An absorbance-based fiber-optic sensor for CO2(aq) measurement in porewaters of sea floor sediments

We present results of the construction, testing and calibration of a sensor designed for in situ measurement of CO2(aq) concentrations in porewaters of sea floor sediments. The sensor relies on pH-dependent absorbance changes of a dye-containing solution enclosed within a gas-permeable membrane. It is rugged enough to withstand insertion several centimeters in sediments without damage, and small enough to make measurements separated spatially by ≤ 2 mm. Response time at 20 °C is a few minutes. Calibration results imply that the sensor is capable of resolving differences in CO2(aq) concentration of ± 8%, or about 2–3 × 10−6 M, and is stable over time scales of several hours. Preliminary results from two sea floor locations in the equatorial Atlantic of depths 3300 and 4700 m are qualitatively consistent with simultaneous, adjacent pH electrode measurements. Sensor performance could be improved by adding a system for in situ reagent delivery, and by monitoring a non-absorbing wavelength in the dye absorbance spectrum to decrease dependence on the dye concentration of the internal solution.