Experimental paleotemperature equation for planktonic foraminifera

Small live individuals of Globigerinoides sacculifer which were cultured in the laboratory reached maturity and produced garnets. Fifty to ninety percent of their skeleton weight was deposited under controlled water temperature (14° to 30°C) and water isotopic composition, and a correction was made to account for the isotopic composition of the original skeleton using control groups. Comparison of. the actual growth temperatures with the calculated temperature based on paleotemperature equations for inorganic CaCO₃ indicate that the foraminifera precipitate their CaCO₃ in isotopic equilibrium. Comparison with equations developed for biogenic calcite give a similarly good fit. Linear regression with Craig's (1965) equation yields: t = -0.07 + 1.01 t ̂ (r= 0.95) where t is the actual growth temperature and t ̂ Is the calculated paleotemperature. The intercept and the slope of this linear equation show that the familiar paleotemperature equation developed originally for mollusca carbonate, is equally applicable for the planktonic foraminifer G. sacculifer. Second order regression of the culture temperature and the delta difference (δ¹⁸Oc - δ¹⁸Ow) yield a correlation coefficient of r = 0.95: t ̂ = 17.0 - 4.52(δ¹⁸Oc - δ¹⁸Ow) + 0.03(δ¹⁸Oc - δ¹⁸Ow)² t ̂, δ¹⁸Oc and δ¹⁸Ow are the estimated temperature, the isotopic composition of the shell carbonate and the sea water respectively. A possible cause for nonequilibnum isotopic compositions reported earlier for living planktonic foraminifera is the improper combustion of the organic matter.