The kinetic oxygen cycle in chemical sediments

Introduction

Temperature is probably the most fundamental parameter in Earth sciences, but it has been vigorously debated for most of Earth’s history. Temperature derived from carbonate 18O/16O isotope ratios (δ18Oc) can be biased by:

1. Kinetic effects
2. Alteration
3. Late precipitation of secondary carbonates

Methodology

KinO shows that each of these mechanisms is expected to follow a different trajectory in δ18Oc vs. δ17Oc space. Within this triple oxygen isotope space, different precipitation mechanisms and different types of kinetic effects fall on characteristic slopes. This allows for corrections of these effects, enabling the attainment of absolute paleo-temperatures.

Oxygen isotopes are classically analyzed in materials presumed to form in near-equilibrium with water, mainly aiming to attain paleo-temperatures. KinO follows a completely different paradigm by specifically focusing on materials with large kinetic isotope effects. This approach allows for quantitative estimates of fundamental parameters other than temperature, such as paleo-pH.

Kinetic Effects in Sulfate

Sulfate δ18OSO4 is in notorious disequilibrium with water. While sulfite (SO32-) equilibrates with water, the final oxidation step to sulfate (SO42-) induces a kinetic effect. KinO demonstrates how the respective kinetic effects can be identified and corrected for, providing insight into:

• The formation mechanisms
• The respective physicochemical conditions (e.g., pH)
• Formation temperatures

This approach can provide an alteration-resistant paleo-thermometer for the Archean.

Applications

The methodological development proposed in KinO will be employed to understand marine C and O isotope excursions in the geological record, including:

1. The Paleocene-Eocene Thermal Maximum (PETM)
2. The Neoproterozoic Shurham anomaly

The latter represents the largest negative carbon excursion on Earth and precedes the Cambrian explosion of life. The PETM represents the most recent equivalent of a CO2 and CH4 induced climatic excursion associated with the sixth largest mass extinction.