Ocean Temperature Changes Across the Eocene-Oligocene Boundary

古气候研究的主要挑战之一是了解从白垩纪和古近纪的无冰，高二氧化碳的“温室世界”到我们今天生活的寒冷，低二氧化碳的“冰库世界”的转变。在这一长期气候演变过程中，最引人注目的事件之一是南极大陆永久冰盖的相对快速增长和大约3400万年前（始新世-渐新世，或E-O边界）的海平面下降。据认为，这一事件是由大气二氧化碳水平下降和/或海洋环流变化引起的。显然，一个关键的数据是在这一转变过程中海洋温度的高分辨率记录;为了实现这一目标，我们将开发赤道太平洋ODP站点1218沉积物中的地球化学记录。但是，我们解读研究中心1218 /和类似研究中心/的此类记录的能力受到许多因素的限制。获取海洋表面温度记录的经典方法是分析生活在海洋表面沃茨的有孔虫碳酸钙壳的氧同位素组成。不幸的是，越来越清楚的是，这样的记录通常会受到损害，因为有孔虫外壳在海洋深处溶解和重结晶，有效地消除了原始的温度信号。基于底栖有孔虫氧同位素的底层水温记录更可靠;然而，这些记录很难解释，因为氧同位素受温度和全球冰量的影响，而全球冰量在E-O边界上明显变化。因此，要获得高分辨率的海洋温度记录，需要采用新的方法。这些工具之一是基于海洋微生物中细胞脂肪的组成/TEX 86代理。这些化合物中的环与生物体细胞膜的结构有关，因此环的数量随着生长环境的温度而增加。由于该代用品基于有机化合物，因此不容易受到影响基于碳酸钙壳的代用品的相同溶解问题的影响。为了获得底层水温，我们将测量分配到底栖有孔虫碳酸钙壳中的镁的量（Mg/Ca比）。Mg/Ca比值不受冰量的影响，并已被用来开发深海冷却的长期记录;但它们也受到海洋中碳酸盐物种分布的影响，并且在E-O边界处似乎也发生了显着变化。因此，我们将测量Mg/Ca比和B/Ca比，它们反映了碳酸盐离子的分布;它们可以一起用来破译深海中的温度和碱度变化。这两种方法将并行应用于跨越E-O边界的25个样本。这将使我们能够确定在这一重大气候事件中赤道太平洋的深度和深度究竟冷却了多少。它还将使我们能够确定系统中是否存在任何负反馈;例如，冰盖的形成是否减缓了侵蚀并使二氧化碳浓度再次增加？最后，通过比较深海和浅海冷却，我们将能够评估海洋环流变化的模型。

One of the major challenges in palaeoclimate research is understanding the transition from the ice-free, high carbon dioxide 'greenhouse world' of the Cretaceous and Paleogene to the colder, low carbon dioxide 'icehouse world' in which we live today. During this long-term climate evolution, one of the most dramatic events was the relatively rapid growth of permanent ice sheets on the Antarctic continent and co-occurring drop in sea level about 34 million years ago (the Eocene-Oligocene, or E-O, boundary). It is thought that this event was caused by a decrease in atmospheric carbon dioxide levels and/or a change in ocean circulation. Obviously, a crucial piece of data is a high-resolution record of ocean temperatures during this transition; to achieve this, we will develop geochemical records in sediments from ODP Site 1218 in the equatorial Pacific. However, our ability to interpret such records from Site 1218 / and comparable sites / is limited by a number of factors. The classical approach for obtaining sea surface temperature records is by analysing the oxygen isotopic composition in the calcium carbonate shells of foraminifera living in the surface waters of the ocean. Unfortunately, it is becoming clear that such records are typically compromised, because the foraminifera shells dissolve and recrystallise in the deep part of the ocean, effectively erasing the original temperature signal. Bottom water temperature records, based on oxygen isotopes in benthic foraminifera, are more reliable; these records, however, are hard to interpret because the oxygen isotopes are affected by both temperature and global ice volume / which obviously changed dramatically across the E-O boundary. Thus, to obtain high resolution ocean temperature records requires the application of new approaches. One of these tools is based on the composition of cellular fats in marine microorganisms / the TEX86 proxy. The rings in these compounds are related to the structure of the organism's cell membrane, such that the number of rings increases with the temperature of the growth environment. Because this proxy is based on organic compounds, it is not susceptible to the same dissolution concerns that affect proxies based on calcium carbonate shells. To obtain bottom water temperatures we will measure the amount of magnesium that has partitioned into the benthic foraminiferal calcium carbonate shells (Mg/Ca ratios). Mg/Ca ratios are not affected by ice volume and have been used to develop long-term records of deep ocean cooling; but they are also affected by the distribution of carbonate species in the ocean and that appeared to change dramatically at the E-O boundary as well. Therefore, we will measure both the Mg/Ca ratios and B/Ca ratios, which reflect carbonate ion distributions; together they can be used to decipher both the temperature and alkalinity change in the deep ocean. These two approaches will be applied in parallel for twenty-five samples spanning the E-O boundary. This will allow us to determine exactly how much the deep and shallow Equatorial Pacific cooled across this major climate event. It will also allow us to determine if there were any negative feedbacks in the system; for example, did the build up of ice sheets slow down erosion and allow carbon dioxide concentrations to build up again? Finally, by comparing deep and shallow ocean cooling, we will be able to evaluate models of ocean circulation change.