Excess pCO2 in Surface Seawater of the Arabian Gulf

Abstract

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 calculated in surface seawater averaged 459 ± 61 matm and was supersaturated with respect to the atmosphere. The region was degassing CO 2 to the atmosphere and the flux was about 1.25 mmol C m -2 d -1 . The origin of this excess CO 2 must be due to CaCO 3 precipitation. The horizontal relationship between salinity-normalized total alkalinity (NTA) and dissolved inorganic carbon (NDIC) showed that CaCO 3 formation was more important, relative to net biological productivity, than in the open ocean. The tracer Alk* has values primarily determined by CaCO 3 formation and values of Alk* ranged from -50 to -310 mmol kg -1 , which is consistent with substantial CaCO 3 formation. DAlk* increased with increasing distance northward from Hormuz. The rate of calcification calculated from the air-sea flux of CO 2 (5.6 mmol C kg -1 y -1 ) and from DAlk* (5.9 mmol C kg -1 y -1 ) agreed well. However, CaCO 3 formation by net calcification in coral reefs is unlikely as they have limited distribution and have been severely damaged by past coral bleaching. There are high concentrations of excess particulate Ca in the water column that cannot be accounted for by input of CaCO 3 -rich Qatari dust. Carbonate forming plankton are absent in the water column. We propose that abiological, heterogeneous calcite precipitation (HCP) may be occurring. The mechanism is unknown but nucleation by CaCO 3 -rich Qatari dust may assist this process.