Using species-specific paleotemperature equations with foraminifera: a case study in the Southern California Bight

Species-specific paleotemperature equations were used to reconstruct a record of temperature from foraminiferal δ 18O values over the last 25 kyr in the Southern California Bight. The equations yield similar temperatures for the δ 18O values of Globigerina bulloides and Neogloboquadrina pachyderma. In contrast, applying a single paleotemperature equation to G. bulloides and N. pachyderma δ 18O yields different temperatures, which has been used to suggest that these species record the surface-to-thermocline temperature gradient. In Santa Barbara Basin, an isotopically distinct morphotype of G. bulloides dominates during glacial intervals and yields temperatures that appear too cold when using a paleotemperature equation calibrated for the morphotype common today. When a more appropriate paleotemperature equation is used for glacial G. bulloides, we obtain more realistic glacial temperatures. Glacial–interglacial temperature differences (G–I Δ T) calculated in the present study indicate significant cooling (∼8–10°C) throughout the Southern California Bight during the last glacial maximum (LGM). The magnitude of glacial cooling varies from ∼8°C near the middle of the Southern California Bight (Tanner Basin and San Nicolas Basin) to ∼9°C in the north (Santa Barbara Basin) and ∼9.5–10°C in the south (Velero Basin and No Name Basin). Our temperature calculations agree well with previous estimates based on the modern analog technique. In contrast, studies using N. pachyderma coiling ratios, U k′ 37 indices, and transfer functions estimate considerably warmer LGM temperatures and smaller G–I Δ T.