Abstract Dissolved organic carbon (DOC) contains organic acid charge groups that contribute organic alkalinity (OrgAlk) to total alkalinity (TA). These effects are often ignored or treated as a calculation uncertainty in many aquatic CO2 studies. This study evaluated OrgAlk variability, sources, and characteristics in estuarine waters exchanged tidally with a groundwater-influenced salt marsh in the northeast USA. OrgAlk provided a biogeochemical link between organic and inorganic carbon cycling through its direct effects on pH, and thus CO2 system speciation and buffer capacity. Two main charge groups were identified including carboxylic and phenolic or amine groups. Terrestrial groundwater and in-situ production within salt marsh peat contributed OrgAlk to the tidal creek, with the former being a more significant source. Groundwater entering the marsh complex contained exceptionally high OrgAlk (> 150 µmol kg−1), and these compounds were preferentially preserved within the DOC pool during groundwater transport and mixing with coastal water. OrgAlk:DOC ratios in groundwater and marsh-influenced water varied across space and time. This highlights the insufficiency of using a fixed proportion of DOC to account for organic acid charge groups. Accounting for OrgAlk altered H+ concentrations by ∼1–41 nmol kg−1 (equivalent to a pH change of ∼0.03–0.26), pCO2 by ∼30–1600 μatm and buffer capacity by ∼0.00–0.14 mmol kg−1 at the relative OrgAlk contributions of 0.9–4.3% of TA observed in the marsh-influenced tidal water. Thus, OrgAlk may have a significant influence on coastal inorganic carbon cycling. Further theoretical calculations confirm that these concentrations of OrgAlk would have sizable impacts on both carbonate speciation and, ultimately, air-sea CO2 fluxes in different coastal environments, ranging from estuarine to shelf waters. A new conceptual model linking organic and inorganic carbon cycling for coastal waters is proposed to highlight the sources and sinks of organic acid charge groups, as well as their biogeochemical behaviors and mechanistic control on the CO2 system.

中文翻译：

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有机和无机碳循环之间的重要生物地球化学联系：有机碱度对受潮间带盐沼影响的沿海水域碳酸盐化学的影响

摘要 溶解有机碳 (DOC) 包含有机酸电荷基团，这些电荷基团将有机碱度 (OrgAlk) 贡献给总碱度 (TA)。在许多水生 CO2 研究中，这些影响通常被忽略或视为计算不确定性。本研究评估了与美国东北部受地下水影响的盐沼进行潮汐交换的河口水域的 OrgAlk 变异性、来源和特征。OrgAlk 通过直接影响 pH 值，从而提供了有机和无机碳循环之间的生物地球化学联系，进而影响 CO2 系统形态和缓冲能力。确定了两个主要的电荷基团，包括羧基和酚基或胺基团。陆地地下水和盐沼泥炭中的原位生产为潮汐小溪贡献了 OrgAlk，前者是更重要的来源。进入沼泽复合体的地下水含有异常高的 OrgAlk (> 150 µmol kg-1)，这些化合物在地下水运输和与沿海水混合期间优先保留在 DOC 池中。地下水和受沼泽影响的水中的 OrgAlk:DOC 比率随空间和时间而变化。这突出了使用固定比例的 DOC 来解释有机酸电荷基团的不足。考虑到 OrgAlk 改变的 H+ 浓度约 1-41 nmol kg-1（相当于 pH 变化约 0.03-0.26），pCO2 变化约 30-1600 μatm，缓冲容量变化约 0.00-0.14 mmol kg-1在受沼泽影响的潮水中观察到的有机碱贡献占 TA 的 0.9-4.3%。因此，OrgAlk 可能对沿海无机碳循环产生重大影响。进一步的理论计算证实，这些 OrgAlk 浓度将对碳酸盐物种形成以及最终对不同沿海环境（从河口到陆架水域）中的海气二氧化碳通量产生相当大的影响。提出了一种连接沿海水域有机和无机碳循环的新概念模型，以突出有机酸电荷基团的源和汇，以及它们对 CO2 系统的生物地球化学行为和机制控制。