Oligocene deep ocean oxygen isotope variations primarily driven by temperature.

Area of Science:

• Paleoclimatology
• Oceanography
• Glaciology

Background:

• Oxygen isotopes in marine calcite (δ¹⁸Ocalcite) are crucial for reconstructing past climate and Antarctic ice sheet behavior.
• Previous interpretations suggested a highly dynamic Oligocene Antarctic ice sheet based on δ¹⁸Ocalcite records.
• These interpretations are challenged by climate models and the potential overestimation of ice volume due to temperature influences on δ¹⁸Ocalcite.

Purpose of the Study:

• To reconstruct Oligocene deep ocean temperature variability independent of seawater chemistry.
• To test whether orbital-paced temperature changes can explain observed δ¹⁸Ocalcite cycles.
• To reassess the proposed dynamic behavior of the Oligocene Antarctic ice sheet.

Main Methods:

• Benthic foraminiferal clumped isotope thermometry was used to determine deep ocean temperatures.
• Analysis focused on the Oligocene epoch (34-23 million years ago).
• Orbital variability, specifically eccentricity, was investigated for its influence on temperature.

Main Results:

• The first Oligocene record of orbital variability in deep ocean temperature was established.
• Large, eccentricity-paced temperature variations of up to 4°C were identified.
• These temperature fluctuations sufficiently explain δ¹⁸Ocalcite cycles without invoking large ice volume changes.

Conclusions:

• Oligocene δ¹⁸Ocalcite cycles can be explained by significant ocean temperature variations, not necessarily continental-scale ice sheet changes.
• The Antarctic ice sheet may have been more stable during the Oligocene than previously thought.
• Deep ocean and global temperatures are sensitive to insolation distribution in warmer, Oligocene-like climates.