Award: OCE-1234388

Because 70% of marine dissolved organic carbon (DOC) is found in the deep ocean, it is important to determine its sources and sinks, and consequently its ultimate role in the global carbon cycle. Unfortunately, the long-term sinks for DOC remain largely unknown. Limited data has suggested that photochemistry is a significant removal process for refractory DOC (RDOC), photooxidizing the deep pool as it eventually circulates through sunlit surface waters. In this project we quantitatively reevaluated this basic question that currently constrains global DOC models: Does photochemistry have a significant role in the removal of the massive amount of refractory DOC that is pooled in the deep sea? Using a solar simulator for field and laboratory irradiation experiments to examined DOC in waters from the North Pacific (Figure 1), we quantified photochemical rates controlling (1) production of carbon monoxide (CO), (2) changes in common optical tracers of organic carbon such as colored dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM), and (3) kinetics of two reactive oxygen species (hydrogen peroxide, HOOH; & superoxide, O2-) that reflect the role of oxygen in DOC photochemistry. Water from this location at the end of the deep ocean circulation conveyor belt has been processed by microbial communities and removed from sunlight for many centuries, providing very low concentrations of DOC that reflects an extensively aged and biologically refractory carbon pool. Taken together, our data indicate that while RDOC reacts initially in sunlight, it is not particularly photochemically reactive over the long term. Previous assumptions about linear accumulation of photochemical products in extended lab irradiations are typically not correct for HOOH, low molecular weight carbonyl compounds, CO, and CO2, (Figure 2) and consequently, previous photochemical production rate estimates and reaction efficiency data based on long-term irradiation experiments both significantly underestimate photochemical reaction rates in oceanic models. Further method development designed to link our CO/HOOH data to the largest photochemical carbon product, CO2, have resulted in a new HOOH/CO2 proxy. Consequently, our HOOH data from this project will allow proxy evaluation of CO2 production to go with direct CO production measurements, giving better photochemical estimates for the dominant photo-oxidation products for DOC/CDOM. New data for reactive oxygen species (ROS) (Figure 3) have provided new data on O2- reaction pathways in blue water systems with potential implication for surface redox chemistry. Additional shipboard irradiations for high-resolution mass spectrometry analysis also showed that deep sea DOC composition is chemically transformed by photoreactions (Figure 4), becoming more similar to surface DOC in the sunlit surface ocean. Farther to our direct examination of DOC photochemistry for this project, our struggles to verify proxies for CO2 photoproduction in blue seawater have led to development of a novel isotope enrichment method that will fundamentally change the direct evaluation of CO2 photochemistry directly in seawater at very low photon dose levels and with no need for chemical modification of samples prior to irradiation as is commonly done for analytical reasons. Broader impacts included K-12 visits to local schools to present their science, a cruise-related website entitled "ask the oceanographer" and a live blog during their cruise. Grade school students decorated Styrofoam cups that were then shrunk in the N. Pacific at depth and delivered back to the class where we discussed Pressure in the Deep Sea. Two undergraduate and four graduate students obtained research experience at sea on the Gulf of Alaska cruise with data resulting in publications for all students involved and supporting 4 Ph.D. theses. The funding impacted fields beyond marine organic chemistry. CDOM & EEMs profile data provide new information to help with the description of deep circulation in the Gulf of Alaska. HOOH and superoxide data from this effort inform related work on surface metal redox chemistry and the possible role of ROS on nitrogen budgets through inhibition of marine archaea. Last Modified: 11/27/2018 Submitted by: Patricia M Medeiros