Thesis Defense Seminar - Christian Amos

COASTAL AND MARINE SYSTEM SCIENCE PROGRAM
DEPARTMENT OF PHYSICAL AND ENVIRONMENTAL SCIENCES
TEXAS A&M UNIVERSITY-CORPUS CHRISTI

TITLE: Carbonate Chemistry in the Mission-Aransas Estuary and its Controlling Factors

GRADUATE COMMITTEE: Dr. Xinping Hu (Chair), Dr. James Silliman, Dr. Patrick Larkin

ABSTRACT
Estuaries are bodies of water that connect rivers with the ocean. The difference between the salt content of freshwater and the ocean causes a salinity gradient. Unlike ocean water, estuarine waters can have a complex composition, leading to high variability due to either point or non-point sources. While estuaries are relatively poorly studied in the context of ocean acidification, they are a key factor in carbon dioxide (CO2) degassing back into the atmosphere because rivers carry high levels of CO2 and remineralization of terrestrially derived organic matter also elevated estuarine CO2 levels. Estuarine carbonate chemistry can vary due to many processes and mechanisms, and one of the prominent variabilities is caused by hydrological variability. Meanwhile, estuaries are large reservoirs of “blue carbon”, an organic carbon pool that includes salt marshes, mangroves, and seagrasses, which sequester substantial amounts of carbon due to their high productivity and generally high burial rates in these habitats. Global warming has supported the expansion of mangrove habitats to higher latitudes due to rising temperatures, although extreme weather events could decimate these habitats. In February 2021, there was an extreme freeze (Winter Storm Uri), which affected most of the Texas coast. The freeze lasted from February 13-18, with the wind chill as low as -15°C. Because of this extreme event, substantial mangrove dieback occurred in south Texas estuaries. Mangrove kill and shedding could supply adjacent estuaries with excess organic carbon, which may lead to the production of CO2 upon remineralization, making the affected estuaries more net heterotrophic, hence a stronger CO2 source compared to prior years without freeze impact. This study examines the semiarid Mission-Aransas Estuary in the northwestern Gulf of Mexico for spatial and temporal variations in carbonate chemistry parameters (total alkalinity, total dissolved inorganic carbon, pH, CO2 partial pressure or pCO2, carbonate saturation state) from June 2018-December 2021. The result from the period of a deep freeze that occurred on estuarine carbonate chemistry, when compared with the “normal” years without this extreme event. The freeze event corresponded with a subsequent increase in CO2 outgassing. There was a significant difference between thermal and non-thermal components of pCO2, and the non-thermal (biological) component, i.e., enhanced remineralization, was the cause of this excess outgassing.  These findings allow the elucidation of the controlling factors on estuarine carbonate chemistry in the context of climate change.