Lenton Timothy M, Dahl Tais W, Daines Stuart J, Mills Benjamin J W, Ozaki Kazumi, Saltzman Matthew R, Porada Philipp
Earth System Science, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4QE, United Kingdom;
Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark;
Proc Natl Acad Sci U S A. 2016 Aug 30;113(35):9704-9. doi: 10.1073/pnas.1604787113. Epub 2016 Aug 15.
The progressive oxygenation of the Earth's atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O2) first approached modern levels (∼21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435-392 Ma, and the appearance of fossil charcoal indicates O2 >15-17% by 420-400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burial-the net long-term source of O2 We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today's global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2‰ increase in the carbonate carbon isotope (δ(13)C) record by ∼445 Ma, and predict a corresponding rise in O2 to present levels by 420-400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O2 levels.
地球大气的逐步氧化对生命演化至关重要，但大气氧（O₂）首次接近现代水平（约21%）的时间和方式之谜仍未解开。氧化还原替代数据表明，在4.35 - 3.92亿年前深海已被氧化，而化石木炭的出现表明在4.20 - 4.00亿年前O₂>15 - 17%。然而，现有模型未能预测此时的氧化作用。在此我们表明，最早从约4.70亿年前开始在陆地表面定殖的植物，通过大幅增加全球有机碳埋藏——O₂的长期净来源，导致了这次古生代中期的氧化事件。我们使用基于性状的生态生理模型预测，到约4.45亿年前，隐花植物植被覆盖可能已达到当今全球陆地净初级生产力的约30%。来自现代苔藓植物的数据表明，这种丰富的早期植物物质的摩尔碳磷比（约2000）比海洋生物量（约100）高得多，以至于给定的磷风化通量能够支持更多的有机碳埋藏。此外，最近的实验表明，早期植物选择性地增加了从岩石中风化出来的磷（相对于碱度）的通量。在一个长期生物地球化学循环模型中综合这些效应，我们重现了到约4.45亿年前碳酸盐碳同位素（δ¹³C）记录持续增加2‰的情况，并预测到4.20 - 4.00亿年前O₂相应上升到当前水平，这与地球化学数据一致。这次氧气上升代表了调节机制的永久性转变，转变为一种由火灾介导的负反馈稳定高O₂水平的机制。
