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

Abstract

Marine sediments play critical roles in the global carbon cycle, including the remineralization of organic carbon and the dissolution or precipitation of CaCO 3 , processes which can produce or consume CO 2 , HCO 3 -, CO 3 2-, and H + (Emerson and Hedges 2003;Martin and Sayles 1994). 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. The feeding, burrowing, and irrigation activities of these bottom-dwelling animals create three-dimensional and time-dependent patterns of biogeochemical reactions within sediments, and greatly complicate quantification and estimation of material fluxes (Aller 1980(Aller , 2001. In-situ, high-resolution measurements of the distributions of pCO 2 are thus essential for conceptualizing and modeling heterogeneous biogeochemical reactions, microbial metabolism, and solute transport in bioturbated deposits. Recently, we reported on a planar pCO 2 fluorosensor, which allows high-resolution two-dimensional mapping of pCO 2 in sediments and overlying water (Zhu et al. 2006a). This sensor uses 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS) as the fluorescence indicator, ethyl cellulose as polymer matrix, and has a response time of ~2.5 min and a recovery time of ~5 min (over 0-5 matm). While a significant advance, it cannot respond rapidly enough to measure CO 2 dynamics associated with high frequency transport-reaction processes such as burrow irrigation. To better study real-time pCO 2 variation in bioturbated sediment, we designed and report here on an improved planar pCO 2 sensor with a faster response.The first pCO 2 sensor was developed in 1957 based on potentiometric electrodes (Stow et al. 1957;Severinghaus and Bradley 1958). 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-A rapid response, planar fluorosensor for measuring twodimensional pCO 2 distributions and dynamics in marine sediments Qingzhi Zhu* and Robert C. AllerSchool of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, USA Abstract CO 2 distributions in marine sediments are closely coupled to early diagenetic reactions and transport processes. An existing planar optode designed to measure pCO 2 in sediments and overlying water was modified to substantially lower response times and to allow for enhanced resolution of CO 2 dynamics. Sensing foils are fabricated by entrapping fluorescent indicator 8-hydroxy-1,3,6-pyrenetrisulfonic acid-tet...

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