A large number of studies have sought explanations for glacial/interglacial atmospheric carbon dioxide (CO2) cycles via ocean processes that transfer CO2 to/from the deep ocean. While research continues to pursue such mechanisms, recent studies have introduced new observations that require consideration of geologic processes as well. One recent hypothesis that incorporates geologic processes (Stott and Timmermann, 2011) posited that changes in ocean temperatures, which are fixed at high latitudes, regulate CO2-hydrate stability at intermediate depths in the Pacific Ocean and in doing so, modulate the accumulation and release of CO2-rich fluids that accumulate within the sediments that blanket hydrothermal systems. However, without the availability of cores from the sites near known hydrothermal systems in the western Pacific, it has not been possible to adequately test this hypothesis.
This EAGER project, led by the faculty member at the University of Southern California who co-authored the concept paper, will fund collaboration with a team of German scientists to collect sediment cores from the western tropical Pacific, near Mindinao. These materials should enable testing of the controversial hydrothermal CO2 hypothesis. If a geologic source for the old carbon signal could be identified and conclusively linked to the water column signals, it could potentially transform the way we think of the connections between the solid Earth and climate. Funding supports participation in the research expedition for two members of the principal investigator's research group, along with generation of key preliminary data from the cores.
Broader impacts include international collaborations and partial support of a graduate student at the University of Southern California.
In the summer of 2013 two personnel from Dr. Lowell Stott research team participated on a marine sediment coring cruise to the western tropical Pacific. This expedition was supported by the EAGER grant 1344514 to Stott. During the expedition cores were collected from water depths between 500 and 4000m. The purpose of this coring expedition was to investigate the pathways of carbon transport within the ocean during the last glacial/interglacial transition as the concentration of atmospheric CO2 began to rise. With support from this EAGER grant we obtained preliminary 14C age data that has helped us test the hypothesis originally put forth by Stott and colleague Axel Timmermann that calls upon a release of hydrothermal CO2 during deglaciation to explain the rise in atmospheric CO2. The basis for this hypothesis stems from 14C age anomalies originally observed among deep sea benthic foraminifera from cores collected in the eastern tropical Pacific. The 14C age anomalies observed in the shallow cores from the eastern tropical Pacific are much too old to be explained without there being an input of geologic "14C-dead" carbon to the upper ocean. The western Pacific is a location where hydrothermal carbon dioxide has been observed in both liquid and solid form in modern sediments. The storage and the potential release of hydrothermal geologic would help explain the variations in atmospheric CO2 that accompanied glacial cycles. Over the past year Stott's research team at USC processed one of the cores (GeoB1702b) collected during the expedition and completed a stable isotope stratigraphy and 14C age measurements from both benthic and planktonic foraminifera on two cores from the western Pacific (Figure 1). The results of these 14C age determinations from the western Pacific are quite important because they lend support to the Stott and Timmermann 2011 hypothesis. Our preliminary findings indicate there was a reduction in the Δ14C gradient between 500 and 2000 meters in the western tropical Pacific during the last deglaciation as predicted by the Stott and Timmermann hypothesis if geologic carbon was released into the upper ocean (Figure 1). Such a reduction in the Δ14C gradient is reflected in lower Δ14C values at intermediate to shallow water depths, indicative of an input of 14C-depleted carbon as predicted by the Stott and Timmermann hypothesis. Our ongoing effort is to now obtain additional 14C ages from cores collected during the expedition and document a high resolution reconstruction of the water column Δ14C values during the deglaciation. This grant also supported the development of a dissertation project for Mark Nishibyashi who is a student of Stott. Mark participated on the cruise and will be working with Stott to conduct the more comprehensive investigation over the next three years.
