Changes of planktonic foraminiferal assemblages in the Okinawa Trough since the last deglaciation and comparisons of sea surface temperature estimated from five foraminiferal transfer functions

Quantitatively analyzed planktonic foraminiferal data of core A7 were used to discuss the paleoenvironmental changes in the middle Okinawa Trough during the past 18000 years. The planktonic foraminiferal fauna shows apparent changes corresponding well to the deglaciation Holocene phases: At about 18-15 ka, the fauna was dominated by cold water species such as Neogloboquadrina dutertrei and Neogloboquadrina pachyderma; Cold water species decreased rapidly between 15-9.4 ka; After 9.4 ka the fauna was dominated by warm water species such as Pulleniatina obliquiloculata, Globigerinoides ruber and Globigerina glutinata. Millennial scale climate changes during the last deglaciation such as Heinrich 1 (H1), Bølling-Allerød (B/A) and Younger Dryas (YD) were also recorded by planktonic foraminiferal changes, it also shows a series of cold events (at about 2.3-4.6, 5.3, 6.2, 7.3, 8.2 ka) superimposed on a Holocene warming trend. At about 9.4 ka, a sudden change in the relative abundance of warm to cold planktonic foraminiferal species probably indicates a sudden strengthening of the Kuroshio Current in the Okinawa Trough. Five foraminiferal transfer functions were used to estimate the paleo sea-surface temperature (SST) in core A7, through comparisons with SST estimates from two independent methods (U37k and Mg/Ca-based SST), and the reliability of different foraminiferal transfer functions were evaluated. Our results suggest that SIMMAX doesn't show apparent deglaciation Holocene SST changes. The other two transfer functions (MAT and RAM), which are also based on modern analog techniques as SIMMAX, both show small SST variations after 13 ka, without any indication of decreased SST during the YD period. Transfer functions FP-12E and IKM Chen based on Imbrie Kipp method, show general deglaciation Holocene SST trends similar to U37k and Mg/Ca-based SST, however, the deglacial warming is about 1 ka later than the B/A warming, and with no apparent SST decreases during the YD period. We find out that the abnormal SST estimated by FP-12E and IKM Chen during the last deglaciation were probably caused by Globigerina bulloides , the relative abundance of which was probably controlled by productivity during that period. Through getting rid of the productivity effect of G. bulloides in FP-12E and IKM Chen, the two modified transfer functions provide SST estimates comparable to the U37k and Mg/Ca-based SST. Therefore, we concluded that the modified foraminiferal transfer functions of FP-12E and IKM Chen are more suitable for paleo SST estimates in the middle Okinawa Trough.