Carbonate chemistry and CO2 dynamics in the Persian Gulf.

This study examines the carbonate chemistry of the Persian Gulf within the Iranian Exclusive Economic Zone, using datasets collected during the PGE2102 expedition in September 2021. The water column was stratified, with a warm, oxic upper layer (0-25 m: 33.2 °C, salinity 38.9 psu, O2 177 μmol/kg) and a cooler, low-oxygen deep layer (> 25 m: 24.4 °C, salinity 40.2 psu, O2 94.3 μmol/kg). Four water types were identified: Indian Ocean Surface Water (IOSW), Surface Persian Gulf Water (Surface-PG), Deep Persian Gulf Water (Deep-PG), and Northwest Persian Gulf Water (NW-PG). The lowest normalized alkalinity (NAT) was found in NW-PG (2427 ± 29 μmol/kg), suggesting alkalinity loss, while IOSW exhibited the highest NAT (2551 ± 9 μmol/kg). Deep-PG had lower NAT (2460 ± 18 μmol/kg) than surface waters, with surface NAT decreasing westward. Organic matter decomposition in Deep-PG resulted in the lowest pH (7.924 ± 0.030) and highest pCO2 (592.8 ± 47 μatm). Surface waters showed reduced dissolved inorganic carbon (DIC ~2047 μmol/kg) and undersaturated pCO2 (403.9 ± 69 μatm) due to photosynthesis. Hypoxic zones in western and central areas exhibited elevated DIC (up to 2305.6 μmol/kg) and the lowest pH (7.832), reflecting remineralization. Calcium carbonate precipitation contributed to significant alkalinity losses (66.2 μmol/kg), translating to 31 million tons of annual deposition. A ΔOrgC:ΔCaCO3 ratio of 2.6:1 in surface waters suggests higher photosynthetic activity relative to calcification. Despite localized pCO2 hotspots, 85 % of surface waters acted as CO2 sinks, highlighting unique carbonate dynamics shaped by stratification, biogeochemical processes and regional conditions.