Award: OCE-1536597

Collaborative Research: Couple Ocean-Atmosphere Recycling of Refractory Dissolved Organic Carbon in Seawater Project Outcomes Report Breaking wind waves inject air into the ocean, which fragments into bubble plumes that subsequently rise to the sea surface, burst, and inject particles containing sea salt and organic carbon (OC) into the overlying marine atmosphere. It is widely assumed that the particulate OC emitted from the oceans originates from organic matter produced by present-day biological activity in the sunlit surface ocean. We challenge this assumption and propose instead that some if not all of the OC emitted from the oceans by bursting bubbles is very old, cycling through the oceans several times before complete removal. This is not unreasonable, since 95% of the OC in the oceans is thousands of years old, produced by biological activity long ago. This reservoir of ancient carbon, referred to as RDOC, is massive, larger than all terrestrial OC combined. Despite its large size, we know very little about how RDOC is produced or removed from the oceans. The goal of our project was to determine if the bubble bursting process generated by breaking waves at the sea surface is an important process for removing RDOC from the oceans. To accomplish this goal, we conducted a 30 day research cruise in the western North Atlantic Ocean during September and October 2016. We produced marine particles (aerosols) during the cruise using a custom-made marine aerosol generator. Seawater continually flowed through the generator and bubbles were introduced towards the bottom of the seawater reservoir. The range of bubble sizes that were produced mimicked that produced by breaking waves at the sea surface. We collected aerosols that were produced in the generator and subsequently determined the proportion of aerosol OC that was old. During the cruise, we collected aerosol at four stations in the western North Atlantic Ocean. Two stations were in biologically productive waters (coastal Rhode Island and Georges Bank) and two were in the Sargasso Sea, which had comparatively little biological activity. At each station we used radiocarbon dating to measure the age of the OC (i.e., old or modern) in both the aerosol and seawater from which these aerosols were generated. As a reference, we also measured the carbon collected from deep seawater, since this carbon is known to be mostly old. The basis of the method that we used involved counting how much of the naturally occurring radioactive14C isotope was in the sample compared to the 12C carbon isotope, which is the most abundant isotope of carbon found in nature. Several other measurements were made during the cruise to allow us to better interpret the radiocarbon age of OC emitted from the ocean. In the Sargasso Sea and Georges Bank, we determined that a significant fraction, from 19 to 40%, of the aerosol OC emitted into the atmosphere was old. Globally, this process removes between 2 and 20 million metric tons (i.e., 2 - 20 Tg) of RDOC from the oceans each year. This removal rate is comparable to the loss rates of RDOC in hydrothermal systems (1 - 1.3 Tg C/yr), in biologically-mediated processes in the water column (≤ 43 Tg C/yr), and via incorporation into particulate OC in the deep ocean (≤ 9 - 50 Tg C/yr). The discovery of this new process to remove oceanic RDOC is of sufficient magnitude to help resolve a long-standing conundrum regarding losses for ancient marine carbon. This process is also a significant removal pathway for RDOC from the oceans not only in terms of its impact on the cycling of carbon in the oceans, but also with respect to its expected impacts on atmospheric chemistry and climate regulation. These impacts will be sensitive to increases in surface-ocean turbulence predicte Finish d to accompany climate change, since they will result in increased fluxes of RDOC from the oceans to the atmosphere. This project provided opportunities for professional development of nine early career scientists, five from underrepresented groups in the sciences and four of whom are graduate students or postdoctoral fellows. All gained valuable experience through participation in the project and all collaborated in preparing related conference papers and manuscripts for publications. Results from the project also contributed to two PhD dissertations and one MS thesis. All project participants gained new knowledge regarding chemical processes in the surface ocean and lower atmosphere and related measurement techniques and data-analysis tools through interactions with collaborating investigators. The continuing educational development of the PIs and associated investigators has broad reaching potential for enhancing the infrastructure of science. Through public outreach, classroom instruction, interaction with graduate and undergraduate students, collaboration with other colleagues, and publications, knowledge gained from this project will be disseminated to current and future scientists, thereby contributing to continued progress in the field. Last Modified: 03/25/2019 Submitted by: Steven R Beaupre