Collaborative Research: Pacific Ocean stratification since the last ice age: New constraints from benthic foraminifera

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Ocean circulation is thought to have played a key role in the transition from the last ice age to the modern interglacial (Earth's most recent "deglaciation"). However, the lack of a mechanistic explanation for the deglaciation represents a major limitation in our understanding of the ocean-climate system. One way to change global temperature is to alter the amount of CO2 in the atmosphere, and there is mounting evidence that at least part of the last deglacial warming was caused by the release of CO2 from the ocean into the atmosphere. A fundamental shift in the ocean's density structure and circulation could have caused such a carbon release, but there are very few deglacial records of physical seawater properties, particularly from the Pacific Ocean, the largest ocean basin. During the course of this project, multiple high-resolution records of seawater temperature and other physical properties from the Southwest Pacific Ocean will be established using geochemical evidence from marine sediment cores. This collaborative work will strengthen ties between the participating universities and provide practical training for graduate and undergraduate students in the sciences. The project's marine records will extend from the last ice age through the present, providing a detailed history of seawater properties that will shed light on the ocean's role in global climate change. Colder temperatures and higher salinities of abyssal waters in the Pacific likely both contributed to greater seawater density during the last glacial period. However, temperature values and the vertical temperature structure of the glacial ocean across middle to intermediate depths are unconstrained. The timing of changes in seawater temperature and ventilation across these key depth ranges are also unclear. Determining ocean density structure can help constrain past circulation, and a first step in establishing density is to determine temperature. Three sediment cores (~1100, 1600, and 2500 m water depth) will be used to reconstruct seawater temperature and oxygen isotope composition of seawater (delta18OSW) for the past 30,000 years in the southwest Pacific Ocean at ~250 year resolution. High sediment accumulation rates and well-established, stratigraphically-linked tephra chronologies make the targeted sediment cores from New Zealand's Bay of Plenty particularly valuable for assessing rapid changes in this region. Seawater temperature will be reconstructed using Mg/Ca of benthic foraminiferal calcite (Uvigerina) and then combined with delta18O of benthic calcite from the same species to calculate delta18OSW. At a minimum, delta18OSW acts as a conservative water mass tracer, placing constraints on mixing in the ocean interior. This property may also be used to estimate relative changes in salinity, particularly as more estimates of the past relationship between delta18OSW and salinity become available (e.g., from pore-water studies) ultimately bringing us closer to quantifying density. These records will track the pattern of oceanic warming during the last glacial termination, allowing hypotheses regarding ocean de-stratification to be tested. A suite of 50 core-tops will also be used for ground-truthing and refining the Mg/Ca temperature proxy, providing a region-specific temperature sensitivity if appropriate. The project's results could also improve the research community's ability to characterize glacial circulation. For example, glacial temperature profiles could help constrain the solution of last glacial maximum state estimates, which are very sensitive to Southern Ocean structure.