期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：4
页码：995-1000
DOI：10.1073/pnas.1415718112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe history of oxygen at Earth's surface is intimately tied to its production by oxygenic photosynthesis, whereby plants, algae, and cyanobacteria release O2 as a waste product. Despite this metabolism's profound importance, its evolutionary timing is poorly understood. Studies have increasingly revealed a temporal disconnect between evidence for the presence of O2 during weathering as early as 3.0 billion years ago and its atmospheric accumulation 500 million years later. We review this problem and numerically demonstrate that local O2 production and immediate consumption in surface-bound (benthic) microbial ecosystems at profound disequilibrium conditions is the most parsimonious explanation for this delay. Thus, emergence of continental landmass was likely a crucial factor in the earliest oxygenation of Earth's surface environment. The Great Oxidation Event (GOE) is currently viewed as a protracted process during which atmospheric oxygen increased above [~]10-5 times the present atmospheric level (PAL). This threshold represents an estimated upper limit for sulfur isotope mass-independent fractionation (S-MIF), an Archean signature of atmospheric anoxia that begins to disappear from the rock record at 2.45 Ga. However, an increasing number of papers have suggested that the timing for oxidative continental weathering, and by conventional thinking the onset of atmospheric oxygenation, was hundreds of million years earlier than previously thought despite the presence of S-MIF. We suggest that this apparent discrepancy can be resolved by the earliest oxidative-weathering reactions occurring in benthic and soil environments at profound redox disequilibrium with the atmosphere, such as biological soil crusts and freshwater microbial mats covering riverbed, lacustrine, and estuarine sediments. We calculate that oxygenic photosynthesis in these millimeter-thick ecosystems provides sufficient oxidizing equivalents to mobilize sulfate and redox-sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant S-MIF signature. As continental freeboard increased significantly between 3.0 and 2.5 Ga, the chemical and isotopic signatures of benthic oxidative weathering would have become more globally significant from a mass-balance perspective. These observations help reconcile evidence for pre-GOE oxidative weathering with the history of atmospheric chemistry, and support the plausible antiquity of a terrestrial biosphere populated by cyanobacteria well before the GOE.
