Rapid shoaling of aragonite saturation horizon in the northern Indian Ocean

Anthropogenic carbon dioxide (CO2) penetrates up to 1000 m of water column in the Indian Ocean resulting in acidification and corrosion of aragonite skeletal material. The shallowest aragonite saturation horizon (ASH) was observed in the Bay of Bengal (BoB; 219 ± 10 m) within the tropical Indian Ocean. The ASH shoaled at the rate of 6.3 ± 5 and 4.4 ± 3 m yr−1 in the past four decades in the BoB and Arabian Sea respectively. As a result, an increase in total alkalinity (TA) was observed at the rate of 0.5 ± 0.3 and 0.25 ± 0.2 μmol kg−1 yr−1 at the depth of ASH in the BoB, and Arabian Sea respectively. While the shoaling rate of ASH remained the same in the Arabian Sea over the past four decades, in contrast, rapid shoaling was observed in the BoB in the recent decades due to higher accumulation of anthropogenic CO2 in the upper ocean associated with an increase in river discharge and decomposition of riverine organic matter. These two processes decreased the pH resulting in corrosion of aragonite skeletal material and increased TA at the depth of ASH in the BoB. Under a business-as-usual scenario, aragonite-secreting organisms will be seriously affected by the middle of this century in the BoB.