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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. 

With the increasing threat of ocean acidification and the important role of the oceans in the global carbon cycle, highly precise, accurate, and intercomparable determination of inorganic carbon system parameters is required. Thermodynamic relationships enable the system to be fully constrained using a combination of direct measurements and calculations. However, calculations are complicated by many formulations for dissociation constants (over 120 possible combinations). To address these important issues of uncertainty and comparability, we evaluated the various combinations of constants and their (dis)agreement with direct measurements over a range of temperature (−1.9–40 ◦C), practical salinity (15–39) and pCO2 (150–1200 μatm). The results demonstrate that differences between the calculations and measurements are significantly larger than measurement uncertainties, meaning the oft-stated paradigm that one only needs to measure two parameters and the others can be calculated does not apply for climate quality ocean acidification research. The uncertainties in calculated pHt prevent climate quality pHt from being calculated from total alkalinity (TA) and dissolved inorganic carbon (DIC) and should be directly measured instead. However, climate quality TA and DIC can often be calculated using measured pH and DIC or TA respectively. Calculations are notably biased at medium-to-high pCO2 values (~500–800 μatm) implying models underestimate future ocean acidification. Uncertainty in the dissociation constants leads to significant uncertainty in the depth of the aragonite saturation horizon (>500 m in the Southern Ocean) and must be considered when studying calcium carbonate cycling. Significant improvements in the precision of the thermodynamic constants are required to improve pHt calculations. 

Abstract Comprehensive sampling of the carbonate system in estuaries and coastal waters can be difficult and expensive because of the complex and heterogeneous nature of near-shore environments. We show that sample collection by community science programs is a viable strategy for expanding estuarine carbonate system monitoring and prioritizing regions for more targeted assessment. ‘Shell Day’ was a single-day regional water monitoring event coordinating coastal carbonate chemistry observations by 59 community science programs and seven research institutions in the northeastern United States, in which 410 total alkalinity (TA) samples from 86 stations were collected. Field replicates collected at both low and high tides had a mean standard deviation between replicates of 3.6 ± 0.3 µ mol kg −1 ( σ mean ± SE, n = 145) or 0.20 ± 0.02%. This level of precision demonstrates that with adequate protocols for sample collection, handling, storage, and analysis, community science programs are able to collect TA samples leading to high-quality analyses and data. Despite correlations between salinity, temperature, and TA observed at multiple spatial scales, empirical predictions of TA had relatively high root mean square error >48 µ mol kg −1 . Additionally, ten stations displayed tidal variability in TA that was not likely driven by low TA freshwater inputs. As such, TA cannot be predicted accurately from salinity using a single relationship across the northeastern US region, though predictions may be viable at more localized scales where consistent freshwater and seawater endmembers can be defined. There was a high degree of geographic heterogeneity in both mean and tidal variability in TA, and this single-day snapshot sampling identified three patterns driving variation in TA, with certain locations exhibiting increased risk of acidification. The success of Shell Day implies that similar community science based events could be conducted in other regions to not only expand understanding of the coastal carbonate system, but also provide a way to inventory monitoring assets, build partnerships with stakeholders, and expand education and outreach to a broader constituency.