Abstract
AbstractAn accurate description of the carbonate system in pore waters is valuable in studies involving the degradation of sedimentary organic matter, recrystallization of calcium carbonate minerals, calculations of mineral saturation state, and cycling of ions affected by pH. Here, we analyze water chemistry of pore water extracted using centrifugation and Rhizon samplers from hemipelagic sediments in the Gulf of Aqaba, Red Sea, and a shallow salt marsh from Norfolk, England. In both study areas, the data are internally consistent for each pore‐water separation technique, but the measured isotopic composition of the dissolved inorganic carbon (δ13C[DIC]) differs between the two techniques. We performed laboratory experiments that show that both Rhizons and centrifugation are prone to degassing of CO2 enriched with 12C. We suggest that during sampling with Rhizons, air fills the voids left by extracted pore water; combined with the membrane's design to exclude air, some of the aqueous CO2 diffuses into these air bubbles instead of the sampler. Rhizons produce reliable calcium, strontium, manganese, and barium concentration data when soaked in deionized water and then flushed with the sample immediately prior to sampling. However, pore‐water extractions with Rhizons are less reliable for analyses of pH and δ13C[DIC]. Centrifugation produces reliable carbonate chemistry and major element data when tubes are fully filled without headspace and sealed tightly. Working in CO2 low/free atmosphere (e.g., N2 glovebox) enhances the chance of losing CO2 from the sample in both sampling techniques due to increased negative gradient of CO2 between the core and its surrounding.
Highlights
Use of centrifuges is often impractical during ocean research cruises due to a difficulty in balancing the centrifuge, the method often used in pore-water extraction in the Integrated Ocean Drilling Program (IODP) is highpressure squeezing (Reagan et al 2015)
The measured concentrations and rate of change with depth of several analytes are different when pore waters are extracted using centrifugation and Rhizon samplers. This is true when it comes to measurements of δ13C(DIC), which are systematically more negative in the centrifuged samples compared to those obtained using Rhizon samplers (Fig. 1b,d)
Our analyses show that both centrifugation and Rhizon samplers produce adequate data for most studied analytes but centrifugation performs slightly better in many cases
Summary
Past studies comparing different pore-water sampling techniques yielded inconclusive results which highlights that none of the methods available at present is optimal for all cases and all elements of interest. Di Bonito (2005) compared soil pore-water extraction of a large number of trace elements by centrifugation, squeezing, Rhizon samplers, and soil suspensions. On IODP cruises, pore-water extractions comparing squeezing and Rhizon samplers have been explored in several studies (Dickens et al 2007; Schrum et al 2012; Miller et al 2014; Miller et al 2017). Miller et al (2017) suggested that repeat pore-water extractions using Rhizon samplers from the same locations in a core produce internally consistent calcium concentration data
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