Environmental Change and Biotic Impacts of a Transient Greenhouse Warming Event in the Middle Eocene

摘要：由于人类活动，全球大气中二氧化碳浓度（pCO2）正以地球历史上前所未有的速度上升。目前大气中的二氧化碳分压为387 ppmv，超过了地球历史上最近~ 2500万年的浓度报告。为了预测未来地球系统对二氧化碳分压增加的反应，我们必须更好地了解过去在二氧化碳分压高的时间间隔内全球气候和碳循环是如何变化的。始新世（5500 - 3400万年前，Myr）是一个很好的类比，因为它是地质上最近的持续全球变暖的间隔，其特征是高二氧化碳分压（100 - 1000 ppm）。这项研究的重点是最近发现的一个短暂的全球变暖事件，发生在大约4000万年前的中始新世——中始新世气候最佳期（MECO）。在这次事件中，全球气温上升了4到6摄氏度，深海变得更酸，可能是大气中二氧化碳分压暂时增加的结果。这一事件特别有趣，因为MECO的时间和原因不同于始新世报道的其他变暖事件。然而，我们对伴随这一独特事件和相关影响的环境变化所知甚少。例如，气候对二氧化碳分压变化有多敏感？地球上的生物群是如何应对气候变化的？这些问题可以通过测量在海洋沉积物中发现的微化石的地球化学成分来解决。在这项研究中，对两种不同类型的微化石：有孔虫（由碳酸钙构建外壳的单细胞生物）和鱼牙齿化石进行了新的地球化学分析，将用于重建过去的环境变量。浮游有孔虫壳的化学分析将用于重建MECO事件期间海面温度和酸度的变化。浮游有孔虫的反应，例如物种数量、每种物种的数量和壳的大小，也将被记录下来，以评估地表水变化与生物群反应之间的任何联系。还将通过分析底栖有孔虫壳和鱼齿化石的地球化学组成来评估同期底水条件的变化，例如温度和氧浓度。最终，对过去全球变暖事件的研究，如MECO，将提供对二氧化碳分压与全球气候之间联系的更深入的了解，这对于理解地球系统的行为至关重要。这项工作将为高pCO2世界的碳循环运作提供新的见解，并为古气候和碳循环数值模型的配置和测试提供关键数据。这项研究将在卡迪夫大学地球与海洋科学学院进行，该学院正在积极研究地质历史中气候变化的关键时期。我还将与其他研究所的科学家合作，特别是国家海洋学中心、南安普顿和南卡罗来纳大学（见合作细节）。

SUMMARY Global atmospheric carbon dioxide levels (pCO2) are rising at a rate unprecedented in Earth's history as a result of human activity. Current atmospheric pCO2 is 387 ppmv and exceeds concentrations reported for the last ~25 million years of Earth's history. To predict the future response of the Earth's system to increasing pCO2 it is imperative that we develop a better understanding of how global climate and carbon cycling has varied during high pCO2 time intervals in the past. A good analogue is the Eocene Epoch (55-34 million years ago, Myr) because it is the most geologically recent interval of sustained global warmth characterised by high pCO2 (> 1000 ppm). The research proposed focuses on a recently discovered, short-lived global warming event during the middle Eocene approximately 40 million years ago - the Middle Eocene Climatic Optimum (MECO). During this event, global temperatures rose by 4 to 6 degrees celsius and the deep ocean became more acidic, presumably as a consequence of a temporary increase in atmospheric pCO2. This event is particularly interesting because the timing and cause of the MECO differ from other warming events reported in the Eocene. However, we know very little about the environmental changes that accompanied this unique event and the associated impacts. For example - How sensitive is the climate to pCO2 change? How did Earth's biota respond to shifts in climate? These questions can be addressed by measuring the geochemical composition of microfossils that are found in marine sediments. In this study, novel geochemical analyses of two different types of microfossil: foraminifera (single-celled organisms that build their shell from calcium carbonate) and fossil fish teeth, will be used to reconstruct past environmental variables. Chemical analysis of planktonic foraminiferal shells will be used to reconstruct changes in sea surface temperature and acidity during the MECO event. The response of planktonic foraminifera, e.g., number of species, amount of each species and shell size, will also be recorded to assess any links between surface water changes and the response of the biota. Contemporaneous changes in bottom water conditions e.g., temperature and oxygen concentration, will also be assessed by analysing the geochemical composition of benthic foraminiferal shells and fossil fish teeth. Ultimately, study of past global warming events, such as the MECO, will provide a greater understanding of the links between pCO2 and global climate, which is essential to understanding Earth system behaviour. This work will provide new insights into the operation of the carbon cycle in a high pCO2 world and produce key data for the configuration and testing of numerical palaeoclimate and carbon cycle models. The research will be based within the School of Earth and Ocean Sciences at the University of Cardiff, which is actively investigating critical intervals of climate change in the geological past. I will also collaborate with scientists from other institutes notably, the National Oceanography Centre, Southampton and the University of South Carolina (see partnership details).