A culturing study of the planktonic foraminifer Globigerina bulloides has extended a culture-based magnesium-to-calcium temperature calibration down to 6 °C, improving the species’ use in subpolar and polar reconstructions. The findings suggest that existing warm-water equations may underestimate temperature sensitivity in Norwegian Sea specimens and therefore distort cold-ocean reconstructions.
The study addresses a longstanding limitation in paleoceanography. G. bulloides is widely used in marine sediment archives, but most culture-based calibrations linking magnesium-to-calcium ratios to temperature were developed above 14 °C. To test whether those relationships hold in colder waters, the iC3-led team cultured Norwegian Sea specimens at 6 to 13 °C, across salinities of 30.4 to 37.8, and under both coupled and decoupled carbonate chemistry conditions.
“A good proxy is not just a number,” says Adele Westgård, a co-author on the new study. “It is a tested relationship between biology, chemistry and the environment. Our publication helps researchers see where the relationship is strong, and where it needs more caution.”
That principle shaped the experimental design. The team grew living G. bulloides under controlled temperature, salinity, and carbonate chemistry conditions, then used a barium label to identify shell calcite formed during the experiments. Magnesium-to-calcium, sodium-to-calcium, and strontium-to-calcium ratios were then measured chamber by chamber using laser ablation inductively coupled plasma mass spectrometry, allowing the researchers to isolate laboratory-grown material rather than bulk shell calcite.
Magnesium-to-calcium ratios showed a positive exponential relationship with temperature across the 6 to 13 °C range, extending the culture calibration into colder waters. Sodium-to-calcium also varied systematically with temperature, but in the opposite direction. None of the elemental ratios tested showed a significant relationship with salinity, while both sodium-to-calcium and strontium-to-calcium responded to carbonate ion concentration at constant pH, albeit in opposite directions.
“This study reminds us that the shell is a living archive, not a passive recorder,” said Freya Sykes, the study’s first author. “To use shells well, we need to understand both the ocean conditions and the organism that made the shell.” Co-author Mohamed Ezat added: “By combining culturing experiments, geochemistry and paleoceanography, we can better understand the biological and environmental processes behind the climate signals recorded in marine archives.”
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