The development of pH and pCO2 microelectrodes for studying the carbonate chemistry of pore waters near the sediment‐water interface

Abstract

We made stable and rapid-responding pH and pC0, microelectrodes.The pH microelectrode has a -100~pm-diameter half-sphere bulb tip of pH glass, and the pC0, sensor has a 200-300~pm-diameter silicone-membrane covered tip. Because the pH microelectrodes have electrical resistances that arc > 1Oro ohms, all clcctrical connections arc kept scrupulously clean and dry to prevent current leakage during in situ measurements. Suspension effects arc negligible when measuring pH in marine sediments.To demonstrate the performance of the pH and pC0, microelectrodes, WC report in situ microprofiles of pH and pC0, and contiguous profiles of 0, and the formation factor from coastal sediment in San Diego Bay, California. A sharp pH decrease from 7.90 to 7.15 (seawater scale) and a pC0, increase from 578 to 3,000 patm in the top 1 cm of scdimcnt arc interpreted to be primarily the result of HS , Fe2 I , and Mn2 + oxidation by 0, rather than metabolic CO, production. The 02, pH, and pC0, profiles also reveal that there was photosynthetic activity in the top 2 mm of sediment which caused an 0, maximum and a total dissolved inorganic carbon minimum.

A pH plate fluorosensor (optode) for early diagenetic studies of marine sediments

“…Surficial marine sediments, in particular, are characterized by complex heterogeneity and steep gradients in reaction rates, concentration distributions, and mass transport efficiency (Revsbech et al 1980;Reimers et al 1986;Revsbech 1989;Glud et al 1996;Aller et al 1998). Measurements of pH in surficial sedimentary deposits are typically made using H ϩ sensitive glass electrodes (e.g., Revsbech and Jørgensen 1986;Cai and Reimers 1993). Although pH electrodes are ideal for many applications, they are limited in resolving spatial patterns, are subject to matrix-related standardization uncertainties (e.g., liquid junction potential and solution interferences-poisoning), may suffer from electrical interferences and physical damage, and are in some formats relatively expensive (Sillén 1967;Revsbech 1989;Wolfbeis 1991).…”

Dissolved inorganic carbon profiles and fluxes determined using pH and microelectrodes

“…P,.,, is the only other parameter of the CO, system that is readily measurable by microelectrodes. Therefore, Cai and Reimers (1993) and Cai et al (1995) attempted to use a potentiometric Pco2 microelectrode to measure the first in situ PVo2 profiles in marine sediments. These PcO, profiles, however, did not compare well with models constrained by the pH and oxygen microelectrode data at several key depths within the sediment.…”

“…These PcO, profiles, however, did not compare well with models constrained by the pH and oxygen microelectrode data at several key depths within the sediment. Cai and Reimers (1993) and Cai et al (1995) attributed these deviations to the slow response characteristics of the potentiometric P,,, sensor.The goal of this study was to verify that potentiometric PH and PO, microelectrodes, given adequate response times, produce pH and Pcoz data that can be used to derive (1) reliable total dissolved inorganic carbon (DIC) concentrations in pore waters and (2) accurate DIC fluxes across the sediment-water interface. Diffusive fluxes of DIC across the sediment-water interface were calculated using pH and Pco, microelectrode profiles and compared with total fluxes determined directly using laboratory core incubations as the primary test of microelectrode performance.…”

A rapid response, planar fluorosensor for measuring two‐dimensional pCO2 distributions and dynamics in marine sediments

“…This type of pCO 2 sensor combines a pH electrode embedded within a bicarbonate buffer-filled compartment that interacts with the environment through a gas-permeable membrane. Such sensors have been used to profile pCO 2 distributions vertically in marine sediments and to study biogeochemical reactions near the sediment surface (Cai and Reimers 1993;Zhao and Cai 1997). They are limited by relatively slow response, short lifetime, and an arduous fabrica-tion process.…”

“…As a consequence, deposits near the sediment-water surface (0-5 cm depth) are typically characterized by steep vertical gradients of CO 2 , pH, and O 2 (Cai and Reimers 1993;Reimers et al 1996;Hales and Emerson 1997;de Beer et al 1997). This zone is also commonly inhabited by macrofauna.…”