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Foram grain boundaries can reset δ18O in ~100 years

Oxygen exchanges along foraminifera nanocrystallite grain boundaries fast enough that even glassy tests can be isotopically biased.

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What diffusion path resets foraminifera δ18O without texture change?

Rapid oxygen grain-boundary diffusion among nanocrystallites.

How much oxygen exchanged after 54 days at 90 °C?

Over 1% of all oxygen in the test, with no visible texture change.

How fast do grain boundaries equilibrate at 0 °C?

About 100 years with surrounding pore fluids.

Why is ‘glassy’ screening insufficient?

Visually pristine tests can still exchange isotopes along grain boundaries.

What organism and order were incubated?

Benthic hyaline Ammonia (Rotaliida).

What fraction of oxygen sits in grain boundaries?

About 2.4%, after which 190 °C kinetics switch to slower lattice diffusion.

How do biogenic grain-boundary rates compare with abiotic calcite?

Substantially faster in foraminifera tests than in abiogenic carbonates.

Why is lattice diffusion too slow?

Even the highest literature D would raise δ18O only ~2‰ in 8 days at 190 °C, far below the measured exchange.

Did oxidative cleaning stop enrichment?

No; cleaned tests were only slightly less enriched, so organics are not the whole story.

How long is the foram δ18O paleotemperature archive?

About 120 million years of deep-water and sea-surface temperatures.

What grain-boundary volume fraction is implied for 20–50 nm nanocrystallites?

Boundaries may be 2–9% of biocarbonate volume.