Establishment of the western Pacific warm pool during the Pliocene: Evidence from planktic foraminifera, oxygen isotopes, and Mg/Ca ratios

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Abstract

This study presents new evidence of when and how the Western Pacific Warm Pool (WPWP) was established in its present form. We analyzed planktic foraminifera, oxygen isotopes, and Mg/Ca ratios in upper Miocene through Pleistocene sediments collected at Deep Sea Drilling Program (DSDP) Site 292. These data were then compared with those reported from Ocean Drilling Program (ODP) Site 806. Both drilling sites are located in the western Pacific Ocean. DSDP Site 292 is located in the northern margin of the modern WPWP and ODP Site 806 near the center of the WPWP. Three stages of development in surface-water conditions are identified in the region using planktic foraminferal data. During the initial stage, from 8.5 to 4.4 Ma, Site 806 was overlain by warm surface water but Site 292 was not, as indicated by the differences in faunal compositions and sea-surface temperature (SST) between the two sites. In addition, the vertical thermal gradient at Site 292 was weak during this period, as indicated by the small differences in the δ18O values between Globigerinoides sacculifer and Pulleniatina spp. During stage two, from 4.4 to 3.6 Ma, the SST at Site 292 rapidly increased to 27 °C, but the vertical thermal gradient had not yet be strengthened, as shown by Mg/Ca ratios and the presence of both mixed-layer dwellers and thermocline dwellers. Finally, a warm mixed layer with a high SST ca. 28 °C and a strong vertical thermal gradient were established at Site 292 by 3.6 Ma. This event is marked by the dominance of mixed-layer dwellers, a high and stable SST, and a larger differences in the δ18O values between G. sacculifer and Pulleniatina spp. Thus, evidence of surface-water evolution in the western Pacific suggests that Site 292 came under the influence of the WPWP at 3.6 Ma. The northward expansion of the WPWP from 4.4 to 3.6 Ma and the establishment of the modern WPWP by 3.6 Ma appear to be closely related to the closure of the Indonesian and Central American seaways.

Introduction

The modern Western Pacific Warm Pool (WPWP) occupies much of the tropical–subtropical region of the western Pacific and eastern Indian oceans and has an average annual sea-surface temperature (SST) of above 28 °C (Yan et al., 1992, Webster and Palmer, 1997; Fig. 1). It is maintained by easterly trade winds and is composed of a thick, warm, mixed layer. As a result, the thermocline deepens in the western equatorial Pacific (> 200 m) and shoals gradually toward the eastern equatorial Pacific (~ 50 m). Fluctuations in the temperature and size of the WPWP are important factors influencing the El Niño/Southern Oscillation (ENSO) and have a global impact through atmospheric teleconnections.

Several studies have shown that the evolutionary history of the WPWP was closely related to the closure of the Indonesian and Central American seaways during the Miocene and Pliocene (e.g., Keller, 1985, Kennett et al., 1985, Srinivasan and Sinha, 1998, Chaisson and Ravelo, 2000, Jian et al., 2006, Li et al., 2006). Biogeographic data (Kennett et al., 1985) and isotopic data (Keller, 1985) of planktic foraminifera from three time slices in the Miocene showed the progressive stimulation of surface and subsurface circulation in the equatorial Pacific and the gradual elevation of the Equatorial Under Currents (EUC). This earlier works focused on the effects of the constriction of the Indonesian Seaway. Using planktic foraminiferal data from the western and eastern equatorial Pacific, Chaisson and Ravelo (2000) suggested the development of the modern east–west thermocline in the equatorial Pacific from 4.5 to 4 Ma related to the closure of the Central American Seaway. Jian et al. (2006) studied planktic foraminiferal records in the South China Sea and suggested (1) the early formation of the WPWP from 11.5 to 10.6 Ma related to the closure of the Indonesian Seaway, and (2) the final formation of the WPWP from 4 to 3.2 Ma linked to the closure of the Central American Seaway. These studies imply a stepwise development of the WPWP due to the closure of the Indonesian and Central American seaways. However, the timing and process of the establishment of the modern WPWP remains unclear. For example, recent investigations have yielded contradictory conclusions regarding oceanic conditions in the equatorial Pacific during the early Pliocene warm period. Using oxygen isotope and magnesium-to-calcium (Mg/Ca) ratio of planktic foraminifera in the western and eastern equatorial Pacific, Rickaby and Halloran (2005) suggested cool, La Niña conditions during this warm period. Alternatively, others have suggested that the Pliocene warm period was characterized by permanent El Niño-like conditions (Wara et al., 2005, Ravelo et al., 2006).

