Abstract
Careful extraction and analysis of porewater from sediment cores are critical for the investigation of small-scale biogeochemical processes. Firstly, small sample volumes and high spatial resolution are required. Secondly, several chemical species in the anaerobic porewater are sensitive to oxidation when brought in contact with ambient air. Here we present the combination of a special sampling technique and an analytical method for the porewater extraction of a varved sediment core from Lake Baldegg in central Switzerland, using MicroRhizon samplers and a portable capillary electrophoresis (CE) instrument. MicroRhizon filter tubes of 1 mm diameter and 20 mm length are suitable for fast retrieval of particle-free porewater samples directly from the sediment core. Since the time-span between sampling and analysis is less than 20 seconds, oxygen-sensitive Fe(ii) can be analyzed in one go together with Na(+), K(+), Ca(2+), Mg(2+), NH4(+), and Mn(ii) without splitting, acidification or dilution of the sample. The major inorganic cations and anions of the sediment porewater can be determined in less than 15 minutes. Detection limits are in the sub-micromolar concentration range. The capillary electrophoresis instrument used in this study requires sample volumes of only 20 μL. These remarkable small sample volumes allow the minimization of disturbance of the sediment cores and a high spatial resolution of the sediment profile, even in sediments with low water content. The equipment is inexpensive, easy to handle, fully portable and therefore suitable for environmental on-site applications.
Highlights
The extraction and analysis of porewater from sediments are some of the most important techniques for the investigation of small scale biogeochemical processes and cycles, e.g. for the estimation of uxes from and to the sediment–water interface, quanti cation of burial and mineralization rates of organic matter and production rates of climate affecting methane, or forSince the 1960s porewater extraction techniques have been improved with regard to simplicity, rapidity, spatial resolution, and integrity of the sediments
Pro ling the sediment–water interface with ion-selective electrodes allowed the measurement of porewater concentration gradients with a spatial resolution of around 25 mm, but were only available for the detection of particular ions in the top few centimeters.[13,14,15]
Blanks (Nanopure water collected with MicroRhizon samplers) and certi ed multielement ion chromatography standard solutions (Fluka, Buchs, Switzerland) were intermittently measured to ensure a high data quality
Summary
Since the 1960s porewater extraction techniques have been improved with regard to simplicity, rapidity, spatial resolution, and integrity of the sediments. Pro ling the sediment–water interface with ion-selective electrodes allowed the measurement of porewater concentration gradients with a spatial resolution of around 25 mm, but were only available for the detection of particular ions in the top few centimeters.[13,14,15] Many of these limitations (artefacts with ex situ methods on the one hand and difficult handling with in situ methods on the other hand) can be overcome by using MicroRhizons (Rhizosphere Research Products, Wageningen), for porewater extraction. The contamination with oxygen is minimal and anaerobic sampling does not require the installation of a glove box.[17] Despite the many advantages of the method, sample handling and analysis remain a challenge due to the small volume, the need for splitting and preservation, and the risk of contamination.[14,22] Field-portable instrumentation can overcome these problems by the immediate analysis of the extracted sample.[23,24,25] a combination of MicroRhizons and a portable capillary electrophoresis (CE) instrument is ideal. The design and performance of the method is presented and exempli ed with sediment cores from a eutrophic lake in Switzerland where porewater concentration measurements were compared with simultaneous analysis by ion chromatography (IC)
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