Violent release of thermogenic gases as a driver in the Paleogene climate & carbon cycle?

古代全球变暖事件与人为气候变化有着根本的不同，但这些事件提供了一个独特的机会来研究气候和碳循环在几个世纪到几千年的时间尺度上是如何运作的。我们可以从古代变暖事件的环境后果中学到很多东西，但要验证这些古代变暖事件与气候预测的相关性，详细重建这些变暖事件的原因至关重要。标志着古新世和始新世的全球变暖事件为失控的温室气候提供了有价值的气候和碳循环响应的“地质实验”。这些事件中最极端的是古新世-始新世极热期（约5600万年前），其标志是在几千年内全球变暖了5摄氏度。有趣的是，大约在同一时期，大规模的火山活动形成了一个“大火成岩省”（LIP），以今天的冰岛为中心，导致北大西洋的开放。在古新世晚期和始新世早期，在数百万年的时间里，这个LIP总共喷发了约1000万立方公里的火山，这些火山喷发从地球深处带上来并释放出的二氧化碳很可能是造成那个时期大气中二氧化碳含量和全球气温上升的原因。此外，挪威近海海床的地震成像揭示了第二种温室气体释放机制，其形式是古代海床上和海床下的100到1000公里宽的陨石坑和烟囱。强烈而广泛的岩浆侵入的短暂阶段加热了海底深处富含碳的沉积层，产生了大量的气体，尤其是甲烷、二氧化碳和水。这些烟囱和火山口是由剧烈的喷发释放出的气体形成的。每一次侵入阶段都迅速激活了许多烟囱和陨石坑，可能会释放出大量的温室气体。然而，直到最近，由于缺乏合适的工具和材料，很难证明大规模的LIP火山活动与更迅速的气候变化之间的联系，更不用说量化其影响了。我们研究了这些喷口对气候的影响，特别是岩浆侵入周围产生的甲烷和二氧化碳，并通过剧烈的喷口释放出来。探险队396号将从其中一个火山口的中心和火山口边缘外获得两个新的岩心。这种独特的材料将使我们能够重建喷口活动的详细历史。年龄诊断化石和稳定的碳同位素将用于重建火山口形成的时间，确定沉积物填充的日期和活动的持续时间。多年来，沉积汞浓度数据已成为重建过去火山活动的工具，我们将利用这一工具进一步了解火山口外的喷发活动。这些分析与其他已建立的工具相结合，以评估热流体对火山口沉积物的影响，并利用热液流体循环信号元素（如铁和锰）的浓度峰值来评估热液羽流对周围海水的影响。总之，这些新数据将显示陨石坑形成的时间、释放气体的时间和强度。我们将这些新数据与现有数据结合起来，重建了排气对气候变化的潜在贡献。这将提高我们对古代变暖事件的理解，为碳循环模型和直接（古）气候预测提供信息。

Ancient global warming events differ fundamentally from anthropogenic climate change, yet these events provide a unique opportunity to study how the climate and carbon cycle operate on timescales of centuries to millennia. We can learn much from the environmental consequences of ancient warming events, but detailed reconstructions of the causes of such ancient warming events are essential to validate their relevance for climate projections. The global warming events that mark the Paleocene and Eocene provide valuable 'geological experiments' of climate and carbon cycle response to a runaway greenhouse climate. The most extreme of these events, the Paleocene-Eocene Thermal Maximum (ca. 56 Million years ago) is marked by 5 degree C of global warming within a few thousands of years. Intriguingly, around the same time, massive volcanic activity, forming a "large igneous province" or LIP, centered on present-day Iceland, led to the opening of the North Atlantic Ocean. A total of ~10 million cubic kms erupted in this LIP over a span of several millions of years in the late Paleocene and early Eocene and CO2 brought up from deep within the Earth and emitted from these eruptions present a likely cause of a rise in atmospheric CO2 levels and global temperatures over that period. Moreover, seismic imaging of the seabed offshore Norway has revealed a second greenhouse gas release mechanism, in the form of 100s to 1000s of km-wide craters and chimneys on and below the ancient seabed. Brief phases of intense and widespread magma intrusion heated carbon-rich sediment layers deep below the seabed producing large volumes of gases, particularly methane, CO2, and water. The chimneys and craters formed when gases were released by violent venting. Each intrusive phase rapidly activated many chimneys and craters at once, emitting potentially large volumes of greenhouse gases. Yet, until recently the absence of suitable tools and material meant it was difficult to demonstrate the connection between the massive LIP volcanism and more rapid climate change, let alone quantify its impact.We study the climatic influence of these vents, particularly with respect to the methane and CO2 generated around magmatic intrusions and released by violent venting. Expedition 396 will obtain two new drill cores from the center of one of the vents, and from just outside its crater rim. This unique material will allow us to reconstruct a detailed history of vent activity. Age-diagnostic fossils and stable carbon isotopes will be used to reconstruct the time of crater formation, to date the sediment infill and the duration of activity. Sedimentary mercury concentration data has been developed as a tool over the past years to reconstruct past volcanic activity, and we will employ this tool to further understand the venting activity from outside the crater. These analyses are paired with other established tools to assess the effects of hot fluids on crater sediments, and utilize concentration spikes in elements that signal circulation of hydrothermal fluids, such as iron and manganese, to assess the influence of the hydrothermal plume on surrounding sea water.Together, these new data will show when the crater formed, for how long it released gases and at what intensity. We use these new data in combination with existing data to reconstruct the potential contribution of venting on climate change. This will improve our understanding of ancient warming events, inform carbon cycle models and - (in)directly - (paleo)climate projections.