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，这可以说是生命史上最重要的三个转折点之一，但人们对这种转变发生的确切原因和方式仍然知之甚少。有一种现象可能推动或至少稳定了向更氧化状态的过渡，那就是产甲烷菌的崩溃。产甲烷菌是太古代时期的一种主要生命形式，通过产生大量甲烷来维持全球缺氧。该项目的目的是验证Konhauser et al. （2009; Nature 458, 750）最近的假设，他将产甲烷菌的减少归因于镍的短缺，镍是这些生物必需的微量营养物质。这些作者认为，生物甲烷产量可能早在氧气上升之前就减弱了，因为海洋Ni浓度在2.7至2.5 Ga之间大幅下降。现代产甲烷菌对Ni稳定同位素的强烈分馏表明，产甲烷菌对Ni的限制可能在沉积记录中留下了同位素指纹。本研究的目的是分析Konhauser等人记录的Ni浓度下降的同一样品的Ni同位素组成，以了解反映产甲烷菌Ni限制开始的同位素变化是否明显。样品将通过微钻从富赤铁矿的带状铁地层中提取，并通过MC-ICP-MS测量同位素组成。研究人员将特别关注年龄在28亿至23亿年之间的样品的Ni浓度与Ni同位素之间的关系，在这段时间内，产甲烷菌可能或可能没有吸收海水中越来越大比例的可用溶解Ni。

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.