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.
Source
Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records
Study at a glance
- Design
- Animal / in-vitro — 18O-exchange incubations of Ammonia tests at chemical equilibrium with calcite, 30–190 °C
- N
- Hours-to-weeks incubations including 54 days at 90 °C; oxidative versus non-oxidative cleaning
- Population
- Calcitic foraminifera tests in 18O-enriched artificial seawater
- Outcome
- Oxygen exchanges along nanocrystallite grain boundaries fast enough that even glassy tests can be isotopically biased
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Inspect source passages
Foram grain boundaries can reset δ18O in ~100 years
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Foraminifera oxygen isotopes provide a continuous paleotemperature record over 120 million years.
Oxygen isotope ratios of calcitic foraminifera tests in ocean sediments constitute the most detailed continuous proxy record of deep-water and sea-surface temperatures over the last 120 million years (Ma) .
About 2.4% of oxygen exchanged after 8 days at 190 °C before the rate dropped.
After 8 days of incubation that resulted in an exchange of about 2.4% of the oxygen in the foraminifera test, the oxygen isotope exchange rate decreased and continued at a much lower rate (Fig.
What they did
Authors incubated Ammonia tests in 18O-enriched artificial seawater at chemical equilibrium with calcite (30–190 °C, hours to weeks), compared oxidative vs non-oxidative cleaning, and used exchange kinetics plus ultrastructure to identify the diffusion path.
What they found
Over 1% of test oxygen exchanged in 54 days at 90 °C without texture change. Kinetics fit grain-boundary diffusion (~2.4% of oxygen in boundaries) far faster than abiotic calcite. Extrapolated to 0 °C, grain boundaries equilibrate with pore fluid in about 100 years, so glassy screening cannot guarantee a pristine paleotemperature.
The limits
What it doesn't show
Experiments use enriched 18O and elevated temperature; the exact bias in buried ocean sediments depends on pore-fluid δ18O and burial history. Grain-boundary widths in biominerals were assumed, not measured, and only Ammonia was incubated.
Key terms
- Grain-boundary diffusion
- Fast isotope transport along nanocrystallite contacts rather than through crystal interiors.
- Glassy test
- Visually pristine foraminifera shell assumed (often wrongly here) to be diagenetically unaltered.
- Vital effects
- Biological fractionation that offsets shell δ18O from inorganic calcite equilibrium.
- Nanocrystallites
- 50–200 nm calcite building blocks of hyaline foraminifera tests.
- Paleotemperature
- Seawater temperature inferred from foraminifera oxygen isotopes.
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Common questions
Why isn’t adsorption the explanation?
Exchanged oxygen exceeds plausible surface monolayers, especially above 90 °C.
Does visual ‘glassy’ screening solve this?
No; exchange occurred without visible texture change.
How much of the test is grain boundary?
About 2.4% of oxygen, matching 65–115 nm nanocrystallites with ~0.5–1 nm boundaries.
What archive is at risk?
The 120-Myr foraminifera δ18O paleotemperature record.
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