IMPULSE: Taking the Pulse of the Icelandic Mantle Plume

地幔是地球最大的组成部分，占地球体积的84%。虽然地幔是固体的，但在地质时期，它像流体一样剧烈地搅动，这一过程被称为地幔对流，是由地球内部放射性衰变的加热和上面的冷却驱动的。地幔对流使整个地球表面变形，形成一个直径数千公里、高度数公里的“动力地形”，形成一个由起伏和洼地组成的环环相扣的格局。动力地形影响洋流格局、陆地表面侵蚀和被侵蚀沉积物的积累，这些影响控制着宝贵的天然矿产资源的分布。火山活动通常也与对流环流中热的上升元素（即地幔柱）有关。最具活力的地幔柱产生了大火成岩省（lip）——熔岩的大量喷发伴随着温室气体的大量释放到大气中。lip与地球历史上对全球气候、生态系统和碳循环的一些最显著的扰动时间一致，包括大规模物种灭绝、海洋缺氧事件和新生代最大的自然全球变暖事件。虽然人们普遍认为，在地质时期（数千万年或更长时间），地幔对流影响了地球表面和气候过程，但这些时间框架太慢，无法解释通常与LIPs同时发生的环境变化的快速发生和持续时间短。但现在越来越多的证据表明，地幔对流、动态地形和火成岩喷发的模式可以在不到100万年的时间内演变。这一理论的关键是一个被称为“热柱脉动”的过程，在这个过程中，地幔中较热和较冷的地幔团随着地幔柱内部的对流循环而移动。最大地幔柱（如冰岛地幔柱）内最热的脉冲可以以超过200毫米/年的速度上升，这比构造板块的运动要快，并且可以导致当地海平面超过1毫米/年的变化，类似于现代平均全球海平面变化。在这样的速度下，过去冰岛地幔柱的脉动可以激活北大西洋LIP产生的温室气体，其速度足以解释古新世-始新世极大热极端全球气候变化事件，这是与人为气候变化最好的自然模拟。然而，由于缺乏关键数据，羽流脉动假说并没有被冰岛或地球其他主要地幔羽流普遍接受。对冰岛附近被称为“v形脊”（VSRs）的海底特征的高质量测量将弥补这一空白，该测量包含了世界上最好的热脉冲记录。我们与其他一些VSRs模型的主要倡导者合作，设计了一个实验，通过测量制造VSRs的地壳的厚度和化学成分来确定VSRs的起源。最近对玄武岩海底进行了一次高质量的地球化学调查，不久将由一个国际钻探项目加以加强。现在，IMPULSE将首次测量几个VSRs下整个地壳的厚度和地震速度的变化（因此整体组成）。我们的试点工作表明，IMPULSE将为一百万年时间框架内的地幔温度波动提供确凿的证据，从而为地幔柱脉动假说提供第一个明确的证据。此外，通过首次正式修正洋中脊构造过程对VSR地壳厚度的复杂影响，我们的新VSR记录将确定地幔温度波动的最短周期。这些结果对于验证地幔对流如何影响地球表面和气候过程的假设至关重要。

The mantle is the largest component of the Earth, comprising 84% of our planet's volume. Although the mantle is solid, over geological time it churns vigorously like a fluid in a process known as mantle convection, driven by heating from radioactive decay in Earth's interior and cooling from above. Mantle convection deforms the Earth's entire surface into an interlocking pattern of swells and depressions known as "dynamic topography", with diameters of several thousand km and heights of several km. Dynamic topography influences oceanic current patterns, land surface erosion and accumulation of the eroded sediment, and these effects are known to control the distribution of valuable natural mineral resources. Volcanic activity also usually occurs in association with the hot, rising elements of the convective circulation, known as mantle plumes. The most vigorous mantle plumes give rise to Large Igneous Provinces (LIPs) - episodic huge outpourings of lava accompanied by voluminous release of greenhouse gases to the atmosphere. LIPs coincide in time with some of the most remarkable perturbations to global climate, ecosystems and the carbon cycle in Earth's history, including mass extinctions, Ocean Anoxic Events, and the largest natural global warming event of Cenozoic time.Whilst it is widely accepted that mantle convection has influenced Earth's surface and climate processes over geological time periods (tens of millions of years or more), these time frames are too slow to explain the rapid onset and short duration of the environmental changes that usually coincide with LIPs. But growing evidence now suggests that patterns of mantle convection, dynamic topography and igneous outpouring can evolve in less than a million years. Key to this theory is a process known as "Thermal Plume Pulsing", in which hotter and cooler blobs of mantle are carried along with the convective circulation within a mantle plume. The hottest pulses within the biggest mantle plumes, such as the Icelandic Mantle Plume, can rise at speeds in excess of 200 mm/yr, which is faster than the motion of tectonic plates, and can cause changes in local sea-level of over 1 mm/yr, similar to modern mean global sea-level change. At such speeds, past pulsing of the Icelandic Mantle Plume could have activated greenhouse gas generation from the North Atlantic LIP rapidly enough to explain the Paleocene-Eocene Thermal Maximum extreme global climate change event, the best natural analogue to anthropogenic climate change.However, the Plume Pulsing hypothesis is not universally accepted for Iceland or Earth's other major mantle plumes as key data is lacking. High-quality measurements of seafloor features near Iceland known as the "V-Shaped Ridges" (VSRs) that comprise the world's best record of the suggested hot pulses will address this gap. Working with the lead advocates of the alternative models for VSRs, we have devised an experiment to determine the origin of the VSRs by measuring both the thickness and the chemical composition of the crust that builds the VSRs. A high-quality geochemical survey of the basaltic seafloor was made recently, and it will soon be augmented by an international drilling project. Now, IMPULSE will measure the variation in thickness and seismic velocity (hence bulk composition) of the entire crust beneath several VSRs for the first time.Our pilot work indicates that IMPULSE will provide firm evidence for fluctuations in mantle temperature on a million-year timeframe to give the first definitive proof of the Mantle Plume Pulsing hypothesis. Furthermore, by formally correcting for the complicating effect of mid-ocean ridge tectonic processes on VSR crustal thickness for the first time, our new VSR record will determine the shortest time period for fluctuations in mantle temperature. These results are crucial to test hypotheses for how mantle convection has influenced Earth's surface and climate proceses.