Award: OCE-1635618

Dissolved organic carbon (DOC) in the oceans represents a pool of this element roughly equal to the CO2 in our atmosphere, and thus understanding the sources and sinks of this material is an important goal in constraining the global carbon cycle. A longstanding question in the DOC field is whether or not photochemistry (reactions driven by light) plays an important role in the conversion of DOC to CO2 and other inorganic carbon forms (DIC).  While some experiments have measured this process in rivers and coastal waters with high DOC and colored dissolved organic matter (CDOM) levels, the vast majority of surface ocean waters have remained unexplored. In fact, CO2 photoproduction rates in ocean waters are extremely difficult to measure, given the much lower concentrations of reactants (DOC/CDOM) and high background DIC levels. Low CO2 photoproduction rates, however, do not necessarily mean zero or trivial rates when scaled up to the vast areas of ?blue water? oceans. This project?s goals were to measure the rates of photochemical DOC conversion to DIC in waters from the central gyres of the North Atlantic and North Pacific Oceans, using a newly-developed more-sensitive method for measuring this parameter. In addition, comparisons with other more indirect methods used to measure photochemical activity and infer DIC production (e.g. measuring proxy products and extrapolating a coastal proxy:CO2 ratio to blue water) were to be made to link this project?s results to previous published literature. 

 

The project?s results indicate that surface ocean DOC is considerably less reactive to light than previously estimated, especially in the summer within the central gyres of the oceans. There is a small portion of marine DOC in deeper waters that is photoreactive, and is likely to be ?burned off? by exposure to light after being confined by stratification in surface layers. Intriguingly, surface waters continue to exhibit significant production of hydrogen peroxide and other reactive oxygen species upon irradiation, yet these species do not appear to have the ability to significantly degrade surface ocean DOC to CO2. In order to gain more insight into this process, we turned to enriching ocean DOC in our samples with extra DOC extracted via 2 different methods from the same waters. These experiments indicted that reactivity was decoupled from the light absorbance of DOC, such that amended samples with much higher levels of color (approaching that of coastal waters) still had very low photo reactivity rates. In short, previous global CO2 photoproduction estimates using coastal production rates and or proxies are likely in error. Based on these results, a manuscript is in prep that will update the global DOC loss rate due to photochemistry.

 

            In terms of Broader impacts, this project was the primary support for a young female graduate student at the University of Georgia, and the results will form the majority of her Thesis work. She is on schedule for her thesis defense in May 2022, having been awarded a University of Georgia Thesis completion grant as well as a departmental outreach award and a Georgia Seagrant fellowship. She has presented her results at a number of local, national and international meetings.  In addition, this project supported a young female postdoctoral fellow, who in 2021 was hired for a tenure-track faculty position at SUNY-ESF. Partial support was also provided for an additional graduate student in Dr. Stubbins? lab, and this student is on track to graduate in 2022 as well. Finally, a number of undergraduate summer interns were supported in years 1 and 2 by this project. In terms of outreach to the public the UGA student was involved in training workshops done by zoom in 2020. A planned aquarium display on photochemistry was brought to the planning and mock-up stage but further progress had to be put on hold due to the departure of Skidaway?s graphics designer as well as Covid-19 restrictions at the aquarium. 

 

 

Last Modified: 11/29/2021
Modified by: Jay Brandes