Icehouse tropical climates and plankton evolution

今天的地球在南极洲和格陵兰岛有主要的冰盖，但情况并非一直如此。计划中的研究重点是渐新世时期（距今3400万至2400万年前）的气候变化。这段时间是特别有趣的，因为它开始于气候的巨大变化。在3400万年前，气候是温暖的，以至于南极洲没有永久的冰，棕榈树一直向北延伸到北极圈。然而，这一温暖时期在3400万年前突然结束，当时南极洲形成了一个巨大的冰盖。一旦南极冰帽形成，它就会在冰相对较少的时期和冰盖非常膨胀的时期之间多次起伏。我之前的工作表明，气候就像一个闪烁的开关，在冰川和温室状态之间的尖端保持平衡，如果我们想要了解一个比现代更温暖的世界，那么渐新世的研究就特别令人兴奋。然而渐新世的气候仍然是一个谜，因为直到现在，还没有好的记录来记录热带地区的气候变化。热带是至关重要的，因为热带和两极之间的温差最终驱动了大气和海洋的循环。不幸的是，热带渐新世沉积物相当稀少，而且大多数以前获得的记录都是不完整的，或者没有保存得足够完好的微化石，无法进行揭示气候所需的那种分析。还有许多问题需要解决，例如；当两极经历这些剧烈的气候波动时，热带地区的情况是怎样的？气候是否在每一阶段的冰形成之前就逐渐变冷，而在冰盖消退之前变暖？地球上的生物群是如何应对气候的巨大变化的？这些问题可以通过分析海洋沉积物微化石的地球化学组成来解决。我研究的是微小的有孔虫化石。它们是单细胞生物，通常由碳酸钙构成骨架。随着时间的推移，死去的有孔虫的壳在海洋沉积物中积累起来，形成了一份长期而有价值的化石记录，古生物学家可以利用这些化石记录来获取有关过去海洋和气候的信息。关于这些微小的有机体，最神奇的事情之一是，它们外壳的化学成分反映了它们生长的水和环境的其他特征。对化石壳的分析可以用来重建气候的许多方面，如海面温度和全球冰量。在过去的几年里，我已经开始系统地识别和获取所有最好的热带样品，这些样品有非常丰富和保存完好的有孔虫，使它们成为化学分析和研究渐新世海洋的理想选择。在波多黎各和东非等地区，还需要更多的收集。对有孔虫化学的详细研究将建立一个关于渐新世气候如何变化的准确图景，并将导致对气候事件和海洋学过程的更好理解。研究剧烈气候变化的间隔，可以更好地了解海洋和气候系统。鉴于目前大气中二氧化碳的增加，预计将导致迅速而深刻的全球变暖，气候知识对社会尤为重要。气候模拟实验表明，渐新世期间地球从温暖到凉爽的气候变化可能与大气中二氧化碳水平的降低有关（在某种程度上，与预测的未来相反）。这些记录将为检验和改进气候模型提供数据，并可能为预测未来从冰盖到热带地区对突然变暖的气候反应提供有用的信息。

The Earth today has major ice caps on Antarctica and Greenland, but this was not always the case. The planned research focuses on climate change during a time period known as the Oligocene (34 to 24 million years ago). This interval of time is of particular interest as it began with a big shift in climate. Before 34 million years ago, the climate was warm - so warm that there was no permanent ice on Antarctica and palm trees stretched as far north as the Arctic Circle. However, this warm period ended abruptly at 34 million years ago when a large ice sheet developed on Antarctica. Once the Antarctic ice cap was established, it waxed and waned many times between periods with relatively little ice and periods with very expansive ice sheets. My previous work has shown that like a flickering switch, the climate was balanced on the cusp between glaciated and greenhouse states, making the Oligocene particularly exciting to research if we want to understand a world somewhat warmer than the modern. However the Oligocene climate remains something of a mystery because, until now, there have been no good records to document climate changes in the tropics. The tropics are vital because it is heat differences between the tropics and poles that ultimately drives the circulation of the atmosphere and oceans. Unfortunately tropical Oligocene sediments are fairly scarce and most previously acquired records are incomplete or do not have sufficiently well-preserved microfossils for the kind of analyses that are required to reveal the climate. Many questions still need to be addressed, such as; What were conditions in the tropics like while the poles were undergoing these dramatic climate swings? Did the climate cool gradually before each phase of ice build up, and warm before the ice caps waned? How did the Earth's biota respond to the big shifts in climate? These questions can be addressed by analysing the geochemical composition of microfossils from marine sediments. I work with tiny microscopic fossils called foraminifera. These are single-celled organisms that commonly build a skeleton made from calcium carbonate. Over time the shells of dead foraminifera accumulate in marine sediments and yield a long and valuable fossil record, which palaeontologists can exploit to gain information on oceans and climate of the past. One of the most marvellous things about these tiny organisms is that the chemistry of their shells reflects the water in which they grew and other features of their environment. Analysis of fossil shells can be employed to reconstruct many aspects of the climate, such as the temperature of the sea surface and global ice volume. In the past few years I have set about to systematically identify and acquire all the best tropical samples that have very abundant and well preserved foraminifera, making them ideal for chemical analyses and to study the oceans of the Oligocene. More collecting is also required, in areas such as Puerto Rico and East Africa. A detailed study of foraminifer chemistry will build an accurate picture of how the Oligocene climate changed and will lead to a greater understanding of climatic events and oceanographic processes. Studying intervals of dramatic climate change, allow a greater understanding of the oceans and climate system. Knowledge of the climate is particularly important to society in light of current increases in atmospheric carbon dioxide which is predicted to cause rapid and profound global warming. Climatic modelling experiments have suggested that the climate shift from a warm to cool Earth during the Oligocene may have been related to a reduction in atmospheric carbon dioxide levels (in a way, the reverse process of what is predicted for the future). These records will provide the data to test and refine climate models and may provide information useful for predicting the future climate response of abrupt warming, from the ice sheets to the tropics.