Constraining the rise of oxygen with oxygen isotopes

Area of Science:

• Geochemistry
• Isotope Geology
• Paleoclimatology

Background:

• Atmospheric oxygenation influenced Earth's sulfur cycle, shifting control from atmospheric chemistry to weathering.
• The sulfur isotope record between 2.5 to 2.3 billion years ago shows mixed signals of anoxia and oxygenation, requiring further investigation.
• Oxygen isotopes in sulfate can provide independent clarification of past atmospheric conditions.

Purpose of the Study:

• To investigate the origin of sulfur isotope anomalies in ancient barites.
• To clarify the atmospheric and weathering conditions during Earth's early oxygenation.
• To establish a diagnostic tool for identifying early oxidative continental weathering.

Main Methods:

• Analysis of sulfur isotope anomalies (Δ³³S) and oxygen isotopes (δ¹⁸O) in sedimentary barites.
• Geochemical and sedimentary evidence interpretation.
• Dating of barite samples from the Turee Creek Basin, Western Australia.

Main Results:

• Discovery of <2.31 Ga barites with positive sulfur isotope anomalies (Δ³³S up to +1.55‰) and low oxygen isotope values (δ¹⁸O down to -19.5‰).
• Identification of sulfide oxidation in meteoric water as the source of these isotopic anomalies.
• Evidence suggesting transfer of sulfur anomalies from a paleo-continent under an oxygenated atmosphere.

Conclusions:

• The combination of low δ¹⁸O and high Δ³³S in sulfates is a reliable indicator of sulfide oxidation in meteoric water.
• This isotopic signature points to incipient oxidative continental weathering around 2.8-2.5 billion years ago or earlier.
• The findings refine our understanding of Earth's early sulfur cycle and atmospheric evolution.