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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 CO2partial 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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Comparison of open cell and single‐step total alkalinity titration methods and implications for organic alkalinity
Abstract An open question in marine carbon chemistry is if organic alkalinity (or some other unidentified species) is present in non‐negligible quantities in the open ocean. If organic alkalinity is indeed present, different methods for total alkalinity (TA) analysis with different titration endpoints could titrate different amounts depending on the dissociation constants (pKa) of the acids present, resulting in meaningful differences or offsets between methods. Two commonly used methods, open‐cell titration with non‐linear least squares fitting and single‐step titration with spectrophotometric endpoint detection, might titrate different amounts of organic alkalinity, if present, depending on their pKa. We test this hypothesis using paired samples collected on two cruises, one in the northwest Pacific and one in the western Arctic, and analyze the TA using both methods. We found the differences to be statistically indistinguishable (∆TA[Open‐Cell−Single‐Step] = 0.5 ± 3.9 μmol kg−1swmean and standard deviationN = 206). Adjustment of the single‐step TA to certified reference material could be obscuring a difference in the methods. The good agreement between methods indicated that the analytical method is not the cause of offsets in Pacific TA identified by the Global Ocean Data Analysis Project version 2. From these results, the presence of organic alkalinity in open ocean waters remains inconclusive but suggests that if present, the concentration is either very low or both methods titrate similar amounts.
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- PAR ID:
- 10672674
- Publisher / Repository:
- Association for the Sciences of Oceanography and Limnology, Wiley Periodicals, LLC
- Date Published:
- Journal Name:
- Limnology and Oceanography: Methods
- Volume:
- 23
- Issue:
- 12
- ISSN:
- 1541-5856
- Page Range / eLocation ID:
- 960 to 972
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/lom3.70002
