DOI: 10.1594/pangaea.974077
title: Pore water data of laboratory incubation experiments on enhanced benthic weathering in organic rich Baltic Sea sediments

description: Enhanced mineral dissolution in the benthic environment is currently discussed as a potential technique for ocean alkalinity enhancement (OAE) to reduce atmospheric carbon dioxide (CO2) levels. This study explores how biogeochemical processes affect the dissolution of alkaline minerals in surface sediments during laboratory incubation experiments (January - May 2022). These involved introducing dunite and calcite to organic-rich sediments from the Baltic Sea under controlled conditions in an oxic environment. The sediment cores were incubated with Baltic Sea bottom water. Eight sediment cores were placed in a rack in an upright position. The bottom water was carefully removed via suction and replaced with a known volume (1.5 l – 2.0 l) of filtered (0.2 µm) Baltic Sea bottom water in order to remove pelagic auto- and heterotrophs and suspended particles. The volume of water added depended on the height of sediment in each core which varied slightly due to the recovery method. After this procedure, a gaseous headspace of ca. 10 cm was left in each core. Furthermore, the cores were equipped with adjustable stirring heads that contained ports for inserting optodes to continuously record pH and oxygen (O2) concentrations in the overlying water. In order to prevent anoxic conditions developing, ambient air was bubbled into the water column. The water column in each core was slowly and continuously flushed with a constant throughflow of 40 µl min-1 from a single reservoir of bottom water. The residence time of the water inside the cores was thus about 4 to 5 weeks. At the end of the experiments, the bottom water was removed via suction and the cores were sliced for pore water analysis. The pore waters were recovered by centrifuging each respective sediment layer in 50 ml falcon tubes at 3000 rpm for 10 minutes. Afterwards, the supernatant water was transferred to polyethylene (PE) vials in an Ar-filled glove bag to minimize contact with oxygen. Samples for TA were analyzed directly after sampling by titration of 1 ml of bottom/pore water with 0.02N HCl. Titration was ended when a stable purple color appeared. During titration, the sample was degassed by continuous bubbling with nitrogen to remove any generated CO2 and H2S. The acid was standardized using an IAPSO seawater standard. Anion element concentrations (SO42-, Cl-, Br-) were determined using ion chromatography (IC, METROHM 761 Compact, conductivity mode). Acidified sub-samples (30 μl suprapure HNO3- + 3 ml sample) were prepared for analyses of major and trace elements (Si, Na, K, Li, B, Mg, Ca, Sr, Mn, Ni and Fe) by inductively coupled plasma optical emission spectroscopy (ICP-OES, Varian 720-ES). In addition to the parameters listed above, pore waters were analyzed for sulfite (H2S) and Fe2+. For the analysis of dissolved Fe2+ concentrations, sub-samples of 1 ml were taken within the glove bag, immediately stabilized with ascorbic acid and analyzed within 30 minutes after complexation with 20 μl of Ferrozin. For H2S, an aliquot of pore water was diluted with appropriate amounts of oxygen-free artificial seawater and the H2S was fixed by immediate addition of zinc acetate gelatin solution.
description: Additional information regarding the experimental set-up:* Laboratory: GEOMAR cool-lab (12.5 °C)* Treatments, added to sediment core of 10 cm diameter: Dunite (4.4781 g, manufacturer Sibelco), Calcite (7.8618 g, manufacturer unknown).* Baltic sea water sampled in Boknis Eck (Salinity: 21, Temperature: 4.2 °C)* Start of the experiments: 2022-01-14 (morning)* End of the experiments: 2022-05-07 (afternoon)