Chemolithoautotrophic denitrification intensifies nitrogen loss in the Eastern Arabian Sea Shelf waters during sulphidic events

•Denitrification in the Western Indian continental shelf (WICS) waters during late summer monsoon was sulphide-driven, occurred at the mean rate of 2.69 µM N2 d−1, outcompeted chemoorganotrophic denitrification by 2-8 times and intensified the N loss.•Detection of S0 plume through satellite imagery, stimulation of N2 and N2O production by sulphide, and consumption of NOx− and S2− at a ratio close to the theoretical value suggested the denitrification being carried out by chemolithoautotrophs.•High N2O production in the sulphide-spiked 15N incubations suggested that the autotrophic denitrification was the prime reason behind high N2O accumulation at the pelagic chemocline during late summer monsoon.•The chemoautotrophic dark CO2 fixation (0.21 g C m−2 d−1) was ∼18% of the photoautotrophic production and accounted for ∼15% of the total column productivity.•The chemolithoautotrophic denitrification rate over the WICS was the second highest among the anoxic coastal systems of the world, and accounted for the net removal of 0.4 Tg of fixed N and 0.57 Tg of sulphide, and fixation of 0.1 Tg of CO2 annually.