Sharpening the U-Th chronometer through technical developments and community implementation

天然碳酸盐（如珊瑚和石笋）中铀的放射性衰变为确定这些材料的年代提供了一种强有力的方法。这种U-Th技术可以追溯到大约50万年前，为我们了解过去气候和环境变化的时间提供了主要途径。在过去的15年中，我们测量U和Th同位素比率和浓度的能力有了相当大的提高，从而使U-Th年龄的不确定性降低了一个数量级。现在的不确定性低至0.1%，即在10万年历史的珊瑚化石或洞穴中，不确定性为100年。精确度的提高使我们能够回答范围更广、范围更广的问题，这对我们理解更新世气候和海平面变化的机制至关重要。但它也暴露了一个问题。用于测量U和Th的示踪剂溶液的校准是在每个实验室使用不同的技术独立进行的，并且已经非常清楚，所得到的U-Th年龄虽然令人印象深刻地精确，但在不同实验室之间的精度水平并不一致。现在，实验室间的偏差已经超过了典型的年龄不确定性。这种实验室间不确定度的原因是由于缺乏校准目的和长期评估实验室间协议的合适材料。这种校准最广泛使用的材料之一，HU-1，最近被证明在不同实验室使用的溶液之间存在0.5%的差异。而且不存在广泛分布和特征明确的“年龄标准”，这些标准可以被所有U-Th实验室分析，以促进实验室间一致性的量化。我们建议采取一系列行动来解决国际U-Th年代学界的这些不足。我们将开发一系列U-Th校准溶液，其成分由第一性原理计量学（即从高纯度U和Th金属的称重和溶解）可知。我们将使用这些解决方案来校准三个英国和一个海外实验室使用的示踪剂，每个实验室都在U-Th工作方面享有良好的声誉。然后，我们将继续开发四种不同的“年龄解决方案”，方法是将U和Th同位素按比例混合在一起，模拟社区分析的典型成分。这些“年龄溶液”的组成将使用最新的、精确校准的示踪剂来测量，这样所有的成分都将被知道，并可追溯到基本的质量测量。这些“年龄解决方案”也将免费提供给全球所有要求它们的U-Th实验室，我们将生产足够的解决方案，以便它们可以使用20年。我们将协调一项实验室间的比较工作，这样，我们将首次能够量化不同实验室产生的数据的一致水平。作为一个社区，我们将希望确保实验室间差异的程度最小化，因此，如果实验室发现他们的结果不准确，他们将能够使用年龄解决方案，其组成是众所周知的，以提高他们的结果的准确性。这项工作在国际U-Th社区得到了非常广泛的支持，我们收到了来自33个实验室的支持信，占全世界此类实验室的绝大多数。用于生成日期的U-Th数据的数学处理也存在局限性。我们提出的分析工作将与这些新的数据精简模板的制定相一致。总的来说，这些活动将显著和永久地提高碳酸盐U-Th年代的可靠性——我们对更新世气候的许多知识都是基于这些年代。

The radioactive decay of uranium incorporated in natural carbonates (e.g. corals and stalagmites) provides a powerful way of dating these materials. Such U-Th techniques extend to about half a million years ago and provide the major way in which we can learn about the timing of past climate and environmental change. Over the past 15 years there has been considerable improvement in our ability to measure U and Th isotope ratios and concentrations resulting in a reduction of U-Th age uncertainties by an order of magnitude. Uncertainties are now as low as 0.1%, or 100 years in the age of fossil coral or speleothem that is 100,000 years old. This increase in precision has enabled a wide and expanding range of questions to be answered and is critical to our understanding of the mechanisms of Pleistocene climate and sea-level change. But it has also exposed a problem. Calibration of the tracer solutions used to make U and Th measurements is performed independently in each laboratory using differing techniques and it has become abundantly clear that resulting U-Th ages, while impressively precise, do not agree at this level of precision from one lab to another. There is now inter-laboratory bias at a level that exceeds typical quoted age uncertainty. The cause of this inter-laboratory uncertainty is due to a lack of suitable materials for both calibration purposes and for long-term assessment inter-laboratory agreement. One of the most widely used materials for such calibration, HU-1, has recently been demonstrated to vary, by up to 0.5% between the solutions used in different laboratories. And there exists no widely distributed and well characterised 'age standards' that could be analysed by all of the U-Th laboratories to facilitate quantification of inter-laboratory agreement. We propose a series of actions to address these short fallings in the international U-Th chronology community. We will develop a series of U-Th calibration solutions who's composition is known from first principles metrology (i.e. from the weighing and dissolution of high-purity U and Th metal). We will use these solutions to calibrate the tracers used in three UK and one overseas laboratories - each with a well established reputation for U-Th work. We will then proceed to do develop four different 'age solutions' by taking the U and Th isotopes and mixing them together in proportions that mimic typical compositions analysed by the community. The composition of these 'age solutions' will be measured using the newly and precisely calibrated tracers, so that all compositions will be known and traceable to basic measurements of mass. These 'age solutions' will also be made freely available to all U-Th laboratories who request them worldwide, and we will produce enough of the solutions so that they will last the community 20 years. We will co-ordinate an inter-laboratory comparison exercise so that, for the first time, we will be able to quantify the level to which dates produced in different laboratories agree. As a community we will want to ensure that the level of inter-laboratory variation is minimised, so if labs find their results to be inaccurate they will be able to use the age-solutions, whose compositions are well known, to improve the accuracy of their results. There is very widespread support for this effort in the international U-Th community and we have letters of support from 33 laboratories - the vast majority of all such laboratories worldwide. There are also limitations with the mathematical treatment of U-Th data used to produce dates. Our proposed analytical efforts will be made in concert with the development of these new data reduction template. Overall, these activities will provide dramatic and permanent increase in the reliability of U-Th dates of carbonates - the dates on which so much of our knowledge of Pleistocene climate is based.