A paleoceanography concept defining records and proxies used to reconstruct past ocean conditions and climate variability. It governs how microfossils, sediments, and geochemical signals are dated, correlated, and translated into environmental estimates. It does not provide a single climate variable without calibration and uncertainty assessment for the proxy and chronology. It materially affects understanding of past circulation, temperature, and ice volume by preserving time-resolved signals in cores. The concept is generally stable, though chronological methods and proxy interpretations are refined over time.

Principle

Mg incorporation into biogenic carbonates increases with temperature; Mg/Ca ratios are converted to temperature using species-specific or regional calibration equations, while correcting for seawater Mg/Ca changes and vital effects.

Demonstration

Planktonic foraminifera from a Holocene core yield Mg/Ca ratios that, after cleaning and species-specific calibration and accounting for salinity and dissolution, produce an SST estimate aligned with instrumental records.

Misapplication

Neglecting diagenetic overgrowths, species-specific vital effects, or past seawater Mg/Ca variations leads to misleading temperatures; using uncleaned samples that contain contaminant clays or carbonates biases Mg/Ca high.

Consequence

When properly processed and calibrated, Mg/Ca paleothermometry provides seasonally sensitive quantitative temperature reconstructions and can complement isotope-based proxies to separate temperature from ice-volume signals.

Reversal

A reversal would be to treat Mg/Ca ratios as direct proxies of seawater chemistry rather than temperature, ignoring the strong temperature dependence of Mg incorporation in many taxa.

Boundary

Applicable primarily to carbonate-secreting organisms with well-characterized calibrations; not appropriate for siliciclastic carbonates, heavily recrystallized material, or samples lacking taxonomic control.

Semantic Tension

Tension exists between Mg/Ca approaches emphasizing species-specific calibrations and those favoring pooled multi-species calibrations for signal averaging; there's also tension between Mg/Ca and δ18O interpretations of temperature versus ice volume.

Synthesis

Mg/Ca paleothermometry converts cleaned, taxonomically constrained biogenic carbonate Mg/Ca ratios through calibrated relationships into quantitative past seawater temperatures, provided diagenesis, vital effects, and seawater chemistry changes are addressed.