Precisely dated records of sea level and environmental change from Tahiti.

了解过去海平面的变化是一个重要问题。海平面平均气候给出了地球过去气候变化的概念。海平面还控制着陆地的范围、海洋的循环以及自然环境的许多其他特征。了解过去的海平面也有助于我们预测未来的变化——这对生活在海岸附近的人来说显然很重要！我们对过去海平面的了解大部分来自珊瑚。许多种类的珊瑚只生长在非常浅的海水中，所以在当前海平面以上或以下发现的珊瑚化石告诉我们，海洋曾经处于不同的高度（相对于陆地）。我们的实验室专门使用铀向其他元素（特别是钍）的衰变来测定自然样品（包括珊瑚）的年代。这些“u系列”技术是测定珊瑚年代的主要方法，但实际上它们在大多数珊瑚化石上都失败了。在提议的工作中，我们将努力理解为什么约会失败，并找到解决问题的方法。然后，我们将确定海平面变化的年代，以及地球过去环境其他方面的变化。这项工作将在塔希提岛附近钻探期间回收的一套珊瑚上进行。我们的建议包括五个单独的目标：首先，我们将开发已经化学改变的化石珊瑚的年代测定，以便更多的样本可以测定年代。我们将分析一系列样品类型（包括珊瑚、其他碳酸盐和水），以了解珊瑚礁中同位素的运动。我们将测量一系列不同的同位素，以了解珊瑚变化的本质，以及纠正这种变化的方法。许多同位素的使用也提供了对未改变珊瑚年龄的可靠性的检查。其次，我们将使用我们的新技术来测量过去海平面变化的数量和时间。我们将重点关注与最后两个海平面高点有关的变化。这项工作将提供关于如何在长时间尺度上控制地球气候的重要新信息。例如，它将告诉我们气候变化与已知地球轨道变化之间的关系。第三，我们将利用先进的分析能力，大幅增加u系列定年珊瑚的数量。我们开发了测量U和Th的新方法，比以前的方法更精确、更快。通过与另外两个研究小组合作，我们将帮助更好地限制过去2万年的海平面历史。这项工作还将识别在同一时期在一定水深范围内生长的珊瑚，从而重建水柱特征。第四，在珊瑚U-Th定年中计算了过去海水中铀的同位素组成。通常假定这一成分是恒定的，初始（234U/238U）不同于现在海水的珊瑚被改变了。我们将利用我们对珊瑚变化的了解，以及我们收集的大型数据集，得出海水（234U/238U）的历史。这将有助于未来的气候年代测定，并将提供有关大陆过去风化过程的信息。最后一个相当不同的目标与重建过去的条件有关，例如平均海洋温度和盐度。我们将与现有的合作者合作，开发新的化学工具来评估这些过去的条件。我们将特别研究测量过去海洋盐度的工具。

Understanding changes in past sealevel is an important issue. Sealevel averages climate to give an idea of changes in Earth's past climate. Sealevel also controls the extent of land, the circulation of the ocean, and many other features of the natural environment. Understanding past sealevel also helps us to predict future changes - obviously important to anyone living near the coast! Much of our understanding of past sealevel comes from corals. Many species of corals only grow in very shallow seawater so the presence of a fossil coral above or below present sealevel tells us that the sea used to be at a different height (relative to the land). Our laboratory specialises in dating natural samples, including corals, using the decay of uranium to other elements, particularly thorium. These, 'U-series', techniques are the main way of dating corals, but they actually fail on the majority of fossil coral. In the proposed work, we will strive to understand why dating fails, and to find ways around the problems. We will then date changes in sealevel, and changes in other aspects of Earth's past environment. This work will be performed on a suite of corals to be recovered during drilling near Tahiti. Our proposal consists of five separate goals: First, we will develop dating of fossil corals which have been chemically altered so that more samples can be dated. We will analyse a range of sample types (including corals, other carbonates, and waters) to understand movement of isotopes in the reef. And we will measure a range of different isotopes to understand the nature of coral alteration, and ways to correct for it. The use of many isotopes also provides a check on the reliability of ages from unaltered corals. Second, we will use our new techniques to measure the amount and timing of sealevel change in the past. We will focus attention on changes associated with the last two sealevel highstands. This work will provide important new information about how Earth's climate is controlled on long timescales. It will tell us, for instance, how changes in climate relate to known changes in the Earth's orbit. Thirdly, we will make use of advances in analytical capability to dramatically increase the number of corals dated by U-series. We have developed new ways to measure U and Th which are more precise and much faster than earlier methods. By collaborating with two other research groups, we will help to better constrain sealevel history for the last 20 thousand years. This work will also identify corals that grew in a range of water depths during the same period allowing reconstruction of water column features. Fourthly, the isotope composition of uranium in past seawater is calculated during U-Th dating of corals. It is normally assumed that this composition is constant and that corals with initial (234U/238U) different from present seawater are altered. We will use our understanding of coral alteration, and the large dataset we collect, to come up with a history for seawater (234U/238U). This will help with future dating of climate, and will provide information about past weathering processes on the continents. A final rather different objective related to reconstruction of conditions in the past such as average ocean temperature and salinity. Working with an existing collaborator, we will develop new chemical tools to assess these past conditions. In particular we will investigate tools to measure the salinity of the past oceans.