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广东快乐十分走势图:Limited oxygen production in the Mesoarchean ocean
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Episodic development of “oxygen oases” during the Archean Eon characterizes the hundreds of millions of years transition to permanent oxygenation in the atmosphere–hydrosphere system at the Great Oxidation Event (～2.4–2.3 Ga). One of these well-characterized oxygen oases is recorded in Mesoarchean sediments of the Pongola Supergroup. We show that in contrast to the Neoarchean, biological oxygen production in a shallow ocean having Mo-based nitrogen fixation was not sufficient to result in a dissolved nitrogen reservoir that would carry the isotopic effects of an aerobic nitrogen cycle. Nevertheless, it appears that low concentrations of bioavailable phosphorus, rather than nitrogen, suppressed the growth and expansion of oxygenic photosynthesizers and may explain why pervasive and permanent oxygenation was delayed during the Archean Eon.
The Archean Eon was a time of predominantly anoxic Earth surface conditions, where anaerobic processes controlled bioessential element cycles. In contrast to “oxygen oases” well documented for the Neoarchean [2.8 to 2.5 billion years ago (Ga)], the magnitude, spatial extent, and underlying causes of possible Mesoarchean (3.2 to 2.8 Ga) surface-ocean oxygenation remain controversial. Here, we report δ15N and δ13C values coupled with local seawater redox data for Mesoarchean shales of the Mozaan Group (Pongola Supergroup, South Africa) that were deposited during an episode of enhanced Mn (oxyhydr)oxide precipitation between ～2.95 and 2.85 Ga. Iron and Mn redox systematics are consistent with an oxygen oasis in the Mesoarchean anoxic ocean, but δ15N data indicate a Mo-based diazotrophic biosphere with no compelling evidence for a significant aerobic nitrogen cycle. We propose that in contrast to the Neoarchean, dissolved O2 levels were either too low or too limited in extent to develop a large and stable nitrate reservoir in the Mesoarchean ocean. Since biological N2 fixation was evidently active in this environment, the growth and proliferation of O2-producing organisms were likely suppressed by nutrients other than nitrogen (e.g., phosphorus), which would have limited the expansion of oxygenated conditions during the Mesoarchean.
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Author contributions: F.O.O., A.H., R.S., and A.B. designed research; F.O.O., A.H., and A.B. performed research; F.O.O. contributed new reagents/analytic tools; F.O.O., A.H., J.E.S., S.W.P., E.E.S., M.B., and A.B. analyzed data; and F.O.O., A.H., J.E.S., S.W.P., E.E.S., R.S., B.E., M.W., M.B., and A.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1818762116/-/DCSupplemental.
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