Characterizing mantle input during the Cenomanian/Turonian oceanic anoxic event using osmium isotopes

地球科学的基本目标是重建地质历史，以便了解过去的事件如何塑造了地球的演化。来自地球古代海洋的岩石沉积物为过去的事件提供了线索。地球的历史周期性地记录了海洋在短时间内（50万年到100万年）突然耗尽氧气（缺氧）的证据。据信，海洋缺氧事件与关键洋流环流的中断、气候扰动和/或强烈的岩浆活动有关，这些活动导致了富含碳的黑色岩石的沉积。这些全球海洋缺氧事件的原因仍然是一个激烈争论的话题，解决这个问题将对我们理解为什么海洋缺氧事件仅在白垩纪很常见具有深远的意义。我们建议研究的一个特定的海洋缺氧事件发生在9350万年前的塞诺曼尼亚-土鲁尼亚地质时期的边界。研究古代海洋的地球科学家依靠海洋沉积物中的化学信号来重建海洋过程和环境，例如碳、氧和锶的同位素。从海洋沉积物中测定的锇同位素也可以用来监测大陆风化和岩浆活动进入海洋的输入和通量的变化。我们目前的理解表明，9350万年前的地球与今天的地球有着相似的板块构造，我们已经很好地确定了洋流循环路径，并且单一的锇同位素组成表明锇主要来自地壳的碎屑。然而，就在Cenomanian-Turonian边界海洋缺氧事件之前，最近的一项研究表明，海洋中的锇来自强烈的岩浆活动，这可能引发了全球海洋缺氧事件。然而，与最近的研究相比，我们研究的两个剖面的锇同位素值显示，这些时间相关值是完全不同的，这表明海洋中的锇同位素收支有很大的不同。因此与今天的海洋非常不同。因此，这提出了一个重要的问题：研究人员迄今为止所依赖的一个地点的锇同位素值能否用于解释全球过程？在两个全球剖面上进行详细的锇同位素分析，将有助于全面了解塞诺曼尼亚- turonian边界海洋缺氧事件期间海洋中锇同位素的均匀性。这将有助于增进对个别海洋区域（例如，对海洋的输入和风化率）和全球洋流的了解。作为直接结果，这项研究将检验岩浆活动是否引发了全球海洋缺氧的假设。特别重要的是，拟议研究的结果将有利于研究海洋以了解地球演化的研究人员。

The fundamental goal of Earth Science is to reconstruct geologic history in order to understand how past events have shaped the evolution of planet Earth. Rock deposits from the Earths ancient oceans hold clues to the past events. The Earth's history periodically records evidence that the ocean's abruptly become depleted in oxygen (anoxic) for short periods of time (0.5 to 1 million years). It is believed oceanic anoxic events are linked to lapses in key oceanic current circulations, climate perturbations, and/or intense magmatic activity that have resulted in the deposition of black carbon-rich rocks. The causes of these global oceanic anoxic events remain a hotly debated topic and resolution of the problem will have profound implications for our understanding of why oceanic anoxic events are common just to the Cretaceous. One specific oceanic anoxic event we propose to study occurred 93.5 million years ago at the boundary of the Cenomanian-Turonian geologic time stages. Geoscientists who study ancient oceans rely on chemical signals in oceanic sediment to reconstruct oceanic processes and environments, for example isotopes of carbon, oxygen and strontium. Isotopes of osmium determined from oceanic sediments can also be used, whereby they monitor changes in the input and flux of continental weathering and magmatic activity in to the oceans. Our current understanding suggests that the Earth 93.5 million years ago had a similar plate tectonic structure to today's Earth for which we have well established ocean current circulation paths and a single osmium isotope composition that suggests that osmium is predominantly derived from detritus from the Earth's crust. However, just prior to the Cenomanian-Turonian boundary oceanic anoxic event a recent study suggests that osmium in the oceans is sourced from intense magmatic activity, which may have triggered a global oceanic anoxic event. However, osmium isotope values from two sections we have studied compared to this recent study show that these time-correlative values are disparate, suggesting that the osmium isotope budget in the ocean was drastically different. Thus very unlike today's ocean. As a result this raises an important question: can osmium isotope values from one location, which researchers have relied on thus far, be used to interpret global processes? Detailed osmium isotope analysis across two global sections will aid in defining a comprehensive understanding of the osmium isotope homogeneity in the ocean during the Cenomanian-Turonian boundary oceanic anoxic event. This will permit an improved understanding of individual oceanic areas (e.g., inputs into the ocean and weathering rates) and global ocean currents. As a direct result the research will test the hypothesis of whether magamtic activity triggered the cause of global oceanic anoxia. Especially significant is that the outcome of the proposed study will benefit researchers who are investigating the ocean to understand the Earths evolution.