Collaborative Research: The role of the Southern Ocean in Late Miocene climate change

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Concern over rising global temperatures makes it important to predict future climate change. However, to predict future climate requires a better understanding of how the climate system works in a warmer world. One way to better understand future warmth is to look at a warm period in the past. The project will explore one of the most recent periods of long-term warmth, the late Miocene, approximately 5 to 8.5 million years ago. The Miocene was characterized by warmer temperatures globally, and particularly in the polar regions. However, currently models are not able to correctly hindcast Miocene temperatures and often underestimate the warming. The ability to predict and understand this warmer world is limited by the availability of observations from the late Miocene. This is particularly true of the Southern Ocean, the area of ocean around Antarctica. This region plays an important role in controlling global temperature changes because it interacts with atmospheric carbon dioxide through physical processes (e.g., ocean mixing) and biological productivity (e.g., photosynthesis). Currently, there are no records of surface ocean temperature or biological productivity from the late Miocene for the Pacific sector of the Southern Ocean. This project will generate records of late Miocene oceanographic conditions in this region, including ocean surface temperature and biological productivity. These data will document how warm the Southern Ocean was during the late Miocene. The data will also provide key insights into how climate in a warmer world responds to changes in atmospheric carbon dioxide. Finally, the study will assess how processes in the Southern Ocean may have contributed to the cooling of the globe at the end of the Miocene. This work will support graduate and undergraduate students in both oceanography and science education. As part of the project, science education MS students and oceanography PhD students will work together to develop expertise in equitable teaching methods and curricular design.The Miocene (23.3 - 5.05 Ma) is critical to understand as it was a period of sustained warmth leading up to the early phase of Northern Hemisphere glaciation. A recent Miocene model comparison study (MioMIP1) indicates that: during the late Miocene the majority of warmth is attributed to atmospheric carbon dioxide (pCO2) forcing; climate sensitivity to pCO2 forcing is state dependent; regional sensitivity to orbital variations in insolation is poorly understood and model-dependent; while a reduced meridional sea surface temperature (SST) gradient is generally a robust feature of warm climates with profound impacts on the hydrologic cycle, it is not well-constrained. One of the largest data-model misfits is in the southern high latitudes where models severely underestimate observed warming. However, this misfit is determined by observations from only a few southern sites, none of which are from the vast region of the central Pacific Southern Ocean (SO). This project will produce the first paleoceanographic records from the late Miocene to early Pliocene (8.3 – 4.0 Ma) in the Central Pacific sector of the SO from a recently drilled site (IODP Expedition 383, Site U1541). These records, covering the Late Miocene Cooling and Late Miocene Carbon Isotopic Shift, will enable the determination of South Pacific meridional SST gradients, an evaluation of long-term climate sensitivity to pCO2 forcing and orbital forcing, an assessment of the links between SST and paleoproductivity variability, and of the evolution of deep-water circulation. The project objectives are to: (a) develop a high-resolution Uk’37-based SST record to constrain the cold southern end member of the meridional SST gradient and to evaluate responses to decreasing pCO2 forcing and potential changes in orbital variability as the mean state changed, and (b) use paleoproductivity proxies to test the hypothesis that enhanced marine productivity during the late Miocene contributed to the removal of atmospheric CO2 leading into the Pliocene. Although not the focus of this project, the study will also use benthic foraminiferal d13C at U1541 to constrain zonal and meridional gradients in deep ocean d13C to inform potential changes in global deep-water circulation. The new late Miocene data from Site U1541, a previously unstudied region, will provide new insight into the workings of the climate system in a warmer 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.