Earth can be studied.