Oldest Known Redox Gradient Discovered

©NASA/UCSD

By analyzing iron isotopes against the uranium content in the jasper rock from the ancient ocean of the Barberton Greenstone Belt in South Africa, scientists have found a defined vertical redox gradient, called a redoxcline, showing a change in the level of oxygenation from the deeper part of the ocean leading to the shallower portion.

While the seawater at deeper level is depleted of oxygen, samples in the photic zone of the ocean, the area where sunlight is able to reach, show low levels of dissolved oxygen. The iron-uranium tests place the oxygenation occurrence at 3.2 billion years ago, 200 millions earlier than previous estimates for ocean redox. The team hypothesizes that oxygen at the photic zone may have been produced through photosynthesis by microorganisms such as cyanobacteria. That then suggests that cyanobacteria also evolved earlier than 3.2 billion years ago.

So far, this discovery stands as the oldest known redoxcline in the ancient oceans, and pushes back the date for the beginnings of ocean redox.

The full paper, "A redox-stratified ocean 3.2 billion years ago" is published in Earth and Planetary Science Letters.

This study was supported by the NASA Astrobiology Institute at the University of Wisconsin-Madison and the DST-NRF Centre of Excellence for Energy and Mineral Resource Analysis (CIMERA) at the University of Johannesburg.

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