ASSESSING DYNAMICS AND TRENDS OF CO2 FLUX ON THE LOUISIANA SHELF
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 15th, 2017, to December 31st 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.
Louisiana Shelf, CO2 Flux, Hurricanes, pCO2, Oceanography
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