Iron and sulfur in the pre-biologic ocean

Area of Science:

• Geochemistry
• Early Earth Science
• Biogeochemical Cycles

Background:

• Reconstructing pre-biologic Earth conditions is crucial for understanding life's origins.
• Geochemical cycles of key elements like iron, oxygen, and sulfur provide insights into early Earth's environment.

Purpose of the Study:

• To develop tentative geochemical cycles for the pre-biologic Earth.
• To infer the relative abundances of oxygen, dissolved iron, and sulfide in the early atmosphere and ocean.

Main Methods:

• Comparative analysis of relative fluxes of oxygen, dissolved iron, and sulfide.
• Assessment of element insolubility to infer environmental concentrations.
• Comparison with geologic records of isotopic compositions.

Main Results:

• Iron flux exceeded oxygen and sulfide fluxes, suggesting abundant dissolved iron.
• Oxygen and sulfide were rare in the early atmosphere and ocean.
• Low sulfate concentrations were due to the absence of continental oxidative weathering.

Conclusions:

• The pre-biologic Earth likely had abundant dissolved iron and scarce oxygen and sulfide.
• Early organisms' sulfur metabolism probably utilized oxidized sulfur species rather than sulfide.
• Findings align with isotopic evidence from sedimentary sulfates and sulfides.