Exploring the roles of ocean circulation and orbital forcing on palaeoceanographic conditions in the southern Tethys during the Late Cretaceous

在地球历史的大部分时间里，大气中的二氧化碳水平和全球平均温度被认为比现在高得多。这段时间被称为“温室”气候。白垩纪(1.45亿至6500万年前)是温室气候的极端末端。当时的气候比现在温暖得多，极地冰层很少，甚至没有，海平面很高。在今天寒冷的阿拉斯加和南极洲等极地地区，恐龙、鳄鱼和热带植物繁衍生息。白垩纪海洋对氧气浓度的变化也很敏感，有时在大片区域变得完全缺乏氧气(所谓的海洋缺氧事件或OAE)。对白垩纪变暖的控制，以及温室气候和短期事件(如OAEs)之间的联系，还不是很清楚。在不同的时间尺度上，有两个因素可能是重要的。在长时间尺度上(数百万年)，大气二氧化碳的变化和大陆的位置是气候的主要驱动因素。大陆的位置会影响海洋环流的模式，海洋环流控制着地球表面周围的热量移动，以及海洋内部的热量储存。在现代世界中，地表水在高纬度地区的海洋中沉到很深的地方，然后填满海洋盆地的最深部分。这种深水团的形成为海洋大部分地区提供了氧气。然而，在极端温暖的时期，如白垩纪，不确定这些过程是否可操作，对气候和海洋环境都有影响。利用在369次探险中采集到的新的深海沉积物岩心，该项目将使用一种元素(钕)作为晚白垩世深水物质的示踪剂，这将使我们能够确定深水来自哪里(高纬度或低纬度？)，这些来源是否随着大陆的移动而随时间变化，以及水团来源与其他气候和环境现象之间的关系，如OAE。在较短的时间尺度上，从10秒到100秒，数千年来，气候受地球轨道变化的控制。这些变化影响了来自太阳的能量的季节和地理分布以及地球接收的总能量，所有这些都可能导致气候振荡。研究这些气候周期的特征有助于理解过去地球上哪些过程在控制区域气候方面起到了重要作用。此外，由于每种周期类型都有其特有的持续时间，因此可以使用计数周期来确定给定海底沉积物厚度所代表的时间量。这反过来又可以通过提供对地质记录中关键点之间经过的时间量的准确估计，例如某些生物的进化和灭绝，来帮助完善地质时间尺度。该项目将在10s到100s的范围内以极高的分辨率(每米超过50次测量)测量369次探险的沉积物岩心的化学和物理特征。然后可以对这些数据进行分析，以确定存在多少个周期以及属于哪种类型。

For much of Earth history atmospheric CO2 levels and average global temperatures are thought to have been much higher than present. These periods of time are known as 'greenhouse' climates. The Cretaceous (145 to 65 million years ago) was an extreme end-member of a greenhouse climate. Then the climate was much warmer than it is today; there was little or no polar ice and sea-levels were high. In polar areas, like Alaska and Antarctica, which are cold today, dinosaurs, crocodiles and tropical plants flourished. The Cretaceous ocean was also sensitive to changes in oxygen concentration, and, at times, became completely devoid of oxygen over widespread areas (so-called 'oceanic anoxic events' or OAEs). The controls on Cretaceous warmth and the links between the greenhouse climate and short-duration events, such as OAEs, are not well understood. Two factors are likely to have been important over different timescales. Over long-timescales (millions of years), changes in atmospheric carbon dioxide and the position of the continents are major drivers of climate. The positions of the continents can affect patterns of ocean circulation, which control the movement of heat around the Earth surface, and the storage of heat in the interior of the ocean. In the modern world, surface water sinks to great depths in the oceans in high-latitude regions and then fills the deepest parts of the ocean basins. This formation of deep-water masses provides oxygen throughout much of the ocean. However, in times of extreme warmth, such as the Cretaceous, it is uncertain whether these processes were operational, with implications for both climate and ocean environments. Using the new deep-sea sediment cores collected during Expedition 369, this project will use an element (neodymium) as a tracer for deep-water masses in the Late Cretaceous, which will allow us to determine where deep-water was coming from (high or low latitudes?), whether these sources changed through time as the continents moved and the relationships between water mass sources and other climatic and environmental phenomena, such as OAEs.On shorter-time scales of 10s to 100s of thousands of years, climate is controlled by variations in Earth's orbit. These variations effect the seasonal and geographic distribution of energy from the sun and the total amount of energy the planet receives, all of which can lead to climatic oscillations. Investigating the signature of these climatic cycles can help understand which processes on Earth were important in controlling regional climates in the past. Furthermore, because each cycle type has a characteristic duration, counting cycles can be used to determine the amount of time represented by a given thickness of sediment on the sea floor. This in turn can help refine the geological time scale by providing accurate estimates of the amount of time that elapsed between key points in the geological record, such as the evolution and extinction of certain organisms. This project will make measurements of the chemical and physical characteristics of the sediment cores from Expedition 369 at extremely high-resolution (more than 50 measurements per m) over 10s to 100s of meters. These data can then be analysed to determine how many cycles are present and of what type.