Disentangling artificial and natural benthic weathering in organic rich Baltic Sea sediments

Abstract

Enhanced mineral dissolution in the benthic environment is currently discussed as a potential technique for ocean alkalinity enhancement (OAE) to reduce atmospheric CO2 levels. This study explores how biogeochemical processes affect the dissolution of alkaline minerals in surface sediments during laboratory incubation experiments. 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. Findings reveal that the addition of calcite increased the benthic alkalinity release from 0.4 μmol cm−2 d−1 (control) to 1.4 μmol cm−2 d−1 (calcite) as well as other weathering products such as calcium. However, these enhanced fluxes returned to lower fluxes after approximately 4 weeks yet still higher than the un-amended controls. Microbial activity appeared to be the primary driver for lowering pore water pH and thus enhanced weathering. In several sediment cores, pH profiles taken at the start of the experiments indicated activity of sulfur oxidizing Beggiatoa spp, which was verified by RNA-profiling of 16S rRNA genes. The pH profiles transitioned to those commonly associated with the activity of cable bacteria as the experiments progressed. The metabolic activity of cable bacteria would explain the significantly lower pH values (~5.6) at sediment depths of 1–3 cm, which would favor substantial calcite dissolution. However, a high abundance of cable bacteria was not reflected in 16S rRNA sequence data. Total alkalinity (TA) fluxes in these cores increased by a factor of ~3, with excess TA/calcium ratios indicating that the enhanced flux originated from calcite dissolution. The dissolution of dunite or the potential formation of secondary minerals could not be identified due to the strong natural flux of silicic acid, likely due to biogenic silica dissolution. Furthermore, no accumulation of potentially harmful metals such as nickel was observed, as highlighted as a potential risk in other studies concerning OAE. Given the complexity of sediment chemistry and changes of the benthic conditions induced by the incubation, it remains challenging to distinguish between natural and enhanced mineral weathering. Further investigation, including the identification of suitable tracers for mineral dissolution, are necessary to assess the feasibility of benthic weathering as a practical approach for OAE and climate change mitigation.

Alkaline mineral addition to anoxic to hypoxic Baltic Sea sediments as a potentially efficient CO2-removal technique

“…By investigating the efficiency of EBW under conditions of hypoxia to anoxia as well as CO 2 enrichment, this study complements a previous study, where calcite and dunite were added to Baltic Sea sediments exposed to oxic, calcite-oversaturated bottom waters (Fuhr et al, 2023). To this end, sediment cores were retrieved from Boknis Eck, a shallow, seasonally hypoxic to anoxic depocenter in Eckernförde Bay in the southwestern Baltic Sea, amended with calcite and dunite and exposed to calcite-undersaturated bottom waters under controlled laboratory conditions with the aim to simulate conditions prevailing during late summer in this part of the Baltic Sea (Melzner et al, 2013).…”

“…The uniformity of pore water solute concentrations in most cores with the initially sampled core suggests that deviations observed in C1 and Dun1 reflect the spatial inhomogeneity of the sediment composition, despite the close proximity of coring locations. In previous experiments conducted under oxic conditions, drastic changes were observed in porewater composition over the course of the experiments (Fuhr et al, 2023). These changes did not occur in the experiments in the present study which is most likely a consequence of the shorter incubation time and the fact that the cores were recovered at an ambient temperature of 10.2 • C, thus closer to the laboratory conditions (12.5 • C), compared to 6.9 • C in the former experiment.…”

“…Two promising minerals are olivine [(Mg,Fe) 2 SiO 4 ], a fast dissolving nesosilicate, with the two endmembers forsterite (Mg 2 SiO 4 ) and fayalite (Fe 2 SiO 4 ), and calcite, a highly abundant calcium carbonate (CaCO 3 ;Rimstidt et al, 2012;Subhas et al, 2015;Renforth and Henderson, 2017;Kremer et al, 2019). Since olivine is the major component of dunite, an ultramafic, igneous rock, and the purification of the mineral is cost intense, dunite instead of olivine would be used in an application scenario (Hochstetter, 1859;Rigopoulos et al, 2018;Fuhr et al, 2023). Still, olivine is the fastest dissolving component of dunite, which is why for the efficiency and efficacy of enhanced dunite weathering, olivine dissolution is the key process (Palandri and Kharaka, 2004;Rimstidt et al, 2012).…”

“…Importantly, these secondary minerals can alter the overall stoichiometry of the dissolution process, leading to disparities between calculated and actual dissolution kinetics (Fuhr et al, 2022). Furthermore, recent studies using sediments from the Baltic Sea have illuminated the distinctive challenges that arise under conditions closer to the natural system (Fuhr et al, 2023).…”

“…In the Baltic Sea, distinguishing between natural and EBW is difficult, given the substantial benthic background fluxes driven by natural CaCO 3 input from continental erosion for calcium and diatom dissolution for silica that can overshadow potential fluxes induced by enhanced benthic weathering (Gasiunaite et al, 2005;Borawska et al, 2022;Wallmann et al, 2022;Fuhr et al, 2023). Furthermore, the dissolution kinetics of both calcite and olivine remain largely unexplored in the brackish waters of the Baltic Sea, introducing uncertainty whether dissolution rates assumed in modeling studies based on standard seawater (Feng et al, 2017;Griffioen, 2017;Fakhraee et al, 2023) can be applied in this specific region.…”