Paleoclimate
Foram grain boundaries can reset δ18O in ~100 years
Open access · cc by · source: Europe PMC
Oxygen exchanges along foraminifera nanocrystallite grain boundaries fast enough that even glassy tests can be isotopically biased.
How this study connects
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Oxygen exchanges along foraminifera nanocrystallite grain boundaries fast enough that even glassy tests can be isotopically biased.
Evidence for the claim as stated.
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.
Key findings
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.
Methodology
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.
Limitations
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.
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