Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records

Area of Science:

• Paleoceanography
• Geochemistry
• Biomineralization

Background:

• Fossil foraminifera tests provide a continuous proxy record of past ocean temperatures.
• Oxygen isotopic composition of foraminifera calcite is a key proxy for paleotemperature reconstruction.
• Previous studies have not fully accounted for post-depositional diagenetic alterations.

Purpose of the Study:

• To investigate the impact of oxygen grain-boundary diffusion on the oxygen isotopic composition of foraminifera tests.
• To quantify the temperature bias introduced by this diffusion process.
• To develop a correction method for paleotemperature records.

Main Methods:

• Incubation of foraminifera tests in 18O-enriched artificial seawater.
• Analysis of oxygen isotopic composition using mass spectrometry.
• Microstructural analysis to detect ultrastructural changes.

Main Results:

• Optically translucent foraminifera calcite tests show measurable alteration in oxygen isotopic composition at low temperatures via grain-boundary diffusion.
• This diffusion occurs rapidly, reaching equilibrium with pore fluids within 100 years under typical oceanic sediment conditions.
• Paleotemperature estimates from 38,400 foraminifera tests were biased by up to +0.86 to -0.46°C.
• No visible ultrastructural changes were observed, indicating a subtle but significant alteration.

Conclusions:

• Oxygen grain-boundary diffusion is a significant factor affecting the accuracy of paleotemperature reconstructions from foraminifera.
• A correctable bias is introduced into paleotemperature records due to this process.
• The findings are applicable to other polycrystalline biocarbonates like corals and mollusks, suggesting a general diagenetic effect.