Global Ocean Spectrophotometric pH Assessment: Consistent Inconsistencies

Abstract

Ocean acidification (OA)or the decrease in seawater pH resulting from ocean uptake of CO2 released by human activitiesstresses ocean ecosystems and is recognized as a Climate and Sustainable Development Goal Indicator that needs to be evaluated and monitored. Monitoring OA-related pH changes requires a high level of precision and accuracy. The two most common ways to quantify seawater pH are to measure it spectrophotometrically or to calculate it from total alkalinity (TA) and dissolved inorganic carbon (DIC). However, despite decades of research, small but important inconsistencies remain between measured and calculated pH. To date, this issue has been circumvented by examining changes only in consistently measured properties. Currently, the oceanographic community is defining new observational strategies for OA and other key aspects of the ocean carbon cycle based on novel sensors and technologies that rely on validation against data records and/or synthesis products. Comparison of measured spectrophotometric pH to calculated pH from TA and DIC measured during the 2000s and 2010s eras reveals that (1) there is an evolution toward a better agreement between measured and calculated pH over time from 0.02 pH units in the 2000s to 0.01 pH units in the 2010s at pH > 7.6; (2) a disagreement greater than 0.01 pH units persists in waters with pH < 7.6, and (3) inconsistencies likely stem from variations in the spectrophotometric pH standard operating procedure (SOP). A reassessment of pH measurement and calculation SOPs and metrology is urgently needed.

An updated version of the global interior ocean biogeochemical data product, GLODAPv2.2020

“…erage (Álvarez et al, 2020;Carter et al, 2018). Outside the North Pacific, we believe, therefore that the pH data are consistent to 0.01.…”

“…3.2.5). Recent literature has demonstrated that internal consistency evaluation procedures are subject to errors owing to incomplete understanding of the thermodynamic constants, major ion concentrations, measurement biases, and potential contribution of organic compounds or other unknown protolytes to alkalinity (Takeshita et al, 2020), which lead to pH-dependent offsets in calculated pH (Álvarez et al, 2020;Carter et al, 2018): these may be interpreted as biases and generate false corrections. The offsets are particularly strong at pH levels below 7.7, when calculated and measured pH are different by on average between 0.01 and 0.02 units.…”

“…-When bottom depths were not given, they were approximated as the deepest sample pressure +10 dbar or extracted from ETOPO1 (Amante and Eakins, 2009), whichever was greater. For GLODAPv2, bottom depths were extracted from the Terrain Base (National Geophysical Data Center/NESDIS/NOAA/U.S.…”

“…We have now chosen to include the discrete f CO 2 values in the product files. This increases transparency and traceability of the product; the f CO 2 data are also highly relevant for ongoing efforts toward resolving recently identified inconsistencies in our understanding of the relationships among the seawater CO 2 chemistry variables (Carter et al, 2018;Fong and Dickson, 2019;Takeshita et al, 2020;Álvarez et al, 2020). A total of 33 924 discrete f CO 2 measurements from 34 cruises conducted between 1983-2014 are now included.…”

Autonomous in situ calibration of ion‐sensitive field effect transistor pH sensors

“…Spatiotemporal mismatch between sensor and bottle sampling adds to uncertainty in independent validation (Bresnahan et al 2014; McLaughlin et al 2017) and a comprehensive discrete sampling program can be painstaking and expensive. Discrete measurements of the marine inorganic carbon system have been refined for decades (Dickson et al 2007; Millero 2007; Riebesell et al 2010); recent work has scrutinized various techniques, uncovering small but measurable effects on pH accuracy from dye impurities as well as a pH‐dependent discrepancy between pH measured spectrophotometrically and estimated from total alkalinity and dissolved inorganic carbon measurements (Liu et al 2011; Carter et al 2018; Fong and Dickson 2019; Álvarez et al 2020; Takeshita et al 2020 a ). All of these issues contribute to increased uncertainty in the pH sensor data and create substantial hurdles in the pursuit of stringent community standards, such as the climate quality goal.…”