42 / 2024-08-30 00:23:19

Excess pCO2 and Carbonate System Geochemistry in Surface Seawater of the Arabian Gulf

pCO2, Corals, Ocean Acidification, Calcification

Session 9 - Global Ocean Changes: Regional Processes and Ecological Impacts

Coral reefs in the Arabian (Persian) Gulf have been severely impacted by degradation of the ecosystem due to climate change and anthropogenic modifications. Bleaching events due to ocean warming have been well described, but ocean acidification has received little attention since a 1977 study by Brewer and Dyrssen (1985). Dissolved inorganic carbon (DIC) and total alkalinity (TA) were sampled in December, 2018 and May, 2019 in the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf.  pCO_2, pH and CO₃^2- were calculated from DIC and TA. TA, DIC and salinity increase in the Gulf due to evaporation after entering through the Strait of Hormuz. Temperature also increases. The pCO₂ in surface seawater averaged 458 ± 62 which was higher than the atmospheric value of 412 ppm. Hence, the Gulf was a source of CO₂ to the atmosphere. A hypothetical model calculation was used to estimate how much pCO₂ could increase due to various processes after entering through the Strait of Hormuz. Increases in T and S, in the absence of biogeochemical processes, would increase pCO₂ to 537 μatm, more than enough to explain the high pCO₂ observed. CO₂ is lost from the Gulf due to gas exchange, decreasing DIC, and reducing pCO₂ to 464 μatm, similar to that observed. The impact of biological processes depends on the process: calcification increases pCO₂ while net primary production decreases pCO₂. Salinity-normalized (to S = 40) total alkalinity (NTA) and dissolved inorganic carbon (NDIC) in surface seawater decrease as waters flow north from Hormuz. The slope suggests that removal of C as CaCO₃, organic matter (CH₂O) or gas exchange (F_CO2) is occurring with a ratio of ΔCaCO₃/(ΔCH₂O or F_CO2) = 1: 2.86. The tracer Alk* has values ranging from -50 to -310 μmol kg^-1 which corresponds to a calcification of 65 μmol kg^-1. Our model calculations indicate that this would increase pCO₂ to 577 matm. Carbonate forming plankton have not been observed in the water column suggesting that calcification occurs in corals. The amount of DIC removed by net primary production is small, consistent with an oligotrophic food web. The high observed pCO₂ reflects a balance between sources due to the impact of increasing T and S on carbonate system equilibrium constants and net calcification and sinks due to CO₂ loss due to gas exchange and net primary production in surface seawater after it enters the Gulf through the Strait of Hormuz.