Ocean Acidification and Carbonate System Geochemistry in the Arabian Gulf

The impacts of ocean acidification are a concern for marine ecology in the Arabian Gulf. Seawater from the Arabian Sea enters the Arabian Gulf through the Strait of Hormuz and its salinity increases due to net evaporation. As a result, Alk and DIC progressively increase. We measured Alk and DIC on samples collected in November 2018, May 2019 and September 2019 at seven stations in the Exclusive Economic Zone of Qatar. Calculated surface P_CO2 averaged 472 matm in 2018 and 447 matm in 2019. The Arabian Gulf is degassing CO₂ at present and will not take up atmospheric CO₂ until 2042. Ocean acidification is not yet an issue in the EEZ of Qatar. The elevated P_CO2 values are due to CaCO₃ formation. Both Alk and DIC increase as surface seawater flows from the Strait of Hormuz into the Arabian Gulf, This is due to the increase in salinity. Normalized NAlk and NDIC were calculated to remove the impact of increasing salinity. Within the Qatar EEZ, NDIC decreased but there was little change in NAlk. This suggests removal of CO₂ by primary production of organic matter with little or no removal by CaCO₃ formation. But relative to the Strait of Hormuz, NAlk and NDIC decrease corresponding to a CaCO₃/OrgC removal ratio of 2/1. We also calculated the nitrate corrected and salinity normalized tracer, Alk*. Values of Alk* were negative, and the change in Alk* relative to Hormuz (DAlk*) indicated that there has been an average decrease of Alk* of -130 mmol kg^-1. This decrease is likely due to CaCO₃ formation but previous studies found no evidence for coccolithophorids. One obvious possibility is that Alk removal is due to CaCO₃ formation in coral reefs. If so the changes seen in Alk* would be due to processes at the sediment-water interface rather than within the water column. However, recent study of the composition of particulate matter found that the average particulate Ca concentration was 3.6%, and was acid soluble. These results suggest that a significant amount of particulate CaCO₃ is present in the water column. One hypothesis is that the particulate Ca comes from carbonate rich atmospheric dust. Using Al as a tracer for dust and the average Ca/Al ratio in average Qatari dust can only explain about 3% of the particulate Ca. An alternative hypothesis is that particulate CaCO₃ may form in the water column due to abiological CaCO₃ formation.