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。这些时间段被称为“温室”气候。白垩纪（1.45亿至6500万年前）是温室气候的极端末期。那时的气候比现在暖和得多；极地几乎没有冰，海平面也很高。在极地地区，比如今天很冷的阿拉斯加和南极洲，恐龙、鳄鱼和热带植物繁盛。白垩纪的海洋对氧浓度的变化也很敏感，有时会在大范围内完全缺氧（所谓的“海洋缺氧事件”或oae）。白垩纪变暖的控制以及温室气候与短时间事件（如oae）之间的联系尚不清楚。在不同的时间尺度上，有两个因素可能很重要。在很长的时间尺度上（数百万年），大气中二氧化碳的变化和大陆的位置是气候的主要驱动因素。大陆的位置可以影响海洋环流的模式，而海洋环流控制着地球表面的热量运动和海洋内部的热量储存。在现代世界，地表水下沉到高纬度地区的海洋深处，然后填满海洋盆地的最深处。这种深水团块的形成为大部分海洋提供了氧气。然而，在极端温暖的时期，如白垩纪，我们不确定这些过程是否有效，这对气候和海洋环境都有影响。利用369号探险期间收集的新的深海沉积物岩心，该项目将使用一种元素（钕）作为晚白垩纪深水团块的示踪剂，这将使我们能够确定深水来自何处（高纬度还是低纬度？），这些来源是否随着大陆的移动而改变，以及水团来源与其他气候和环境现象（如oae）之间的关系。在更短的时间尺度上，比如几十到几十万年，气候是由地球轨道的变化控制的。这些变化影响来自太阳的能量的季节和地理分布以及地球接收的能量总量，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了解地球上哪些过程在过去控制区域气候方面发挥了重要作用。此外，由于每种旋回类型都有一个特征持续时间，计数旋回可以用来确定海底沉积物给定厚度所代表的时间量。这反过来又可以通过提供地质记录中关键点（如某些生物的进化和灭绝）之间经过的时间的准确估计，来帮助完善地质时间尺度。该项目将以极高的分辨率（每米超过50次测量）测量369远征队沉积物岩心的化学和物理特征，测量距离为10到100米。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.