The timing and amplitude of Pleistocene sea-level lowstands and deglacials from U-Th chronology of Great Barrier Reef corals (Expedition 325)

对于寻求了解地球气候的研究人员来说，研究过去的海平面是非常有兴趣的。海平面是陆地上以冰盖形式锁住的水量的代表。因此，对海平面的研究可以告诉我们气候的过去状态--低海平面反映了地球历史上冰盖较大的较冷的冰期；高海平面反映的是较暖的间冰期。我们对现代冰盖将如何应对人为变暖的了解还不全面。围绕冰盖行为的最重要问题之一是确定对变暖做出反应的崩塌速度，这将影响未来海平面上升的速度。解决这一问题的一种方法是检查在第四纪冰川-间冰期过渡的非人为条件下，与已知的冰盖崩塌时期有关的过去海平面上升速度。化石珊瑚提供了对过去海平面的间接测量，从而回答了关于海平面在过去如何(以及何时)变化的问题。珊瑚只生活在海平面以下，也有可能确定它们生长在海平面以下多远。这一水深估计是基于我们发现的珊瑚类型以及我们发现的与它们相关的其他动植物。随着我们找回珊瑚的海拔高度，这种水深估计使我们能够重建过去海平面的高度。这个重建的海平面所代表的时间也可以从珊瑚中测量出来。我们使用生长过程中自然融入珊瑚骨架的铀(及其放射性衰变产物--钍)来计算珊瑚的年龄。这种U-Th测年技术可以确定珊瑚的年龄从1岁到60万岁。然而，在调查过去比现在低得多的海平面(在最近的冰川时期，海平面比现代海平面低约120米)方面存在一个问题。在海平面低的时候生长的珊瑚现在很难接触到，海平面已经上升到现代的水平--它们被深埋在水下。为了解决这一问题并从更深的地方回收样本，将对大堡礁进行IODP钻探探险。这次探险将钻探化石珊瑚礁，这些化石珊瑚礁已经通过之前的现场调查确定了它们的形态。这些化石珊瑚礁位于44至123米的水深，保存着海平面较低的珊瑚档案。我们的计划是从这些化石珊瑚礁中提取样本，使用U-Th测年来确定它们的年龄，然后重建海平面较低时冰川时期的海平面。这些对低海拔海平面的估计将被用于研究驱动地球自然气候系统的机制。在这方面，从低海平面到高海平面变化的时间和速度特别重要，使我们能够测试潜在强迫(地球围绕太阳轨道的变化，或大气二氧化碳浓度等内部气候强迫)和气候响应(海平面)之间的任何超前-滞后关系。

The study of past sea level is of great interest to researchers seeking to understand the Earth's climate. Sea level is a proxy for the amount of water locked up on the land as ice sheets. The study of sea level therefore can tell us about the past state of the climate - low sea levels reflect colder, glacial, periods in Earth history when ice sheets were larger; high sea levels reflect warmer, interglacial, intervals. Our knowledge of how modern ice sheets will respond to anthropogenic warming is not yet comprehensive. One of the most important questions surrounding ice sheet behaviour is the determination of collapse rates in response to warming, which will influence future rates of sea level rise. One way to approach this question is to examine past rates of sea level rise associated with known periods of ice sheets collapse under the non-anthropogenic conditions Quaternary glacial-interglacial transition. Fossil corals provide an indirect measurement of past sea level and therefore answering questions about how (and when) it has changed in the past. Coral only live below sea level and it is also possible to determine how far below sea level they grew. This water depth estimation is based on the types of coral we find and the other fauna and flora we find associated with them. Along with the elevation that we recovered the coral from, this water depth estimation enables us to reconstruct the height of past sea level. The time that this reconstructed sea level represents can also be measured from the coral. We use the uranium (and the product of its radioactive decay - thorium) naturally incorporated into the coral's skeleton during growth to calculate an age for the coral. This U-Th dating technique can find the age of corals from 1 to 600,000 years old. There is, however, a problem in investigating past sea levels which were much lower than present (during the most recent glacial period sea level was c. 120m below modern levels). The corals that grew when sea level was low are hard to access now sea level has risen to its modern level - they are deep underwater. To solve this problem and recover samples from deeper localities, there will be an IODP drilling expedition to the Great Barrier Reef. This Expedition will drill into fossil coral reefs that have been identified by their morphology by a previous site survey. These fossil reefs are at water depths of 44 to 123m and preserve an archive of corals from lower sea levels. It is our plan to take samples from these fossil reefs, use U-Th dating to determine their ages, and then reconstruct sea level for glacial periods while sea level was low. These estimates of low stand sea level will be used to investigate mechanisms that drive the Earth's natural climate system. The timing and rate of changes from low to high sea level are particularly important in this respect, allowing us to test any lead-lag relationship between a potential forcing (changes in the Earth's orbit around the Sun, or internal climate forcing such as atmospheric CO2 concentration) and climatic response (sea level).