Quantification of a subsea CO2 release with lab-on-chip sensors measuring benthic gradients

•A subsea release of CO2 was detected and quantified with a novel approach.•Autonomous lab-on-chip sensors on a lander measured pH and total alkalinity.•The pH signals showed decreases in pH of >0.6 and strong vertical gradients.•pH and hydrodynamic measurements enabled an estimate of CO2 emission rate.•This technique could be adapted to a wide range of in situ benthic gradient studies.