CARAPACE: Calcite-Aragonite transition Across Pacific Atolls from the Cretaceous to the Eocene

甲壳：从白垩纪到始新世横跨太平洋环礁的方解石-文石过渡

• 批准号: NE/W009943/1
• 负责人: Cedric John
• 金额: $ 30.56万
• 依托单位: Queen Mary University of London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2024
• 资助国家: 英国
• 起止时间: 2024 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FW009943%2F1
• 关键词: CARAPACE; Calcite; Aragonite; transition; Across

We propose a new seagoing research campaign in the Mid Pacific and the Emperor's Seamount Chain regions. Specifically, we are interested in the architecture and geometries of reefs situated atop of ancient volcanoes. In mid-ocean setting, corals and other carbonate producers tend to form significant reefs whenever a shallow substrate is available. As postulated by Darwin in the late 1800's, when the volcanic island sinks into the Earth mantle (a process called subsidence) the reefs keep growing and producing sediments, often forming a carbonate island known as an atoll. Sometimes, atolls can have a lagoon in the middle, where the volcano formerly stood. Interestingly, atolls can live for millions of years, but can also die and drown below the surface of the ocean. Drowned atolls are referred to as "guyots".A series of guyots in the mid Pacific piqued our interest. There the production of reefs extends from the Early Cretaceous (about 110 million years ago) to the Oligocene (about 30 million years ago) and to the present. Interestingly, the chemistry of the ocean has changed dramatically during this period: in the Early Cretaceous, the ocean was known as a 'calcitic sea', where the mineral calcite was preferentially precipitated. Somewhere between 50-30 million years ago, the chemistry of the oceans changed and we are now in an 'aragonitic sea', where most of the carbonate production is dominated by the mineral aragonite. Exactly why the chemistry of the ocean has changed remains only partially known, but a probable cause is that global climate had fundamentally changed from the Early Cretaceous warm, ice-free, high CO2 concentration world to the cold, punctuated glaciations and low CO2 concentration world of the recent past.There are two main things we wish to understand with this campaign. The first being the nature of carbonate production across this calcite to aragonite transition, and how the architecture of the carbonate atolls might have adapted in response to this chemical change. Understanding how past carbonates adapted to a different chemistry of the ocean is crucial for predicting how modern climate change will impact corals and other carbonate producers. For instance, will the change in chemistry force carbonate producers such as corals deeper/shallower in the water column? This would impact the geometry of the atoll. Our second objective is to understand by how much global sea-level might have change across the time interval of interest. Current global warming results in the melting of ice sheets at high-latitude, and dramatic sea-level rise. Looking at past example of sea-level changes allows us to calibrate by how much sea-level rises during global warming events. Quantifying the rate of sea-level change can be achieved in carbonates because the reef is growing very close to sea-level, so if we can track the position of the reef and how it changes through time, we can reconstruct global sea-level. For the Cretaceous to Eocene (our window of interest), this is poorly understood. To achieve our two main objectives, we need to collect data on 6 different drowned atolls (guyots); this ensures that we cover the entire geological period of interest, as each atoll is of a slightly different age. The primary data we will use in this research is known as seismic reflection: we send small seismic waves towards the bottom of the sea, and by reconstructing the time of arrival of the reflected seismic waves we can reconstruct the architecture of the atoll.In the future, we plan to select a few 'best' targets based on our data to go back to these atolls and sample the rocks using deep-sea scientific drilling.

我们建议在中太平洋和皇帝海山链区域开展一项新的海上研究活动。具体来说，我们感兴趣的是位于古代火山顶部的珊瑚礁的建筑和几何形状。在海洋中部，只要有浅底，珊瑚和其他碳酸盐生产者往往会形成重要的珊瑚礁。正如达尔文在19世纪后期所假设的那样，当火山岛沉入地幔（一个称为沉降的过程）时，珊瑚礁不断生长并产生沉积物，通常形成一个被称为环礁的碳酸盐岛。有时，环礁的中间可能有一个泻湖，那里曾经是火山的所在地。有趣的是，环礁可以存活数百万年，但也可以在海洋表面以下死亡和溺水。淹没的环礁被称为“盖奥特”。太平洋中部的一系列盖奥特引起了我们的兴趣。在那里，珊瑚礁的生产从早白垩世（约1.1亿年前）延伸到渐新世（约3000万年前）和现在。有趣的是，在这一时期，海洋的化学成分发生了巨大的变化：在早白垩世，海洋被称为“方解石海”，矿物方解石优先沉淀。在5000万到3000万年前的某个地方，海洋的化学性质发生了变化，我们现在处于一个“文石海”，其中大部分碳酸盐的生产都是由矿物文石主导的。海洋的化学成分发生变化的确切原因仍然只有部分人知道，但一个可能的原因是全球气候发生了根本性的变化，从早白垩世温暖，无冰，高CO2浓度的世界到最近的寒冷，间歇性冰川和低CO2浓度的世界。第一个是碳酸盐生产的性质跨越方解石到文石的过渡，以及碳酸盐环礁的结构如何适应这种化学变化。了解过去的碳酸盐如何适应海洋的不同化学性质，对于预测现代气候变化将如何影响珊瑚和其他碳酸盐生产者至关重要。例如，化学变化是否会迫使碳酸盐生产者（如珊瑚）在水柱中更深/更浅？这将影响环礁的几何形状。我们的第二个目标是了解全球海平面在感兴趣的时间间隔内可能发生了多大的变化。目前的全球变暖导致高纬度冰盖融化，海平面急剧上升。查看过去海平面变化的例子，我们可以根据全球变暖事件期间海平面上升的程度进行校准。量化海平面变化的速率可以在碳酸盐中实现，因为珊瑚礁的生长非常接近海平面，所以如果我们可以跟踪珊瑚礁的位置以及它如何随时间变化，我们就可以重建全球海平面。对于白垩纪到始新世（我们感兴趣的窗口），这是知之甚少。为了实现我们的两个主要目标，我们需要收集关于6个不同的被淹没环礁（盖奥特）的数据;这确保我们涵盖整个有关地质时期，因为每个环礁的年龄略有不同。我们将在这项研究中使用的主要数据被称为地震反射：我们向海底发送小地震波，通过重建反射地震波的到达时间，我们可以重建环礁的结构。未来，我们计划根据我们的数据选择几个“最佳”目标，回到这些环礁，并使用深海科学钻探对岩石进行采样。