Organic alkalinity in the global ocean

Abstract

Ocean alkalinity (AT) represents the excess of proton acceptors over proton donors compared to the carbonic acid as a zero proton level. It plays a crucial role in evaluating and predicting the oceanic carbon system including CO2 flux between the atmosphere and the ocean, buffering capacity of seawater, and saturation state of calcium carbonate by using thermodynamic models. The significance of accurate assessment for oceanic carbon system has been amplified with the escalating atmospheric CO2 levels and climate change. However, the current comprehension of ocean alkalinity is incomplete due to the interference of functional groups in organic matter with titration-derived AT measurement. The utilization of these inaccurate AT values in carbonate thermodynamic models results in inaccuracies in estimating various CO2 parameters. This thesis aims to enhance our understanding of AT by exploring the variable impact of dissolved and particulate organic matter on it. The emphasis is on refining methodological approaches to determine dissolved organic matter-driven alkalinity (ADOM) and conducting comprehensive investigations into its distributions in diverse environments, as well as its variability during the decomposition process. The study also emphasizes the investigation of particulate organic matter-driven alkalinity (AT−BIO) in vast oceanic environments. In the first chapter of this investigation, the assessment protocol for NaOH-based back-titration to quantify the involvement of dissolved organic matter (DOM) in AT (ADOM) was modified. The methodology encompassed three consecutive steps for ADOM measurement: 1) sample titration with HCl, 2) removal of CO2 from the sample, and 3) titration of the CO2-removed sample with NaOH. To address the impact of incomplete CO2 removal, a practical approach utilizing artificial seawater was introduced, which resulted in a consistently maintained amount of carbonate alkalinity over a duration exceeding six months. The proposed ADOM evaluation applying back-titration data employed a non-linear least square fitting approach, based on two assumptions: exclusive contribution of proton acceptors from monoprotic acids to ADOM in natural environments and negligible ADOM at pH below 3.5 during back-titration. The evaluation method based on NaOH-titration data to determine ADOM represents a distinguishing modification from the conventional method, which relied solely on HCl-titration. This modification enabled an independent determination of ADOM, excluding the inherent interferences attributed to organic acids within AT. To validate the accuracy of ADOM measurement using the modified back-titration method, this study conducted HCl-titration (AT) and NaOH-based back-titration (ADOM) on a CH3COONa solution, where only acetate ion equally contributes to AT and ADOM. Both results solely caused by the acetate ion demonstrated a 1:1 ratio, confirming the reliability of the ADOM values obtained using the NaOH-based method. The second chapter underscored the significance of systematically evaluating variations in ADOM across diverse regions and temporal distributions within a basis of the modified back-titration method. When applied to open ocean seawaters including certified reference materials, aged local seawater, and deep waters of the East Sea, the method consistently yielded ADOM values approximately 5 μmol kg−1 higher than the value obtained for artificial seawater without ADOM. This small yet significant ADOM value was attributed to the recalcitrant DOM component, which may be widespread in the global ocean. In addition, to assess ADOM in regional and temporal distributions, a total of 56 surface samples from distinct aquatic ecosystems in Korea, encompassing lagoon lakes, estuaries, coastal zones featuring tidal flats and salt marshes, and a total of 95 samples over one and a half years in a coastal environment characterized by the prevalence of macroalgal habitats were evaluated. Examination of ADOM in these field samples revealed considerable variability, ranging from less than 5 μmol kg−1 to 29.05 μmol kg−1. In contrast to AT, which typically exhibited conservative mixing between freshwater and seawater endmembers, the observations of ADOM in each study region implied that the source of ADOM is not solely freshwater, emphasizing the specific region where blooms occur. The estimated dissociation constants, with pKa1 of ~4.5 and pKa2 of ~7.0, suggested a uniformity in the functional groups present within organic matter across diverse environments. The investigation detailed in the third chapter analyzed the stability of ADOM during the phases of DOM release and decomposition in experiments utilizing two macroalgae, Ulva austrails and Sargassum horneri. The variations of ADOM and dissolved organic carbon (DOC) concentrations demonstrated that the ADOM signals significantly followed the variations in DOC. It was confirmed that a substantial portion of ADOM variability correlated with fluctuations in the abundance of carboxyl groups. This study proposed a hypothesis that the capacity of DOM to function as ADOM might depend on whether the DOM is predominantly in a state of decomposition or not. In the fourth chapter, the study evaluated the contributions of phytoplankton and bacteria cells to AT (AT−BIO) in seawater samples sourced from 205 distinct locations, spanning the East Sea, the North Pacific Ocean, the Bering Sea, the Chukchi Sea, and the Arctic Ocean. The methodology for removing particulate matter involved filtering the sample within a limited volume of 150 mL under a weak vacuum pressure (<10 kPa) to minimize cell rupture. The determination of AT−BIO values was based on the comparison between AT values measured for unfiltered versus filtered samples, utilizing a filter with a nominal pore size of 0.45 µm to isolate contributions from particulate matter. The recorded AT−BIO values exhibited a range from 10−19 μmol kg−1 in the East Sea and the North Pacific Ocean, gradually diminishing to 1 μmol kg−1 with increasing distance towards the Arctic Ocean. This investigation underscores non-negligible presence of AT−BIO in coastal and open ocean environments, emphasizing the imperative need for their consideration when evaluating the precision of carbon parameters computed through thermodynamic models dependent on measured AT as a key input parameter. Throughout these four chapters, this thesis assessed the contributions of various organic matter to AT in diverse oceanic environments. I aim for this research to enhance our understanding of seawater alkalinity and, consequently, the carbon dioxide system in the global ocean.