Collaborative Research: Investigating the Influences of Hydrothermal and Respired Carbon in Intermediate Waters of the Equatorial Pacific Ocean During the Last Deglaciation

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The last time Earth warmed by several degrees centigrade was more than 12 thousand years ago. That global warming ended the last Ice Age and paved the way for human expansion and diversification. Scientists have known for decades that this warming was associated with rising concentrations of greenhouse gases, including both carbon dioxide and methane, but have been unable to pinpoint what caused the greenhouses gases to increase. Recent research has shed new light on geologic processes in the ocean that may have influenced the rise in atmospheric carbon dioxide (CO2). Using geochemical methods, scientists have uncovered evidence that CO2 was released from deep sea volcanic provinces in the eastern equatorial Pacific at the end of the last Ice Age. This discovery is important because the eastern equatorial Pacific is one of the primary conduits through which CO2 from the ocean is released to the atmosphere. In the current project the research team will use similar geochemical methods to investigate whether there were other sites in the ocean where volcanic carbon was released at the end of the last Ice Age and whether ocean currents carried more carbon to the eastern equatorial Pacific as the Earth warmed. The results from this research should help answer one of the Grand Challenges in climate science, what regulated the concentration of atmospheric CO2 during one of Earth's largest climatic changes. The project will support one early career researcher and undergraduate and graduate students at both institutions, one of which is a Minority/Hispanic-Serving Institution where most students are first in family attendees.Two hypotheses have been put forth to explain large radiocarbon (14C) excursions and positive pCO2 anomalies in the Eastern Equatorial and Subtropical Pacific (EEP) during the last glacial termination. One hypothesis calls upon ventilation of respired metabolic carbon from a formerly isolated deep-sea reservoir through the Southern Ocean, and advection of that carbon to low latitudes via Antarctic Intermediate Water. This hypothesis has received support from Nd isotope results from the eastern tropical Pacific that document lower values during the Heinrich Stadial 1(HS1) and the Younger Dryas (YD) climate events, interpreted to reflect an increased proportion of southern sourced waters. The other hypothesis calls upon localized releases of geologic carbon from hydrothermal systems in the EEP that increased carbon flux to the surface ocean. This hypothesis has received additional support from a recent study that documented increased accumulation of hydrothermal metals in the marine sediments of the EEP in association with the large 14C excursions. It is possible that both mechanisms contributed to the radiocarbon anomalies and to an increased flux of "old" carbon to the atmosphere through the EEP upwelling system. Testing each hypothesis requires additional data from sites outside of the EEP and particularly from regions in the vicinity of where intermediate waters are formed. The researchers here will test the origin of the "old" carbon in the EEP by studying ventilation (14C) and water mass (eNd, Ba/Ca) proxies at multiple sites that lay in the path of intermediate waters feeding the equatorial Pacific from the North and South. If results from this study do not support old-carbon transfer via intermediate circulation, it will increase the likelihood of a local carbon reservoir as the source of old carbon in the EEP.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.