ASSESSING DYNAMICS AND TRENDS OF CO2 FLUX ON THE LOUISIANA SHELF

Author

Claire McGuireFollow

Date of Award

Summer 6-2023

Degree Type

Masters Thesis

Degree Name

Master of Science (MS)

School

Ocean Science and Engineering

Committee Chair

Christopher Hayes

Committee Chair School

Ocean Science and Engineering

Committee Member 2

Stephan Howden

Committee Member 2 School

Ocean Science and Engineering

Committee Member 3

Chelsea Pederson

Committee Member 3 School

Ocean Science and Engineering

Abstract

The Gulf of Mexico on an annual timescale acts as a sink for ­CO₂. Previous studies have investigated regional-scale trends in the Gulf based on model output. In our investigation of the northern Gulf of Mexico, and especially the area surrounding the Louisiana delta, we found different trends compared with previous studies of the open ocean Gulf and broader shelf region. Predictions of global patterns of the ocean CO₂ cycle require regional studies of unique areas like the Louisiana Shelf. The Louisiana Shelf is a shallow system, has input from river systems, experiences seasonal hypoxia, and is frequently disturbed by hurricanes. This thesis focuses on 5-and-a half-years of continuous surface water CO₂ data from July 15^th, 2017, to December 31^st 2022 to better understand the influences different variables have on the carbon chemistry of the water over varying timescales. Four main hypotheses are addressed in order to establish trends and examine key factors influencing air-sea CO₂flux on the Shelf. We first determined if the Louisiana Shelf followed observed trends of other oceanic areas. We predicted (hypothesis I) that the Shelf would see consistent seasonal variation, with winter and spring having negative fluxes (net CO₂ flux into the ocean) and the summer and fall having less negative fluxes or slightly positive fluxes (or net outgassing). When analyzing the data as a climatology, we found that air-sea CO₂ fluxes during winter and spring were statistically more negative than those in summer and fall. We also predicted that because the region has many dynamic factors, temperature would not be the driving factor of CO₂ flux as it is in some other oceanic regions (hypothesis II). In this study we did find a positive linear correlation between temperature and air-sea CO2 flux, but a relatively weak one (R² = 0.61). The final two hypotheses addressed the potential impact of hurricanes on air-sea CO₂ flux on the Louisiana Shelf, given the fact that hurricane season roughly coincides with the development of CO₂-rich hypoxic bottom water. We predicted (hypothesis III) that the average CO₂ flux in weeks containing a hurricane passage over the Shelf would be higher than the same week in other years with no hurricane passage, and that (hypothesis IV) the weeks containing a storm passage in the summer hypoxic season would have higher fluxes than those in the late fall, after hypoxia has dissipated. In two of the ten storms that occurred during the time-series, we did see storm-weeks with anomalies, but the strongest anomalies are seen on the day of the storm passage, rather than during a full week. With regard to hypothesis IV, we found no significant difference in CO₂ fluxes between storm passages in the summer and those in the fall in our dataset. This is an area for continued research. With more years of data, it will likely be easier to establish climatological baselines against which to compare the daily or weekly response to a storm. Furthermore, with more accompanying data, for instance pH and bottom water data, more process-based hypotheses could be addressed.

Recommended Citation

McGuire, Claire, "ASSESSING DYNAMICS AND TRENDS OF CO2 FLUX ON THE LOUISIANA SHELF" (2023). Master's Theses. 992.
https://aquila.usm.edu/masters_theses/992