Organic alkalinity distributions, characteristics, and application to carbonate system calculations in estuarine and coastal systems

IF 3.8 1区 地球科学 Q1 LIMNOLOGY Limnology and Oceanography Pub Date : 2024-12-16 DOI:10.1002/lno.12761
Christopher W. Hunt, Joseph E. Salisbury, Xuewu Liu, Robert H. Byrne
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Abstract

The capacity of aquatic systems to buffer acidification depends on the sum contributions of various chemical species to total alkalinity (TA). Major TA contributors are inorganic, with carbonate and bicarbonate considered the most important. However, growing evidence shows that many rivers, estuaries, and coastal waters contain dissolved organic molecules with charge sites that create organic alkalinity (OrgAlk). This study describes the first comparison of (1) OrgAlk distributions and (2) acid–base properties in contrasting estuary‐plume systems: the Pleasant (Maine, USA) and the St. John (New Brunswick, CA). The substantial concentrations of OrgAlk in each estuary were sometimes not conservative with salinity and typically associated with very low pH. Two approaches to OrgAlk measurement showed consistent differences, indicating acid–base characteristics inconsistent with the TA definition. The OrgAlk fraction of TA ranged from 78% at low salinity to less than 0.4% in the coastal ocean endmember. Modeling of titration data identified three groups of organic charge sites, with mean acid–base dissociation constants (pKa) of 4.2 (± 0.5), 5.9 (± 0.7) and 8.5 (± 0.2). These represented 21% (± 9%), 8% (± 5%), and 71% (± 11%) of titrated organic charge groups. Including OrgAlk, pKa, and titrated organic charge groups in carbonate system calculations improved estimates of pH. However, low and medium salinity, organic‐rich samples demonstrated persistent offsets in calculated pH, even using dissolved inorganic carbon and CO2 partial pressure as inputs. These offsets show the ongoing challenge of carbonate system intercomparisons in organic rich systems whereby new techniques and further investigations are needed to fully account for OrgAlk in TA titrations.
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水生系统缓冲酸化的能力取决于各种化学物质对总碱度(TA)的贡献总和。总碱度的主要贡献者是无机物,其中碳酸盐和碳酸氢盐被认为是最重要的。然而,越来越多的证据表明,许多河流、河口和沿岸水域都含有溶解的有机分子,这些有机分子带有可产生有机碱度(OrgAlk)的电荷位点。本研究首次比较了(1) 有机碱分布和(2) 不同河口-水体系统的酸碱特性:普莱森特河(美国缅因州)和圣约翰河(加利福尼亚州新不伦瑞克省)。每个河口的 OrgAlk 高浓度有时与盐度无关,通常与极低的 pH 值有关。测量 OrgAlk 的两种方法显示出一致的差异,表明酸碱特性与 TA 定义不一致。在低盐度时,TA 的 OrgAlk 部分占 78%,而在沿岸海洋末段则不到 0.4%。滴定数据模型确定了三组有机电荷位点,其平均酸碱解离常数(pKa)分别为 4.2(± 0.5)、5.9(± 0.7)和 8.5(± 0.2)。它们分别占滴定有机电荷基团的 21%(± 9%)、8%(± 5%)和 71%(± 11%)。将 OrgAlk、pKa 和滴定有机电荷基团纳入碳酸盐系统计算可提高 pH 值的估计值。然而,即使使用溶解无机碳和二氧化碳分压作为输入,中低盐度、富含有机物的样本在计算 pH 值时仍会出现偏移。这些偏差表明,在富含有机物的系统中进行碳酸盐系统相互比较是一项持续的挑战,需要采用新技术并开展进一步研究,才能在 TA 滴定中充分考虑到 OrgAlk。
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来源期刊
Limnology and Oceanography
Limnology and Oceanography 地学-海洋学
CiteScore
8.80
自引率
6.70%
发文量
254
审稿时长
3 months
期刊介绍: Limnology and Oceanography (L&O; print ISSN 0024-3590, online ISSN 1939-5590) publishes original articles, including scholarly reviews, about all aspects of limnology and oceanography. The journal''s unifying theme is the understanding of aquatic systems. Submissions are judged on the originality of their data, interpretations, and ideas, and on the degree to which they can be generalized beyond the particular aquatic system examined. Laboratory and modeling studies must demonstrate relevance to field environments; typically this means that they are bolstered by substantial "real-world" data. Few purely theoretical or purely empirical papers are accepted for review.
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