Foraminifera δ¹⁸O — Interactive Calculator

What Controls Foram Shell δ18O?

Foraminifera shells incorporate oxygen from seawater as they grow, but the δ18O of the shell depends on two independent factors: the temperature of the water at calcification, and the δ18O of the seawater itself (which rises during glacial periods as ¹⁶O gets locked in ice sheets).

T (°C) = 16.9 − 4.0 × (δ18Ocalcite − δ18Oseawater)
Shackleton (1974). δ18Ocalcite vs. VPDB; δ18Oseawater vs. VSMOW. Rearranged: δ18Ocalcite = δ18Osw + (16.9 − T) / 4.0
The two-component problem: A measured change in foram δ18O reflects both a temperature change and an ice-volume change in the seawater. Separating them requires an independent temperature proxy — typically Mg/Ca ratios in the same shells.

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0–5°C = deep/polar water  ·  5–15°C = temperate  ·  20–30°C = tropical surface
Modern ocean ≈ 0‰  ·  Last Glacial Maximum ≈ +1.0–1.2‰  ·  warm Pliocene ≈ −0.3‰
Set to the pre-industrial or modern value at the same site for comparison
+2.23‰
Predicted shell δ¹⁸Ocalcite
+1.73‰
Temperature contribution
0.00‰
Ice-volume contribution
+6.0°C
Temperature anomaly vs. reference

Signal breakdown

Temperature effect
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Ice-volume effect
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Total shell δ¹⁸O
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Mg/Ca cross-check: If Mg/Ca measurements independently constrain the calcification temperature to 10.0°C, then the remaining δ¹⁸O signal not explained by temperature must reflect seawater δ¹⁸O — and therefore global ice volume. Each 0.011‰ change in seawater δ¹⁸O corresponds to approximately 1 m of sea-level change.