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 less than or equal to 2 mm. Response time at 20 degrees 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 x 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. (C) 1997 Elsevier Science B.V.