Terrestrial Methane Cycling During Paleogene Greenhouse Climates

从工业化前到今天，人类活动导致pCO 2和甲烷水平增加。虽然前者的增加主要是由于化石燃料燃烧，但甲烷浓度的增加更为复杂，不仅反映了直接的人类活动，而且反映了气候系统中与温度和水文引起的甲烷排放变化有关的反馈机制。为了解开这些复杂的关系，科学家们越来越多地询问古代气候系统。同样，古气候研究的主要挑战之一是了解甲烷地球化学在控制无冰、高pCO 2温室世界气候中的作用，例如在古近纪早期（约50 Ma）。缺乏甲烷浓度的替代指标是一个问题，因为湿地的甲烷排放受降水和温度的影响，因此它们可能对气候产生重要的正反馈或负反馈。事实上，对过去甲烷水平（pCH 4）的唯一估计来自我们的气候-地球化学模拟，其中GCM驱动了谢菲尔德动态植被模型，从中推导出甲烷通量。这表明，古近纪的pCH 4可能是现代工业化前水平的近6倍，这样的值将产生几乎相当于pCO 2加倍的辐射强迫效应，在CO2水平已经远远高于今天的时间间隔内，这种影响可能特别引人注目。因此，一个更好的了解古近纪pCH 4是至关重要的，了解如何地球变暖的地球上的地球化学过程中运作，并了解在地球history.We的这一重要间隔的气候建议，以改善，扩大和询问这些模型的结果，使用改进的土壤地球化学算法，进行模型的敏感性实验，并比较我们的结果在古代湿地甲烷循环的代理记录。前者将提供一个更好的，面向过程的了解生物微量气体排放，特别是排放的甲烷，氮氧化物和一氧化二氮。敏感性实验将集中在不同的二氧化碳分压水平和操纵大气参数，决定云的形成;总之，这些实验将限制我们的跟踪温室气体估计的不确定性。为了对这些模型进行定性测试，我们将量化脂质生物标志物并确定其碳同位素组成，以估计过去产甲烷和甲烷营养种群的大小，并将其与现代沼泽和全新世泥炭进行比较。我们项目的最后一部分将是确定这些升高的甲烷（和其他痕量气体）浓度对全球变暖起着积极的反馈作用。结合我们的工作，我们将检验这样一个假设，即在始新世温室效应期间，pCO 2、大陆温度和降水的升高导致湿地温室气体排放和大气浓度增加，对气候有显著的反馈作用，迄今为止，大多数模型研究都没有。这项工作对于我们理解温室气候至关重要，但这种综合方法在世界其他任何地方都没有进行;在这里，它由有机地球化学，气候，植被和大气建模以及古植物学和煤岩石学方面的国际专家领导。它将代表我们在理解古代地球化学循环及其对未来全球变暖的潜在反应方面迈出的重要一步。

Human activity has led to an increase in pCO2 and methane levels from pre-industrial times to today. While the former increase is primarily due to fossil fuel burning, the increase in methane concentrations is more complex, reflecting not only direct human activity but also feedback mechanisms in the climate system related to temperature and hydrology-induced changes in methane emissions. To unravel these complex relationships, scientists are increasingly interrogating ancient climate systems. Similarly, one of the major challenges in palaeoclimate research is understanding the role of methane biogeochemistry in governing the climate of ice-free, high-pCO2 greenhouse worlds, such as during the early Paleogene (around 50Ma). The lack of proxies for methane concentrations is problematic, as methane emissions from wetlands are governed by precipitation and temperature, such that they could act as important positive or negative feedbacks on climate. In fact, the only estimates for past methane levels (pCH4) arise from our climate-biogeochemistry simulations wherein GCMs have driven the Sheffield dynamic vegetation model, from which methane fluxes have been derived. These suggest that Paleogene pCH4 could have been almost 6x modern pre-industrial levels, and such values would have had a radiative forcing effect nearly equivalent to a doubling of pCO2, an impact that could have been particularly dramatic during time intervals when CO2 levels were already much higher than today's. Thus, an improved understanding of Paleogene pCH4 is crucial to understanding both how biogeochemical processes operate on a warmer Earth and understanding the climate of this important interval in Earth history.We propose to improve, expand and interrogate those model results using improved soil biogeochemistry algorithms, conducting model sensitivity experiments and comparing our results to proxy records for methane cycling in ancient wetlands. The former will provide a better, process-orientated understanding of biogenic trace gas emissions, particularly the emissions of CH4, NOx and N2O. The sensitivity experiments will focus on varying pCO2 levels and manipulation of atmospheric parameters that dictate cloud formation; together, these experiments will constrain the uncertainty in our trace greenhouse gas estimates. To qualitatively test these models, we will quantify lipid biomarkers and determine their carbon isotopic compositions to estimate the size of past methanogenic and methanotrophic populations, and compare them to modern mires and Holocene peat. The final component of our project will be the determination of how these elevated methane (and other trace gas) concentrations served as a positive feedback on global warming.In combination our work will test the hypothesis that elevated pCO2, continental temperatures and precipitation during the Eocene greenhouse caused increased wetland GHG emissions and atmospheric concentrations with a significant feedback on climate, missing from most modelling studies to date. This work is crucial to our understanding of greenhouse climates but such an integrated approach is not being conducted anywhere else in the world; here, it is being led by international experts in organic geochemistry, climate, vegetation and atmospheric modelling, and palaeobotany and coal petrology. It will represent a major step forward in our understanding of ancient biogeochemical cycles as well as their potential response to future global warming.