Journal article
Ocean alkalinity enhancement reduces silica ballasting during export due to amplified dissolution
Biogeosciences, Vol.23(13), pp.4691-4710
08/07/2026
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Abstract
Ocean alkalinity enhancement (OAE) is a carbon dioxide removal (CDR) technology proposed to store carbon dioxide (CO2) in the ocean on human-relevant time scales. However, depending on OAE intensity, resulting shifts in seawater carbonate chemistry speciation could alter community-driven biomass build-up, particulate stoichiometry, and transformation during particle export. Using mesocosms in the eutrophic North Sea (Helgoland, Germany), we established six alkalinity levels under two dilution scenarios (localized vs. uniform OAE additions) for 39 d. Total alkalinity (TA) was increased to ΔTAmax= 1250 µmol kg−1 (250 µmol TA kg−1 increments) using NaOH with CaCl2 to simulate cation release during calcium-based mineral dissolution, causing strong carbonate chemistry perturbations (e.g., pHmax> 9.25). To compare community-mediated carbon export across equivalent bloom phases, measurements were assessed within mesocosm-specific bloom and export events rather than on fixed sampling days, thereby accounting for OAE-induced shifts in spring bloom timing. During blooms, average phytoplankton biomass (as the concentrations of chlorophyll a and particulate organic carbon in the water column, POCWC) remained unchanged under unequilibrated OAE. In contrast, silica ballasting ratios declined with increasing pHT: suspended biogenic silica to particulate organic carbon ratios (BSiWC : POCWC, where WC = water column) decreased by up to 50 %, while exported BSiSed : POCSed (where Sed = sediment) decreased by 60 %, indicating intensification during sinking. The stronger decline in sinking compared to suspended BSi : POC is consistent with pH-enhanced BSi dissolution during export. Porosity of sinking particles increased with pHT and co-varied with BSiSed : POCSed, suggesting particle-quality traits can modulate dissolution during transit. Organic matter remineralization metrics showed no response to alkalinity addition, and particle sinking velocities did not scale with suspended or sinking silica ballasting ratios. Across dilution scenarios, unequilibrated OAE may reduce silica ballasting, potentially shoaling carbon remineralization, shortening sequestration timescales, and weakening net CO2 removal, while effects of dissolved silica regeneration on diatom productivity remain unresolved. Quantifying how pH-driven BSi dissolution interacts with bloom and export dynamics will be critical for evaluating OAE efficacy and ecological safety.
Details
- Title
- Ocean alkalinity enhancement reduces silica ballasting during export due to amplified dissolution
- Creators
- Philipp Suessle - Universidade dos AçoresJoana Barcelos e Ramos - Universidade dos AçoresNico Manuel Sievers - GEOMAR Helmholtz Centre for Ocean Research KielJulieta Schneider - GEOMAR Helmholtz Centre for Ocean Research KielJuliane Katharina Tammen - GEOMAR Helmholtz Centre for Ocean Research KielLeila Kittu - GEOMAR Helmholtz Centre for Ocean Research KielLaura Marín-Samper - Universidad de Las Palmas de Gran CanariaMaarten Boersma - Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und MeeresforschungKai G Schulz - Southern Cross UniversityUlf Riebesell - GEOMAR Helmholtz Centre for Ocean Research Kiel
- Publication Details
- Biogeosciences, Vol.23(13), pp.4691-4710
- Publisher
- Copernicus GmbH
- Number of pages
- 20
- Grant note
- Bundesministerium fr Bildung, Wissenschaft, Forschung und Technologie: 03F0895A, 03F0895B Fundao para a Cincia e a Tecnologia: UIDB/00153/2020
This study was funded by the Federal Ministry of Education and Research of Germany (BMBF) in the framework of RETAKE (grant/award nos. 03F0895A and 03F0895B), one of the six research consortia of the German Marine Research Alliance (DAM) research mission "Marine carbon sinks in decarbonisation pathways" (CDRmare). Additional support was provided through the Ocean Alk-Align project, funded by Carbon to Sea and the Thistledown Foundation. More support came from the Fundac & atilde;o para a Ciencia e a Tecnologia (FCT, Portugal) with the reference UIDB/00153/2020. JBR is supported by FCT, within Contrato-Programa, Apoio Institucional, Portugal. The article processing charges for this open-access publication were covered by the GEOMAR Helmholtz Centre for Ocean Research Kiel.
- Identifiers
- 991013390052202368
- Academic Unit
- Faculty of Science and Engineering; Science
- Language
- English
- Resource Type
- Journal article