Uncertainty sources for measurable ocean carbonate chemistry variables

Abstract

The ocean carbonate system is critical to monitor because it plays a major role in regulating Earth's climate and marine ecosystems. It is monitored using a variety of measurements, and it is commonly understood that all components of seawater carbonate chemistry can be calculated when at least two carbonate system variables are measured. However, several recent studies have highlighted systematic discrepancies between calculated and directly measured carbonate chemistry variables and these discrepancies have large implications for efforts to measure and quantify the changing ocean carbon cycle. Given this, the Ocean Carbonate System Intercomparison Forum (OCSIF) was formed as a working group through the Ocean Carbon and Biogeochemistry program to coordinate and recommend research to quantify and/or reduce uncertainties and disagreements in measurable seawater carbonate system measurements and calculations, identify unknown or overlooked sources of these uncertainties, and provide recommendations for making progress on community efforts despite these uncertainties. With this paper we aim to (1) summarize recent progress toward quantifying and reducing carbonate system uncertainties; (2) advocate for research to further reduce and better quantify carbonate system measurement uncertainties; (3) present a small amount of new data, metadata, and analysis related to uncertainties in carbonate system measurements; and (4) restate and explain the rationales behind several OCSIF recommendations. We focus on open ocean carbonate chemistry, and caution that the considerations we discuss become further complicated in coastal, estuarine, and sedimentary environments.

Temperature effect on seawater ƒCO2 revisited: theoretical basis, uncertainty analysis, and implications for parameterising carbonic acid equilibrium constants

“…Our parameterisation υp is based on a specific set of carbonate system parameterisations. We know that these are not perfect (e.g., Woosley and Moon, 2023;Carter et al, 2024), so our parameterisation of bh is an intermediate solution. It does agree very well with the available field data (Wanninkhof et al, 2022) upon which it can be tested (and from which it is independent) -indeed, better than any other currently available approach (Sect.…”

“…2) that are not yet fully understood. Even though it is the parameterisation given by the best practice guide (Dickson et al, 2007), it is still well-known that Lueker et al (2000) does not perfectly represent the marine carbonate system to the accuracy with which we can measure it (e.g., Álvarez et al, 2020;Woosley, 2021;Wang et al, 2023;Woosley and Moon, 2023;Carter et al, 2024), and there are outstanding issues to resolve with for example the contribution of organic matter to total alkalinity (e.g., Ulfsbo et al, 2015;Hu, 2020;Kerr et al, 2021) and the borate:chlorinity ratio (e.g., Lee et al, 2010;Fong and Dickson, 2019). Indeed, the parameterised van 't Hoff form agrees slightly better with the Wanninkhof et al (2022) dataset than does the calculation from TC and ƒCO2 with Lueker et al (2000) (Fig.…”

Climatological distribution of ocean acidification indicators along the North American ocean margins

“…This preference order corresponds to the frequency of availability of these measurement types in the underlying data products and is therefore chosen on the basis of maximizing the consistency of the underlying calculations. It has been noted that the stated uncertainty in seawater pH measurements is often low enough to result in reduced uncertainties in calculations using this variable compared to the other variables, but we nevertheless use this ordering to ensure our underlying calculations are maximally consistent, and because of lingering issues with accuracy of pH at depth (Carter et al, 2023).…”

Spectrophotometrically derived seawater CO2‐system assessments: Parameter calculations using pH do not require measurements at standard temperatures

“…Previous studies have generally assumed that internal consistency assessments at 20 C or 25 C are also applicable at lower temperatures, but this suggestion has not yet been thoroughly verified. Specifically, this work assesses (1) differences between measured TA and TA calculated from DIC and pH T at both 25 C and 12 C, (2) differences between measured pH and pH calculated from TA and DIC, and (3) the utility of measuring pH at lower temperatures (i.e., t < 25 C) for use in CO 2 -system calculations, as was proposed by Carter et al (2024).…”