Episodes of reduction in the Western Pacific Warm Pool during Quaternary and Its implications on the Indian Ocean: Planktic foraminiferal evidences

Abstract:

The Pulleniatina Minimum Events (PMEs), defined as the abrupt decline in the abundance of Pu. obliquiloculatadue to changing water conditions, reflect the reduction in the Western Pacific Warm Pool and strength of the Kuroshio Current. We analyzed planktic foraminiferal samples from Sulu Sea (ODP Site 769B) spanning the Quaternary.

The Sulu Sea participates in inter ocean exchange with the South China Sea, Pacific Ocean and Celebes Sea through a number of shallow straits. It is directly affected by the variation of the WPWP. These events can be easily identified by analyzing the relative abundance of key planktic foraminiferal species records.

We discovered seven PMEs, which have been interpreted as a result of the reduction in the WPWP. These events are: PME7 (0.04-0.02 Ma), PME6 (0.16-0.13 Ma), PME5 (0.48-0.44 Ma), PME4 (0.79-0.65 Ma), PME3 (0.90-0.87 Ma), PME2 (1.8-1.36 Ma) and PME1 (2.21-2.08 Ma). The PMEs are associated with an increased abundance of N. dutertrei, which is associated with increase in fertility of the water. We have also found the occurrence of temperate fauna during PME7, PME6, PME5 and PME4, which indicates the influx of cool polar waters in the Sulu Sea. The most plausible mechanism for this event could be the southward advancement of the Arctic front, which caused the southward enhancement of Oyashio Current. The cool nutrient rich waters of the Oyashio Current caused a decline in the SST of the Sulu Sea and increased the fertility.

The PME3, PME2 and PME1 do not show presence of temperate fauna. The very high relative abundance of N. dutertrei indicates the influx of nutrient rich waters from the Western Pacific Ocean via Luzon strait. These events suggest the development of El Niño like conditions.

The PMEs have been correlated with the relative abundance of upwelling indicators (Gg. bulloides and Ga. glutinata) from the equatorial Indian Ocean (ODP Site 716A). Each PME is accompanied by an increase in the relative abundance of upwelling indicators, thereby suggesting enhanced equatorial upwelling due to the convergence of trade winds at the ODP site 716A. This explains the northward migration of ITCZ during the reduction in WPWP due to strengthening of the temperature pressure gradient in the Indian Ocean.