Collaborative Research: Documenting Variations in the Southern Hemisphere Influence on the Western Pacific Warm Pool Thermocline Since 1.5 Ma

In the western tropical Pacific there is an extensive area of warm surface ocean where sea surface temperatures exceed 28°C (or 82.4°F). This region, termed the Western Pacific warm pool (WPWP) releases a vast amount of water vapor and heat into the atmosphere and is thus a key region for regulating global climate and hydrology. Principal Investigators B. Linsley and K. Gibson have recently discovered that in the previous two time periods of global warming during the transition from glacial conditions to interglacial conditions, that the layer of the ocean 100 meters below the warm surface begins warming and reaches peak warming several thousand years before the surface ocean. They have observed this through geochemical analysis of calcareous microfossil shells preserved in deep sea sediment cores recovered from the WPWP. Modern ocean circulation patterns suggest that this subsurface warming is initiated in the Southern Hemisphere. The main objective of this research project will be to extend the geochemical record back in time to better understand the pattern of subsurface western Pacific warm pool warming during other glacial-interglacial transitions over the last 1.5 million years. A key question is whether the subsurface warming in this region of the ocean is a triggering event for the global warming that occurred after each of the multiple glaciations that occurred during this time period, and how changes in the Southern Hemisphere might influence the WPWP and global hydroclimate in our present period of climate change. Outreach, training, and educational activities are broad and include high school, undergraduate, and graduate students as well as the general public. There is a growing body of paleoclimate evidence pointing to the importance of South Pacific forcing of the lower latitude Pacific, and in particular the Western Pacific Warm Pool (WPWP) thermocline. Just below the buoyant surface layer in the WPWP are waters of subtropical origin with ~ 70% supplied from the South Pacific gyre region. This is part of the shallow overturning cell-western boundary current circulation that transmits water at thermocline depths to the WPWP where it mixes with the Northern Hemisphere western boundary current. Much of this subtropical South Pacific water flows as part of the New Guinea Coastal Undercurrent into the Bismarck Sea where our study site, International Ocean Discovery Program (IODP) Site U1486 is located. The South Pacific influence on oceanographic conditions in the Bismarck Sea indicates that sediment cores from this strategic location can be used to reconstruct temporal changes of the southern Hemisphere influence on the WPWP thermocline. Preliminary surface ocean and thermocline-depth foraminifera oxygen isotopes and Mg/Ca results from Site 1486 indicate a 3-4°C warming of the thermocline that started several thousand years before the deglacial transitions at Marine Isotope Stages 2/1 and 6/5. Thermocline temperatures then peaked in the early interglacials at 11 and 126 kyr, followed by abrupt cooling that lasted several thousand years. These are time periods when maximum Earth axial tilt and minimum precession align, suggesting a connection between warming of the WPWP thermocline, maximum obliquity angle and time periods when the summer solstice occurs at perihelion. The Principal Investigators will analyze three species of mixed layer- and thermocline-depth fossil foraminifera shells for oxygen isotopes and Mg/Ca from Site 1486 back to 1.5 Ma at a millennial-scale downcore timestep, spanning the entire mid-Pleistocene transition (MPT). This will allow them to determine when this pattern of early interglacial thermocline warmth developed relative to surface conditions and investigate the following questions: Is there a consistent relationship between thermocline warming in the region of Site 1486 and periods when obliquity angle was at a maximum and precession at a minimum? Are thermocline warming events consistent features of all interglacial periods in the WPWP or just in the latest Pleistocene? Given the importance of the WPWP to global climate and hydrology, the possibility that WPWP thermocline warming prior to and during glacial-interglacial transitions were triggering events with global implications needs to be explored. Since some recent studies have hypothesized a southern hemisphere origin for the dominant phase of the last glacial termination, our results documenting early interglacial warming in the Bismarck Sea sector of the WPWP thermocline may support this idea. Thus, determining the temporal relationship between WPWP thermocline warming events and past deglacial “global warming” events is important in the context of current climate.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.