Bridging the timing gap: connecting Southern Ocean and Antarctic Climate records

鉴于目前对温室气体排放和相关温度上升的担忧，了解全球气候系统的运行机制是很重要的。这种理解可能使我们能够预测人类引起的气候变化和相关的生态系统脆弱性。南大洋在定义地球气候方面起着核心作用，因为在这里，寒冷的深水上升到地表，与大气交换气体和热量。对气候系统最重要的气体之一是二氧化碳（CO2）。由于海洋的碳含量大约是大气的60倍，因此海洋中的一个小扰动就会对气候产生很大的影响。在过去的80万年里，大气中的二氧化碳含量显示出了系统的变化，这是由冰芯中的气体所揭示的，最近的证据表明，二氧化碳含量只会在数百年内迅速增加。我们仍然不知道这些二氧化碳变化是如何以及为什么发生的，但它们的大小和速度表明，它们一定是由深海的变化所驱动的。已经提出的解释过去寒冷（冰期）期间大气二氧化碳浓度较低的机制主要集中在南大洋二氧化碳吸收量增加上。这可以通过增加海冰覆盖和水柱中更分层的结构来实现，这可以防止二氧化碳逃逸到大气中。这一概念得到了模型证据的支持，并预测我们应该在过去寒冷时期在南大洋深处找到古老的富含碳的水域。如果这种分层的水结构被移除，那么随着冰河时代的结束，这些深水将向大气释放二氧化碳。来自冰芯的记录告诉我们，在最后一个冰河时代（“末次冰消”）结束时，二氧化碳的实际上升是分多个步骤发生的。然而，到目前为止，很难从南大洋获得记录来验证上述假设，或者另一种假设，即南方的碳封存是由于浮游海洋植物更积极地吸收二氧化碳而实现的。我们一直缺乏的是一个合适的记录（“档案”），可以直接记录南大洋过去的环境条件，可以解决大约100年或更短的时间增量，类似于保存在冰芯中的记录。在我们的项目中，我们的目标是通过以前所未有的时间分辨率创建南大洋在最后一个冰川期结束和当前暖期（过去25000年）的环流、温度和二氧化碳化学的详细记录，来改变对南大洋在气候变化中的作用的理解。为了实现这一目标，我们将对深海珊瑚化石的骨骼进行地球化学测量，这是一个新的档案，使我们能够在精确和准确的时间尺度上创建过去海洋变化的独特耦合记录。深海珊瑚的骨骼是由它们所生活的海水中的化学成分形成的。这意味着在珊瑚的一生中（约100年），当它们生长时，就会记录下水的质量组成和温度。通过对每个珊瑚化石进行一系列地球化学测量，我们可以重建它生长时的环境条件。对数百个珊瑚重复这个实验，将使我们能够建立自上一个冰河时期以来南大洋行为的第一个直接记录。我们的新记录可以将南大洋环境变化的相对时间与冰芯记录进行比较。因此，它将解决全球气候变化研究中争论最激烈的问题之一，即在数百年至数千年的时间尺度上大气二氧化碳和温度变化的起源。

In light of current concerns over greenhouse-gas emissions and related temperature rise it is important to understand the mechanisms operating in the global climate system. This understanding may allow us to anticipate human-induced climate change and related ecosystem vulnerability. The Southern Ocean plays a central role in defining Earth's climate because it is a location where cold, deep waters rise to the surface and exchange gases and heat with the atmosphere. One of the most important gases for the climate system is carbon dioxide (CO2). Since the oceans contain about 60 times more carbon than the atmosphere, it only takes a small perturbation in the ocean to have a large climate impact. Atmospheric CO2 levels have shown systematic changes over the past 800,000 years as revealed by gasses trapped in ice cores, and recent evidence has come to light that shows that CO2 can increase rapidly over only hundreds of years. We still do not know how and why these changes in CO2 occur but their size and speed suggests that they must have been driven by changes in the deep ocean.Mechanisms that have been put forward to explain lower atmospheric CO2 concentrations during past cold (glacial) periods focus on increased CO2 uptake in the Southern Ocean. This could have been achieved by a combination of increased sea ice cover and a more layered structure in the water column, which prevents CO2 from escaping to the atmosphere. The concept is supported by modeling evidence and predicts that we should find old, carbon-rich waters in the deep Southern Ocean during past cold times. If this layered water structure was removed, then these deep waters would release CO2 to the atmosphere as ice ages came to a close. Records from ice cores show us that the actual rise of CO2 during the end of the last ice age ('last deglaciation') happened in multiple steps. So far, however, it has been very difficult to obtain records from the Southern Ocean to test the hypothesis posed above, or the alternative hypothesis that carbon sequestration in the South was achieved due to more active CO2 uptake by planktonic marine plants. What we have been lacking is a suitable recorder ('archive') of past environmental conditions directly in the Southern Ocean that can resolve time increments of about 100 years or less, similar to in the record preserved in ice cores.With our project we aim to transform understanding of the Southern Ocean's role in climate change by creating detailed records of the circulation, temperature, and CO2 chemistry of the Southern Ocean at the end of the last ice age and into the current warm period (past 25,000 years) at unprecedented temporal resolution. To achieve this we will make geochemical measurements on the skeletons of fossil deep-sea corals, a novel archive that allows us to create unique coupled records of past oceanographic change on a precise and accurate timescale. The skeletons of deep-sea corals are formed using the chemical ingredients of the seawater that they live in. This means that during the lifetime of a coral (~100 years) a record of water mass composition and temperature is captured as they grow. By performing a suite of geochemical measurements on each fossil coral, we can reconstruct environmental conditions at the time it grew. Repeating this exercise for hundreds of corals will allow us to construct the first directly dated record of the Southern Ocean's behavior since the last ice age. Our new record will allow comparison of the relative timing of environmental changes in the Southern Ocean with those of ice core records. It will therefore address one of the most hotly debated questions in global climate change research, the origin of changes in atmospheric CO2 and temperature on time scales of hundreds to thousands of years.