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.

我们建议在太平洋中部和英皇海山链地区开展新的海上研究活动。具体来说，我们对古代火山顶部的珊瑚礁的建筑和几何形状很感兴趣。在海洋中部环境中，只要有浅层基质，珊瑚和其他碳酸盐生产者就会形成重要的珊瑚礁。正如达尔文在19世纪晚期所假设的那样，当火山岛沉入地幔时（这一过程被称为沉降），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屿。有时，环礁中间会有一个泻湖，那是火山以前所在的地方。有趣的是，环礁可以存活数百万年，但也可能在海洋表面下死亡和淹死。被淹没的环礁被称为“guyots”。太平洋中部的一系列海盗激起了我们的兴趣。那里的珊瑚礁从早白垩纪（约1.1亿年前）到渐新世（约3000万年前）一直延伸到现在。有趣的是，海洋的化学成分在这一时期发生了巨大的变化：在早白垩纪，海洋被称为“方解石海”，在那里矿物方解石优先沉淀。大约在5000万到3000万年前，海洋的化学成分发生了变化，我们现在处于一个“文石海”，这里的碳酸盐主要由文石矿物组成。海洋的化学成分发生变化的确切原因尚不完全清楚，但一个可能的原因是，全球气候已经从白垩纪早期温暖、无冰、二氧化碳浓度高的世界发生了根本性的变化，而在最近的过去，全球气候已经从温暖、无冰、二氧化碳浓度高的世界转变为寒冷、间断的冰川期和二氧化碳浓度低的世界。我们希望通过这次活动了解两件主要的事情。第一个是方解石到文石转变过程中碳酸盐生产的性质，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成分，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珊瑚等碳酸盐生产者在水柱中变深/变浅？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我们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地区冰盖融化，海平面急剧上升。看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度可以在碳酸盐中实现，因为珊瑚礁的生长非常接近海平面，所以如果我们能追踪珊瑚礁的位置以及它是如何随时间变化的，我们就能重建全球海平面。对于白垩纪到始新世（我们感兴趣的窗口），这一点知之甚少。为了实现我们的两个主要目标，我们需要收集6个不同溺亡环礁（guyots）的数据；这确保了我们涵盖了整个感兴趣的地质时期，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到达的时间，我们可以重建环礁的结构。在未来，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取样。