To investigate when and how the modern WPWP was established, we analyzed planktic foraminiferal assemblages, oxygen isotopes, and Mg/Ca ratios from Deep Sea Drilling Program (DSDP) Site 292 in the western Pacific and compared them with the results of Chaisson (1995), Chaisson and Ravelo (2000), and Wara et al. (2005) from Ocean Drilling Program (ODP) Site 806 located in the western equatorial Pacific. DSDP Site 292 is located in the northern margin of the modern WPWP and ODP Site 806 lies near the center of the WPWP (Fig. 1). Therefore, comparison of data from Sites 292 and 806 is ideally suited for investigating expansion of the modern WPWP.

Section snippets

Materials and methods

DSDP Site 292 was drilled on the southeastern part of the Benham Rise (15°49.11′N, 124°39.05′E, water depth 2943 m) in the western margin of the West Philippine Sea (Ingle et al., 1975; Fig. 1). The area of Site 292 is directly influenced by the WPWP (Fig. 1), except during El Niño events when the WPWP migrates eastward due to weakened easterly winds. The modern, average annual SST at Site 292 is 28.5 °C, the salinity is 34.4‰, and the depth of the thermocline is approximately 180 m at the

Depth rankings of planktic foraminiferal species

Planktic foraminifera live in the upper water column, with distinct depth habitats (Bé, 1977, Ravelo and Fairbanks, 1992, Watkins et al., 1996). The δ18O value of planktic foraminiferal shells calcified in isotopic equilibrium is indicative of the temperature and isotopic composition of the water in which they lived. Therefore, comparisons of the δ18O values from different species enable us to assign relative depth rankings to planktic foraminifera (Keller, 1985, Gasperi and Kennett, 1992,

Surface-water evolution at Site 292 based on comparisons to Site 806

Comparison of planktic foraminiferal census records from DSDP Site 292 to those of ODP Site 806 (Chaisson, 1995, Chaisson and Ravelo, 2000, Wara et al., 2005) in the western Pacific (Fig. 1) suggest that three stages occurred in the paleoceanographic history of the western Pacific over the past 8.5 myr.

In the initial phase I, two Miocene species, G. nepenthes and S. seminulina, dominated the assemblage at Site 292 (Fig. 3a). In contrast, before 4.4 Ma, the assemblage at Site 806 was

Conclusions

This study presents planktic foraminiferal data from DSDP Site 292 and ODP Site 806 that record several phases of the development toward the modern WPWP by the mid-Pliocene. Variations in planktic foraminiferal assemblage, oxygen isotope, and Mg/Ca ratio data show three stages in the surface-water evolution of the western Pacific initiating at 8.5 Ma. From 8.5 to 4.4 Ma, SST values based on Mg/Ca ratios in G. sacculifer and oxygen isotope data from G. sacculifer and Pulleniatina spp. suggest

Acknowledgments

Review and comments by Dr. Stephen Gallagher and an anonymous reviewer significantly helped to improve the manuscript. We thank Dr. Y. Iryu of Tohoku University (present address: Nagoya University) for the use of the stable isotope facilities at Tohoku University, and Dr. T. Yamada and Dr. R. Asami of Tohoku University for supporting the stable isotope analyses. We also thank Mr. H. Yamamoto and Mr. M. Takada of Marine Works Japan, Ltd. for their assistance with the microelement analyses at the

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