Beyond the Nadir: Early Cretaceous-Cenozoic archives of the northern Equatorial Atlantic Gateway

这项建议寻求根据地震反射数据建立地层框架，为未来在西非进行海洋钻探做准备。这项钻探计划旨在了解(1)南美洲和非洲分离的时间，当北大西洋和南大西洋的大洋盆地首次连接起来时，形成了“赤道大西洋门户”；(2)地球历史上极端温暖时期的海洋条件(温度、化学成分、氧含量)；以及(3)了解大约6500万年前潜在陨石撞击的环境后果。第一个目标是了解赤道大西洋门户(EAG)的开放，这是重要的，因为由于海洋环流条件的变化，这一事件对地球的气候和天气模式产生了重大影响。然而，我们对这一事件的理解有很大的不确定性，特别是当它形成时，因为它发生在‘白垩纪正常超时’(CNR)--一个地球磁场异常稳定的时期。磁场反转对于测定洋壳的年龄很重要，但在北冰洋核磁共振期间这是不可能的。我们的地震反射数据，再加上勘探井中的生物地层数据，将为导致EAG开通的构造事件提供新的约束。它还将突出我们可以钻探、直接采样和测定记录这些事件的沉积物的地区，以及记录大陆分离的最古老的洋壳。它还将对EAG通道开放期间发生的构造事件序列和门户的长期加深提供新的见解。第二个目标是了解地球白垩纪和新生代的海洋状态。特别是在白垩纪时期，全球气候和大西洋海洋比现在温暖得多，其特点是严重缺氧(低氧水平)和有机碳埋藏。这些事件被称为海洋缺氧事件(OAEs)，似乎是由大规模火山喷发产生的强烈温室气体排放引发的，持续了数十万年到可能100万年。尽管它们已经被钻探了很多次，但我们并不清楚它们的范围有多广--例如，有多少水柱受到了影响--或者这些沉积物中记录的反馈过程。我们计划沿着深度断面确定钻探地点，并在地震数据表明它们异常厚的地方，获得关于这些事件的重要新信息。最后，我们的目标是建立在现有IODP建议(1004-APL)的基础上，以了解白垩纪末期陨石撞击的后果。纳迪尔陨石坑与墨西哥杀死恐龙的奇克苏鲁布陨石坑形成时间大致相同，可能与那次事件有关。陨石坑已经在2D地震数据上被识别出来，但新获得的3D数据将更清楚地阐明陨石坑的形态和特征，使我们能够建立这种事件(山体滑坡、喷射物、海啸)的环境后果的更强大的模型。这也将突出替代(对于建议1004-APL)或额外的(对于新的提议)钻探地点，以测试撞击坑假说并了解其后果。钻探陨石坑将是必要的，以测试这一特征是否真的是由撞击坑形成的，如果是的话，测试陨石坑的准确年龄。这将使我们能够准确地确定它的年代，并测试希克苏鲁布事件实际上是一个双星系统(两颗小行星)，还是在数十万年或数百万年的过程中属于一个寿命更长的“撞击星团”的一部分。

This proposal seeks to build a stratigraphic framework from seismic reflection data to prepare for a future ocean drilling proposal in West Africa. This drilling proposal will aim to understand (1) the timing of separation of South America and Africa, forming the 'Equatorial Atlantic Gateway' when the ocean basins of the North Atlantic and South Atlantic were connected for the first time; (2) ocean conditions (temperature, chemistry, oxygen content) during extreme warm periods in Earth history; and (3) to understand the environmental consequences of a potential meteorite impact ~65 million years ago.The first objective, to understand the opening of the Equatorial Atlantic Gateway (EAG), is important as this event had a major influence on Earth's climate and weather patterns, because of changing ocean circulation conditions. However, there are major uncertainties in our understanding of this event, particularly when it formed, as it occurred some time during the 'Cretaceous Normal Superchron' (CNR) - a period of unusual stability in Earth's magnetic field. Magnetic field reversals are important for dating the age of ocean crust, but this is not possible during the CNR. Our seismic reflection data, coupled with biostratigraphic data from exploration boreholes, will provide new constraints on the tectonic events that led to the EAG opening. It will also highlight areas where we can drill and directly sample and date the sediments that record these events, as well as the oldest ocean crust that records the separation of the continents. It will also provide new insights into the sequence of tectonic events that occurred during the opening of the EAG and the long term deepening of the gateway through time.The second objective is to understand the state of the Earth's oceans during the Cretaceous period and Cenozoic. During the Cretaceous period in particular, the global climate and the ocean of the Atlantic was much warmer than in the present day, and it was characterised by periods of intense anoxia (low oxygen levels) and burial of organic carbon. These events, called Oceanic Anoxic Events (OAEs), appear to have been triggered by episodes of intense greenhouse gas emissions from massive volcanic eruptions and lasted hundreds of thousands to perhaps a million years. Although they've been drilled many times, we don't have good information on how extensive they were - how much of the water column was affected for example - or on feedback processes recorded in these sediments. We plan to identify sites for drilling along a depth transect, and where seismic data indicates that they are exceptionally thick, to obtain important new information on these events.Finally, we aim to build on an existing IODP proposal (1004-APL) to understand the consequences of a meteorite impact at the end of the Cretaceous period. The Nadir Crater formed at approximately the same time as the dinosaur-killing Chicxulub Crater in Mexico and may be related to that event. The crater has been identified on 2D seismic data but newly acquired 3D data will illuminate the crater morphology and character much more clearly, allowing us to build more robust models of the environmental consequences of this event (landslides, ejecta, tsunamis). This will also highlight alternative (for proposal 1004-APL) or additional (for the new proposal) drill sites to test the impact crater hypothesis and understand its consequences. Drilling the crater will be necessary to test whether this feature was indeed formed by an impact crater and, if so, the precise age of the crater. This will allow us to date it precisely and to test whether the Chicxulub event was actually a binary system (two asteroids) or part of a longer lived 'impact cluster' over a course of hundreds of thousands or several million years.