Award: OCE-1536605

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 modern organic matter produced by present-day biological activity in the photic zone. 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 an unreasonable assumption 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 this old carbon 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 was an important, new process to remove 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 by the bursting bubbles, and subsequently determined what fraction of aerosol OC was old and how much was modern. During the cruise, we collected aerosol particles at four stations in the western North Atlantic Ocean, two in biologically productive waters (coastal Rhode Island and Georges Bank) and two in waters in the Sargasso Sea that had very little biological activity. At each station we measured the age of the carbon (i.e., old or modern) in both the aerosol and seawater from which these aerosol were generated. As a reference, we also measured the background levels of carbon from the deep sea, since this carbon is known to be mostly old. The basis of the method we used involved counting how much 14-C carbon isotope was in the sample compared to the 12-C carbon isotope, the main isotope of carbon found in nature. Several other measurements were made during the cruise to allow us to better interpret the carbon-dating age of the OC emitted from the ocean. We determined that in the Sargasso Sea and Georges Bank a significant fraction, from 19 to 40%, of the particulate OC that was emitted into the atmosphere was old. Globally, this process removes 2 - 20 x 1012 grams (i.e., 2 - 20 Tg) of RDOC from the oceans on an annual basis. This removal rate is comparable to the rate of RDOC loss in hydrothermal systems (1 - 1.3 Tg C/yr), the biologically-mediated loss in the water column (≤ 43 Tg C/yr), and the loss via incorporation into particulate organic carbon 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 predicted 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: 02/19/2019 Submitted by: David J Kieber