Reconstruction of oceanographic properties and atmospheric circulation at the marginal regions of the Western Pacific Warm Pool during the late Quaternary

Abstract

Evolution of surface ocean environment in the eastern margin of the Western Pacific Warm Pool and its forcing mechanisms during the late Quaternary were investigated using various biogenic and lithogenic components in the northwestern and central equatorial Pacific where paleoclimatic records are rare due to paucity of sedimentary archives. To better understand the coupled behavior and interactions between the atmosphere and surface ocean dynamics, the provenance of eolian mineral dust and the composition of biogenic microfossils were investigated together. For the last 600 kyr, eolian mineral dust deposited in the northwest Pacific, represented by a sediment core (12°30N, 135°00E) from the Philippine Sea, has been supplied predominantly from the central Asian deserts (e.g., Taklimakan desert). It highlights that the prevailing wind system, rather than seasonal monsoon, has been the main transporting agent of eolian dust from Asian inland to the North Pacific. In contrast, eolian mineral dust deposited in a central equatorial Pacific site (5°53N, 177°26W) reveals Australian/South American dust affinity before 0.8 Ma but gradually increasing Asian dust signal after 0.8 Ma. The change in dust provenance was accompanied by an increased dust flux and a decrease in surface productivity and salinity. These changes can be explained by the southward movement of the Intertropical Convergence Zone (ITCZ) and North Equatorial Counter Current (NECC) and the direct influence of these features on the site after 0.8 Ma. The southward movement of the ITCZ is in phase with the warming trend observed in upwelling regions of the equatorial Pacific and Atlantic between 0.9 and 0.8 Ma. The southward movement of the ITCZ from 0.9 to 0.8 Ma indicates more significant cooling in the Northern Hemisphere (NH) than in the Southern Hemisphere across the Mid-Pleistocene Transition, which is supportive of the interpretation that the NH ice sheet expanded significantly and stabilized after 0.9 Ma. For the last deglacial period, water column structure and temperatures were investigated in the central equatorial Pacific (6°40′N, 177°28′W) from a multiple-core by using oxygen isotopic compositions and Mg/Ca ratios of three planktplankticonic foraminifer species (G. sacculifer, P. obliquiloculata, and G. tumida) that dwell in mixed layer, upper thermocline, and lower thermocline, respectively. Only the upper thermocline temperatures have risen as high as 3°C during the last deglaciation between 18 and 12 ka, and then decreased to the present level since then. It can be explained by weakening of water column stratification in the upper part of thermocline and shallowing of P. obliquiloculata habitat at this time. Such change coincides the period of decreased pole to equator temperature gradients in the SH and following NH cooling events, the Heinrich Stadial 1 (HS1) and Younger Dryas (YD), resulting in intensification of northeast Trades and southward migration of the ITCZ. Intensified wind-driven mixing and reduced precipitation during the last deglacial period likely had caused relatively nutrient-rich surface water condition without a significant surface cooling and migration of P. obliquiloculata to shallower depth. This interpretation leads to a conclusion that the ITCZ was placed far south to the study site during the last deglaciation and the study site was influenced by northeast Trades that prevails north of the ITCZ. This thesis emphasizes the strong control of atmospheric processes on the surface oceanography at the study site, i.e., north central equatorial Pacific, on the long-term global climate change.