Enhanced silicate weathering in permeable sediments from the North Sea – a laboratory study using flow-through reactors

The Earth’s climate is increasingly warming due to ongoing anthropogenic carbon dioxide (CO2) emissions. In order to mitigate the human-made climate change and to meet the Paris Agreement goals of limiting the warming below 2°C, active carbon dioxide removal (CDR) from the atmosphere is of great importance in addition to massive CO2 emission reductions. A possible CDR method is rock weathering and the associated dissolution of silicate minerals in the ocean, which leads to marine alkalinity enhancement and, thus, an enhanced flux of CO2 from the atmosphere into the ocean. In the framework of the project RETAKE, a consortium of the German Marine Research Alliance (DAM) research mission CDRmare, we investigate the potential, feasibility and side effects of silicate mineral dissolution in high-energy coastal environments where strong currents and advection of seawater through permeable sediments have been proposed to accelerate weathering of silicate rocks [1]. Permeable sediments are generally characterized by advective pore-water flow. Under advective conditions, higher weathering rates than those found in diffusion-controlled depositional settings are expected since the reaction products are rapidly removed and the formation of authigenic mineral coatings on mineral grains is prevented. Using flow-through sediment columns, advective pore-water fluxes through the sediment as they prevail in natural permeable beach and coastal deposits can be simulated [2,3]. Here, we present data from laboratory experiments with flow-through reactors that are filled with permeable sandy sediments from the North Sea, Germany, amended with fine-grained dunite (0.063-0.180 mm), mainly composed of olivine (~ 90 %). The flow-through experiments are conducted under oxic conditions whereby air-saturated natural seawater is continuously pumped through the reactors for 160 days. Our results demonstrate an increase in both alkalinity and dissolved inorganic carbon (DIC) of up to 4 mM in the reactors with dunite addition while the alkalinity and DIC concentrations in the control reactors (without dunite addition) are close to background seawater values of 2.3 mM. However, since dunite contains relatively high amounts of nickel (0.3 wt%), enhanced weathering may also be associated with an increased release of this potentially toxic trace metal. Indeed, the nickel concentrations in the effluent water of the dunite-amended sediment columns are increased by up to 900 nM. Silica and phosphate concentrations are elevated compared to the seawater values in both the control and the dunite-amended reactors. While the silica concentrations in the dunite-amended reactors are higher by up to 10 µM compared to the control, the opposite pattern is observed for phosphate. The slightly lower phosphate concentrations in the dunite reactors might be related to the precipitation of authigenic minerals, for example, iron phosphates or to adsorption of phosphate onto mineral grains. To identify possible authigenic minerals as potential sinks for the reaction products, the solid phase will be sampled and the chemical and mineralogical composition is analyzed after the experiments are terminated.   [1] Meysman and Montserrat, 2017. Biol. Lett. 13: 20160905. [2] Ahmerkamp et al., 2020. Sci. Rep. 10: 3573. [3] Zhou et al., 2023. Sci. Total Environ. 865: 161168.