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Physical and biogeochemical controls of sea surface pCO2 along the North American Atlantic Coastal Ocean Margins on different time scales
Date
2025
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Coastal ocean margins play a crucial role in the global carbon cycle, acting as significant carbon sinks and highly productive ecosystems while being particularly vulnerable to climate change and ocean acidification. Despite their importance, there remains a critical knowledge gap in understanding the complex carbon dynamics and acidification processes in these regions, especially in the North American Atlantic Coastal Ocean Margin (NAACOM), which encompasses diverse and economically vital coastal ecosystems. This dissertation aims to investigate the complex physical and biogeochemical processes controlling inorganic carbon chemistry and ocean acidification processes along the NAACOM on different spatial and temporal scales. Through a series of studies, I address critical gaps in understanding the spatial and temporal variability of surface ocean partial pressure of CO2 (pCO2) and air-sea CO2 fluxes in these regions. ☐ I first evaluate the reliability of multiple pCO2 products in the Gulf of Mexico (GoMx). Our analysis reveals significant spatial and seasonal variations in pCO2 due to complex local non-thermal dynamics, particularly in the Louisiana Shelf and Western Florida Shelf. I find that the GoMx is CO2-neutral overall, but with notable spatial differences in pCO2 trends. However, despite the fact that all the reconstructed data products are capable of providing a reasonable estimated annual mean pCO2 climatology for the GoMx, they are unable to simulate the long-term trend of pCO2 within the GoMx. ☐ To address the limitations in existing pCO2 products, I develop a new regional reconstructed pCO2 product (ReCAD-NAACOM-pCO2) for the entire NAACOM. This product uses a novel two-step approach combining random forest and linear regression. Trained on Surface Ocean CO2 Atlas (SOCAT) observations and various environmental variables, ReCAD-NAACOM-pCO2 demonstrates high accuracy and robustness in capturing regional-scale variations, seasonal cycles, and decadal trends of pCO2 from 1993 to 2021. ☐ Utilizing this new product, I investigate the spatial and seasonal variability of pCO2 and air-sea fluxes across the NAACOM. I reveal a south-to-north decreasing gradient in pCO2, primarily driven by the meridional temperature gradient and modulated by regional processes. In southern areas, seasonal thermal cycles dominate pCO2 seasonality, while in northern areas, biological processes play a more significant role. Our analysis identifies the NAACOM as a CO2 sink, with a mean flux of -0.63 ± 0.19 mol C m-2 yr-1 (-10.14 ± 3.00 Tg C yr-1) in the narrow margin (depth < 200m) and -0.60 ± 0.21 mol C m-2 yr-1 (-24.24 ± 8.31 Tg C yr-1) in the wide margin (distance from coast < 400km). This wide-margin CO2 uptake is around 60% lower than previously reported values, emphasizing the importance of accurate regional assessments. ☐ Finally, I explore the impact of Gulf Stream variability on coastal acidification along the U.S. East Coast. Our findings show unexpectedly rapid acidification, with pCO2 increasing faster than atmospheric CO2 in both the Mid and South Atlantic Bights (MAB and SAB) during 2002-2021, accompanied by a rapid decline in the dissolution saturation of pH and calcium carbonate saturation state (Ω). Specifically, during this period, the sea surface pCO2 of the SAB shelf grew 60% faster than the atmosphere pCO2 and 28% faster than the MAB. I attribute this acceleration to an overall extraordinary regional warming background, and additionally to reduced biological carbon removal and decreased buffer capacity on the SAB shelf — all linked to Gulf Stream dynamics. By incorporating Gulf Stream dynamic parameters into a multiple linear regression model and Earth System Models, I provide revised estimates for past and future pCO2 levels in these regions. I show that the rapid pCO2 contrast between SAB and MAB can be traced back to the 1990s and will continue to be larger in the future due to a weakening Gulf Stream. ☐ This dissertation significantly advances our understanding of carbon dynamics in the NAACOM, providing crucial insights for improved coastal carbon cycle modeling. Our findings highlight the complex interplay of physical, biological, and chemical processes in shaping coastal carbon dynamics and acidification patterns. The development of the ReCAD-NAACOM-pCO2 product offers a valuable tool for future research in this area. ☐ Furthermore, our research underscores the urgent need for enhanced ocean acidification monitoring in Gulf Stream-influenced areas and other Western Boundary Current regions. The rapid coastal acidification observed along the U.S. East Coast has important implications for marine ecosystem management and policy-making. As coastal regions face increasing pressures from climate change, our work provides critical information to support informed decision-making and adaptation strategies. ☐ Keywords: North American Atlantic Coastal Ocean Margin; pCO2; Ocean acidification; Gulf Stream
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Keywords
Gulf Stream, Ocean acidification, Coastal ocean margins, Ecosystems