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

大约2.5亿年前，地球上80% -95%的物种在6亿多年来最大规模的物种灭绝中灭绝。这一世界性的事件影响了陆地和海洋生物，但我们还不知道它是如何发生的。为了确定大规模灭绝的原因，重要的是确定其速率以及与可能的因果事件（如全球气候变化、小行星撞击和火山活动）的时间关系。确定岩石的绝对年龄是所有地球科学家面临的一个基本挑战。最好的测年方法是放射性测定法，其中矿物颗粒或晶体的形成年龄是通过晶体内放射性同位素的衰变和它们的放射性子同位素的生长来确定的。当新的矿物形成时，这些同位素时钟会被重置，并与母体同位素结合。沉积物和沉积岩大部分是由其他地方形成的颗粒构成的，因此测定沉积颗粒的年代并不能给出沉积的年代。因此，大多数沉积物不能直接测定年代，而是通过与其他地区的化石进行比较来评估它们的相对年龄。不幸的是，很难把来自世界不同地区的海洋和陆地岩石联系起来，因为它们没有共同的化石。因此，许多大陆沉积岩的年龄知之甚少（最多+/- 1000万年），我们无法判断二叠纪/三叠纪（P-T）灭绝是在同一时间发生还是在很长一段时间内发生。我们也不能明确地确定物种灭绝与火山活动等事件之间的关系。如果能够开发出一种精确的大陆沉积岩测年方法，就可以构建出可靠的年表来研究P-T灭绝和地球历史上的其他关键事件。最近，有证据表明，包括天然放射性同位素147Sm和176Lu在内的微量元素在死后会迅速融入骨骼。骨在死亡后以<100ka的时间尺度再结晶，一旦再结晶，表现为一个化学封闭系统，不允许进一步的元素交换。因此，化石骨中的Sm-Nd和Lu-Hf同位素钟可能记录了再结晶后的时间。由于许多地质层序的再结晶速度很快，因此再结晶年龄可以作为寄主沉积物沉积年龄的代表。我们的目标是利用这些新知识来开发一种基于化石骨的Lu-Hf和Sm-Nd系统的地球计时器。然而，骨是一种复杂的组织，同一块骨的不同部位在人死后可能表现不同。因此，在确定日期之前，确定微量元素摄取的速率和性质以及每块骨头的化学稳定性是很重要的。我们已经确定了可以评估骨中微量元素摄取史的标准。一项初步研究表明，这些标准可以正确地识别出适合定年的个体骨骼，并且使用Lu-Hf和Sm-Nd方法对这些骨骼进行定年的初步尝试获得了与已知年龄一致的年龄，误差在1Ma以内。我们将扩展这一试点研究，以确定在何种条件下骨骼可以用作放射性测年的基质，以测试我们筛选方案的有效性，并最终建立沉积序列的绝对年表。我们将通过确定沉积序列中骨骼的年龄来评估我们技术的准确性和分辨率，包括年龄受到独立约束的P-T序列。然后，我们将应用我们的方法来确定具有较差分辨日期的重要P-T序列的年龄。这项技术的成功发展将提供一种方法来确定含骨沉积序列的绝对年龄，并将提供一个时间框架，在这个框架内，可以研究地球上生命所经历的最深刻危机的速度和性质。

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.