Terrestrial Methane Cycling During Paleogene Greenhouse Climates (NERC grant NE/J008591/1)

Supplementary material for published paper, Early Paleogene wildfires in peat-forming environments at Schoningen, Germany by BE Robson et al, http://doi.org/10.1016/j.palaeo.2015.07.016 NERC grant abstract: 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.

补充材料，发表于论文《德国Schoningen地区早中新世泥炭形成环境中的森林火灾》by BE Robson 等人，http://doi.org/10.1016/j.palaeo.2015.07.016 NERC项目摘要：人类活动导致自工业革命以来大气中二氧化碳（pCO2）和甲烷（CH4）浓度不断上升。其中，pCO2浓度上升主要归因于化石燃料的燃烧，而CH4浓度的增加则更为复杂，这不仅反映了人类活动的直接影响，还体现了气候系统中与温度和水分变化相关的甲烷排放反馈机制。为了揭示这些复杂的关联，科学家们正日益深入探究古代气候系统。同样，古气候研究的主要挑战之一在于理解甲烷生物地球化学在调控无冰、高pCO2温室世界（如早中新世约50Ma）气候中的作用。由于缺乏CH4浓度的替代指标，这一问题变得尤为棘手，因为湿地CH4排放受降水和温度影响，它们可能对气候产生重要的正反馈或负反馈作用。实际上，过去CH4水平（pCH4）的唯一估计源于我们的气候生物地球化学模拟，其中GCMs驱动了Sheffield动态植被模型，从而推导出CH4通量。这些模拟表明，早中新世的pCH4可能接近现代前工业化水平的6倍，这样的数值将产生与pCO2加倍相当的辐射强迫效应，这种影响在CO2水平已远高于今日的时间段中尤为显著。因此，对早中新世pCH4的深入了解对于理解在更温暖的地球上生物地球化学过程如何运作，以及理解地球历史上这一重要时段的气候至关重要。我们计划通过改进土壤生物地球化学算法、进行模型敏感性实验，并将我们的结果与古代湿地CH4循环的替代记录进行比较，以改进、扩展并检验这些模型结果。前者将提供对生物源温室气体排放（特别是CH4、NOx和N2O排放）的更深入、以过程为导向的理解。敏感性实验将集中于pCO2水平的变动和对决定云形成的大气参数的操作；这些实验共同将限制我们对温室气体估计的不确定性。为了定性地检验这些模型，我们将量化脂质生物标志物并确定其碳同位素组成，以估计过去产甲烷菌和甲烷氧化菌的种群规模，并将它们与现代泥炭沼泽和全新世泥炭进行比较。我们项目的最后部分将是确定这些升高的CH4（及其他温室气体）浓度如何作为全球变暖的正反馈。综合我们的工作将检验假设，即古新世温室时期的升高pCO2、大陆温度和降水导致了湿地温室气体排放和大气浓度的增加，这对气候产生了显著的反馈作用，而这一点在迄今为止的大多数模拟研究中都未被考虑。这项工作对于理解温室气候至关重要，但这样的综合方法在世界其他地方尚未实施；在这里，它由有机地球化学、气候、植被和大气模拟以及古植物学和煤成岩学领域的国际专家领导。这将是我们在理解古代生物地球化学循环及其对未来全球变暖潜在响应的认识上迈出的重大一步。