Abstract
Information on marine CO2 system variability has been limited along the Inside Passage of the Pacific Northwest coast of North America despite the region’s rich biodiversity, abundant fisheries, and developing aquaculture industry. Beginning in 2017, the Alaska Marine Highway System M/V Columbia has served as a platform for surface underway data collection while conducting twice weekly ~1600-km transits between Bellingham, Washington and Skagway, Alaska. This dataset allowed for the assessment of marine CO2 system patterns along the Inside Passage, including quantification of the relative importance of key drivers in shaping pCO2 variability. Surface water pH and aragonite saturation state (Ωarag) were determined using the pCO2 data with alkalinity from a regional salinity-based relationship, which was evaluated with discrete seawater samples and underway pH measurements. Low pH and corrosive (Ωarag < 1) Ωarag conditions were seen during winter and in persistent tidal mixing zones, and corrosive Ωarag values were also seen in areas that receive significant glacial melt in summer. The time-of-detection was computed and revealed that tidal mixing zones may be sentinel observing sites with relatively short time spans of observation needed to capture secular trends in seawater pCO2 equivalent to the contemporary atmospheric CO2 increase. Finally, anthropogenic CO2 was estimated and showed notable time and space variability. We theoretically considered the change in hydrogen ion concentration ([H+]), pH, and Ωarag over the industrial era and to an atmospheric pCO2 level consistent with a 1.5 °C warmer climate and revealed greater changes in [H+] and pH in winter as opposed to larger Ωarag change in summer. In addition, the contemporary acidification signal everywhere along the Inside Passage exceeded the global average, with Johnstone Strait and the Salish Sea standing out as potential bellwethers for biological OA impacts. In theory, roughly half the acidification signal experienced thus far over the industrial era may be expected over the coming 15 years with an atmospheric CO2 trajectory that continues to be shaped by fossil-fuel development.
Highlights
Atmospheric carbon dioxide (CO2) has increased from 278 ppm in 1765 to 412 ppm in 2019 due to the emissions of 40 CO2 from fossil fuel combustion and land use change, which combined have mobilized a total of 700 Gt of carbon that otherwise would have remained locked in long-term geological reservoirs (Friedlingstein et al, 2020)
Seasonal warming in most regions began in April and occurred earlier in the Salish Sea, which was consistent with satellite observations that have identified earlier seasonal warming in this region relative to coastal areas to the north (Jackson et al, 2015)
Surface salinity was fresher throughout the year in the Salish Sea, variability in salinity was larger in the Alexander Archipelago (Figures 1 and 2) where seasonal freshwater delivery to the coastal ocean contributes 41% of the freshwater input to the Gulf of Alaska (Edwards et al, 2020)
Summary
Atmospheric carbon dioxide (CO2) has increased from 278 ppm in 1765 to 412 ppm in 2019 due to the emissions of 40 CO2 from fossil fuel combustion and land use change, which combined have mobilized a total of 700 Gt of carbon that otherwise would have remained locked in long-term geological reservoirs (Friedlingstein et al, 2020). Over the industrial era, an estimated 160 Gt of this carbon pool has transferred into the ocean, known as the oceanic anthropogenic CO2 component (Sabine et al, 2004), and has led to changes in the marine CO2 system including reduced carbonate ion concentration ([CO32]) and pH, and increased hydrogen ion concentration ([H+]) and CO2 partial pressure (pCO2) These marine CO2 system changes 45 are collectively referred to as “ocean acidification” (Caldeira and Wickett, 2003;Doney et al, 2009;Feely et al, 2004a;Feely et al, 2009), and two recent assessments estimate an average pH decline for the global surface ocean on the order of 0.1 units (Jiang et al, 2019;Lauvset et al, 2020). These assessments of global average pH and Ωarag decline are based on calculations of anthropogenic CO2 content, long-term change in both pH and Ωarag resulting from anthropogenic CO2 input has been captured in prominent open ocean time series datasets (Bates et al, 2014;Doney et al, 55 2020)
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