Atmospheric Science

Accounting for Delay

See allHide authors and affiliations

Science  02 Oct 2009:
Vol. 326, Issue 5949, pp. 21
DOI: 10.1126/science.326_21c

Earth's early atmosphere was lacking in oxygen for nearly 2 billion years; a variety of data imply that the concentration of the oxidizing gas began to rise abruptly about 2.4 billion years ago. The degree to which biological processes may have contributed to this rise remains subject to debate—some evidence suggests that cyanobacteria, which produce oxygen as a by-product of photosynthesis, appeared at least several hundred million years earlier, but why then the delay? Two isotopic studies from rocks several hundred million years older than the prominent oxidation event imply that some feedbacks may have been involved. Godfrey and Falkowski show that nitrogen isotopes record a type of nitrogen cycling in the oceans requiring the presence of free oxygen. In turn, this cycling would have depleted inorganic nitrogen required by cyanobacteria or plankton for growth, limiting their oxygen production. Frei et al. report a chromium isotope record, which traces oxidative weathering. Their data implicate small and transient amounts of oxygen in the atmosphere and ocean during this time. Together these and other data imply that the final abrupt oxidation of Earth's atmosphere reflected some fits and starts over the previous several hundred million years.

Nat. Geosci. 2, 10.1038/ngeo633 (2009); Nature 461, 250 (2009).

Navigate This Article