Pulleniatina Minimum Events from the Sulu Sea as evidence for Reduction in the Western Pacific Warm Pool during Quaternary

The Western Pacific Warm Pool (WPWP) significantly affects the heat budget and associated climate over the Indian Ocean region. The reduction in the WPWP is manifested in the form of reduced Indonesian Throughflow (ITF) and may be well represented by the census count and oxygen isotope records of planktic foraminifera.

The present study is an attempt to reconstruct the episodes of reduction in the WPWP during the Quaternary, on the basis of extremely low relative abundance of planktic foraminifera Pulleniatina (Pu.) obliquiloculata from the ODP Hole 769B in the Sulu Sea. This species is a thermocline dweller, that thrives in tropical to warm subtropical latitudes. It is considered to be an indicator of Kuroshio Current and shows a direct correlation with the expansion of the WPWP.

In the ODP Hole 769B, Pu. obliquiloculata shows a low relative abundance during the Quaternary, except a few instances of significant increase, so much so that it comprises almost 50% of the entire faunal assemblage. The rising trend is characteristic of high SST and an expansion in WPWP. We have identified seven distinct events of sharp decline in the relative abundance of Pu. obliquiloculata (<5%) and named these events as Pulleniatina Minimum Events (PMEs). These events are: PME-1- (2.21-2.08 Ma); PME- 2 (1.8-1.36 Ma); PME- 3 (0.9-0.87 Ma); PME-4 (0.79-0.65 Ma), PME-5 (0.48-0.44 Ma), PME-6 (0.16-0.13 Ma) and PME-7 (0.04-0.02 Ma) in descending stratigraphic order. We interpret these events to mark be the result of the reduction in the WPWP. We have also found the occurrence of temperate fauna during the stratigraphically younger last five PMEs (PME7 to PME3), which indicate reduction in the Western Pacific Warm Pool (WPWP) probably caused by glaciations. The glacial events probably enhanced the Oyashio Current, which caused the influx of cool fertile waters in the Sulu Sea. The evidence of the increased fertility in the Sulu Sea is marked by the increased relative abundance of Neogloboquadrina dutertrei, a fertility indicator species, during the PME7 to PME3. The PME2 and PME1 show no presence of temperate fauna. These events of reduction in the WPWP may be attributed to the development of El Niño like conditions.

The PMEs were also correlated with the five PL events recorded by Sinha et al (2006) from ODP Hole 763A in the Eastern Indian Ocean. These PL events: PL-1- (2.22 Ma); PL-2 (1.83 Ma); PL-3 (0.68 Ma), PL-4 (0.45 Ma) and PL-5 (0.04 Ma), represented reduced strength of ITF either due to the glacial events (PL-3 to PL-5 events) or due to ENSO induced changes (reduction) in the WPWP (PL-1-and PL-2 events).

The PMEs show striking correlation with the PL events, giving testimony to episodes of reduction in the WPWP during Quaternary.