Characterising the Nd-isotopic composition of Cretaceous intermediate- and deep-water masses using fossil fish remains

科学家和政界人士都在广泛讨论温室气体浓度增加和全球变暖对地球构成的威胁。我们应对这一威胁的能力在很大程度上得益于对我们星球气候历史的透彻了解。大约在1.44亿至6500万年前的白垩纪时期，地球气候比现在温暖得多，大气中的二氧化碳水平比今天的水平高出两到三倍以上。两极几乎没有冰，恐龙、鳄鱼和热带植物生活在南极洲和阿拉斯加等高纬度地区。显然，白垩纪代表了全球变暖的“自然”例子，并提供了有关我们的星球可能如何应对未来由石油和煤炭燃烧驱动的人为变暖的有价值的信息。从大约1.2亿到9000万年前，浮游生物和植物(从陆地冲进来)产生的大量碳被埋在北大西洋底部的低氧条件下。这些沉积物肯定对碳循环产生了重大影响，并可能通过储存碳来帮助调节大气中的二氧化碳含量。然而，这种情况在大约9000万年前就结束了，含有更多氧气的海水填满了北大西洋的底部，从而阻止了碳的埋藏。9000万年前，北大西洋是一片与世隔绝的海洋，由陆地(连接非洲和南美洲)与南大西洋隔开。在9000万年前之后，两个大洋之间可能存在着一种联系，使得水从南向北混合。有人认为，这种深水连接导致了北大西洋氧气条件的变化。此外，这一事件可能导致海洋环流增加，从而将热量从热带输送到更凉爽的高纬度地区。为了检测深水环流的变化是否影响了北大西洋沉积物的沉积，有必要有一种方法来对每个海洋的深水进行指纹识别。钕(一种符号为ND的重金属)以七种形式(或同位素)存在，从岩石中被侵蚀后被河流输送到海洋中。其中一种同位素--143Nd在太平洋、大西洋和印度洋之间有很大的不同，因为每个海洋都被不同类型的岩石包围，这些岩石中都含有不同数量的143Nd。这些差异表现为143Nd值与144Nd值之比。目前，太平洋的比值最高，而大西洋的比值最低。因此，如果在一个单独的地点，随着时间的推移，从低到高的值变化，这可能表明在该地点存在的太平洋水的比例增加。在过去的地质学中，我们可以用鱼的牙齿和骨头来估计底水的ND同位素的值。鱼死后沉入海底腐烂。通过海底的化学反应，鱼的牙齿和骨骼获得ND的比例与它们所在的海水相同。通过大洋钻探回收的深海沉积物中往往含有鱼类遗骸，可以用来构建海水Nd同位素比值的时间记录。我计划从太平洋、大西洋和印度洋为白垩纪建立这样的记录，这将使我们能够首先看到海洋在这个时候是否有独特的特征价值(与现代海洋相同)。然后，我将制作几条来自北大西洋的记录，看看大约9000万年前氧气条件的变化是否是由于非洲最终从南美洲分离而导致的海洋环流增加所致。这项研究将对理解海洋环流如何调节全球碳循环和全球温度具有重要意义。

The threat posed to our planet by increased greenhouse gas concentrations and global warming is being widely discussed by scientists and politicians alike. Our ability to deal with this threat is greatly aided by a thorough understanding our planet's climate history. Approximately 144 to 65 million years ago, during the Cretaceous period, the Earth's climate was considerable warmer than present, with atmospheric CO2 levels more than two to three times higher than present-day levels. There was little ice at the poles, and dinosaurs, crocodiles and tropical plants living in high latitudes regions like Antarctica and Alaska. Clearly, the Cretaceous represents a 'natural' example of global warming and provides valuable information concerning how our Planet might respond to future anthropogenic warming, driven by the burning of oil and coal. From ~120 to 90 million years ago large amounts of carbon produced by planktonic organisms and plants (that were washed-in from the land) were buried at the bottom of the North Atlantic in low-oxygen conditions. These sediments must have had a significant impact on the carbon-cycle and may have helped regulate the amount of CO2 in the atmosphere by storing carbon. However, this situation ended about 90 million years ago and waters containing much more oxygen filled the bottom of the North Atlantic, thereby preventing the burial of carbon. Prior to 90 million years ago, the North Atlantic was an isolated ocean, separated by land (that connected Africa and South America) from the South Atlantic. After 90 million years ago a connection may have existed between the two oceans allowing water to mix from south to north. It has been suggested that this deep-water connection caused the change in oxygen conditions in the North Atlantic. Additionally, this event may have allowed increased ocean circulation that would have transported heat away from the tropics to cooler high latitudes. In order to detect whether changes in deep-water circulation affected the deposition of sediments in the North Atlantic, it is necessary to have a method of 'fingerprinting' the deep-water from each ocean. Neodymium (a heavy metal with the symbol Nd) occurs in seven forms (or isotopes) and is transported to the oceans by rivers after it has been eroded from rocks. One of the isotopes, 143Nd, varies substantially between the Pacific, Atlantic and Indian Oceans because each ocean is surrounded by different rock-types that all have varying amounts of 143Nd in them. These differences are expressed as the ratio of 143Nd to 144Nd. At the present day, the Pacific Ocean has the highest ratio values, whereas the Atlantic has the lowest values. Thus, if at a single site there is a change from lower to higher values with time, this may indicate an increase in the proportion of Pacific Ocean water present at this site. In the geological past we can use fish teeth and bones to estimate bottom-water Nd-isotope values. After death, fish sink to the sea-floor and decay. Through a chemical reaction at the sea-floor fish teeth and bones gain Nd in the same ratio as the sea-water in which they lie. Deep-sea sediments recovered by ocean drilling often contain fish remains that can be used to construct records through time of seawater Nd-isotope ratios. I plan to construct such records from the Pacific, Atlantic and Indian Oceans for the Cretaceous that will allow us, firstly, to see if the the oceans had distinct, characteristic values at this time (in the same way as the modern oceans). I will then produce several records from the North Atlantic to see if the change in oxygen conditions approximately 90 million years ago was caused by increased ocean circulation due to the final separation of Africa from South America. This research will be important for understanding how ocean circulation regulates the global carbon cycle and global temperatures.