Assessing the role of methanogens in driving earth oxygenation using Ni isotopes

大约24亿年前，氧气首次在大气和海洋表层水中大量而持久地积累，可以说是生命史上三个最重要的转折点之一，但这种变化到底是如何发生的，人们仍然知之甚少。可能推动或至少稳定了向氧化程度更高的状态转变的一个现象是产甲烷菌的崩溃，产甲烷菌是太古宙大部分地区的一种主要生命形式，通过产生大量甲烷来维持全球缺氧。这个项目的目标是检验Konhauser等人最近的假设。(2009；自然，458,750)，他将产甲烷菌的减少归因于镍的短缺，镍是这些生物必不可少的微量营养。这些作者认为，在氧气上升之前，生物甲烷的产量可能已经下降了很久，因为海洋中的镍浓度在2.7到2.5Ga之间大幅下降。最近发现的现代产甲烷菌强烈分馏稳定的镍同位素表明，产甲烷菌对镍的限制可能在沉积记录中留下了同位素指纹。这项研究的目的是分析Konhauser等人在同一样品中的镍同位素组成。记录了镍浓度的下降，以查看反映甲烷菌镍限制开始的同位素位移是否明显。将用微型钻头从条带状铁建造中富含赤铁矿的地层中提取样品，并用MC-ICP-MS测量同位素组成。调查人员将特别关注年龄在2.8至23亿年之间的样品的镍浓度与镍同位素之间的关系，在这段时间内，产甲烷菌可能会或可能不会同化海水中越来越大比例的有效溶解镍。

The first substantial and long-lasting accumulation of oxygen in the atmosphere and surface waters of the oceans ~2.4 billion years ago was arguably one of the three most important turning points in the history of life, but exactly why and how this transformation took place remains poorly understood. One phenomenon that likely drove or at least stabilized the transition to a more oxidizing state was collapse of methanogens, a dominant form of life throughout much of the Archean Eon that sustained global anoxia through production of abundant methane. The objective of this project is to test the recent hypothesis of Konhauser et al. (2009; Nature 458, 750), who attributed the decline of methanogens to a shortage of nickel, an essential trace nutrient for these organisms. These authors argued that biogenic methane production might have waned well before the rise of oxygen, because oceanic Ni concentrations decreased substantially between 2.7 and 2.5 Ga. The recent discovery that modern methanogens strongly fractionate stable isotopes of Ni suggests that Ni limitation of methanogens may have left an isotopic fingerprint in the sedimentary record. The aim of this study is to analyze Ni isotope compositions of the same samples in which Konhauser et al. documented the Ni concentration decline, to see if an isotopic shift reflecting the onset of Ni limitation of methanogens is evident. Samples will be extracted from the hematite-rich layers of banded iron formations by microdrill, and isotopic compositions will be measured by MC-ICP-MS. Investigator will focus in particular on relationships between Ni concentrations and Ni isotopes for samples with ages between 2.8 and 2.3 billion years, the interval over which methanogens may or may not have assimilated increasingly large proportions of available dissolved Ni in seawater.