Foraminiferal Geochemistry and Paleoceanographic Proxies

Foraminifera are unicellular marine protists that produce calcareous shells or tests whose chemical composition records past ocean conditions. The incorporation of trace elements and stable isotopes into foraminiferal calcite varies systematically with environmental parameters such as temperature, salinity, carbonate chemistry and oxygenation. Measurements of Mg/Ca ratios in shell walls serve as a palaeothermometer, whilst oxygen isotope ratios (δ18O) integrate sea surface temperature and global ice volume. Emerging proxies, including manganese-to-calcium (Mn/Ca) ratios, boron-to-calcium (B/Ca) and species-specific trace-element banding, extend reconstructions to low-oxygen conditions, salinity changes and carbonate system perturbations. Rigorous calibration and correction for diagenetic alteration, notably grain-boundary diffusion, are essential to preserve signal fidelity. A multiproxy strategy enables disentangling of overlapping influences of temperature, seawater chemistry and biological “vital effects” on test geochemistry. These reconstructions underpin our understanding of climate variability, polar amplification, deoxygenation events and oceanic carbon cycling across geological timescales.

Research from Nature Portfolio

Recent studies have refined sea surface temperature reconstructions by combining Mg/Ca and δ18Oc measurements in polar planktonic foraminifera. A multiproxy calibration isolating seawater carbonate chemistry demonstrates that conventional Mg/Ca thermometry may underestimate polar amplification by up to 3 °C during glacial intervals without accounting for carbonate system variability. Additionally, experiments investigating post-burial alteration reveal that rapid oxygen grain-boundary diffusion in glassy foraminiferal calcite can bias δ18O-derived temperatures by as much as +0.86 to −0.46 °C. Correcting for this diffusion process across large datasets of fossil tests improves the precision of temperature reconstructions and can be extended to other biogenic carbonates with similar nanocrystalline textures.

Foraminiferal Geochemistry and Paleoceanographic Proxies publication trend

The graph below shows the total number of articles in foraminiferal geochemistry and paleoceanographic proxies across all publications each year (not limited to Nature Index journals).

Technical terms

Foraminiferal test: The calcite shell produced by foraminifera, recording environmental chemistry.

Mg/Ca ratio: The proportion of magnesium to calcium in calcite, used to infer past seawater temperatures.

δ18O: The ratio of 18O to 16O in carbonate, reflecting temperature and global ice volume changes.

Mn/Ca ratio: The manganese to calcium ratio in calcite, employed as a proxy for redox and oxygenation conditions.

Grain-boundary diffusion: Post-depositional exchange of isotopes along crystal boundaries that can alter original geochemical signals.

Multiproxy approach: The combined use of multiple geochemical indicators to isolate environmental variables and improve reconstruction accuracy.

References

1. A solution for constraining past marine Polar Amplification. Nature Communications (2024).
2. Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records. Communications Earth & Environment (2023).
3. 500 million years of foraminiferal calcification. Earth-Science Reviews (2023).
4. Single chamber Mg/Ca analyses of Globigerinoides ruber for paleo-proxy calibration using femtosecond LA-ICP-MS. Scientific Data (2024).
5. Benthic Foraminiferal Mn/Ca as Low‐Oxygen Proxy in Fjord Sediments. Global Biogeochemical Cycles (2023).