Controls on the oxygen and carbon isotopic composition of planktonic foraminifera: Implications for paleoceanographic reconstructions

Living planktonic foraminifera were cultured in the laboratory to quantify the influence of temperature and additional factors on the oxygen and carbon isotopic composition of their shells. New temperature: δ18O relationships (paleotemperature equations) for O. universa (symbiotic) and G. bulloides (non-symbiotic) explain field data from plankton tows and sediment traps better than previously published equations. O. universa relationships vary with irradiance and [CO₃2−], and those for G. bulloides vary with ontogeny (shell size). Most previously published paleotemperature equations overestimate temperature by 1.0–2.5°C relative to the O. universa equations and by 3–5°C relative to the G. bulloides equations when ambient temperature varies from 15–25°C.; G. bulloides δ13C decreases 0.11‰ °C−1, probably due to increased incorporation of respired CO₂ at higher metabolic rates and temperatures. In contrast, O. universa δ13C is independent of temperature when grown under low light (LL) and increases slightly with temperature (0.05‰ °C−1) when grown under high light (HL), possibly due to temperature enhanced symbiont photosynthetic rates.; HL O. universa δ18O is not influenced by phosphate concentration variations in the range of 0.8–2.7 μmol kg−1. HL O. universa δ 13C increases ∼0.4‰ when phosphate concentration increases from 0.8–1.4 μmol kg−1. Therefore, variations in surface ocean nutrients will not affect isotopic temperature calculations, but they could influence $d13CSCO2$ reconstructions.; Species-specific paleotemperature equations calibrated here yield similar average temperatures in the Southern California Bight for G. bulloides, N. pachyderma, and O. universa over the last 25 kyr. Glacial G. bulloides in Santa Barbara Basin are an encrusted morphotype that gives LGM temperatures that are too cold when using the new G. bulloides paleotemperature equation, whereas using the LL O. universa equation gives more realistic glacial temperatures. Calculated glacial-interglacial temperature differences (G-I ΔT) for the Southern California Bight are ∼5.5°C in the north and west and ∼8.5–9.5°C in the south and east. The glacial temperatures and G-I ΔT calculated here agree well with previous estimates using the modern analog technique (MAT) and N. pachyderma coiling ratios. Previous studies using alkenone unsaturation ratios $Uk\prime 37$ and faunal transfer functions predict much warmer LGM temperatures and smaller G-I ΔT.