Glacial rock flour is a potential source for marine carbon dioxide removal by stimulating phytoplankton growth

Glacial rock flour (GRF) is a fine-grained silicate mineral formed below the Greenland Ice Sheet where the bedrock is abraded to a fine powder. GRF is transported by meltwater into fjords and coastal waters and its dissolution in seawater is part of the natural cycling of material between continents and the ocean. It is present in large sedimentary deposits along the coast of Greenland. However, due to the relatively small size distribution of GRF (d₅₀ ~ 2-5 µm) it has a relatively long residence time in the coastal surface layers and significant amounts reach the open ocean as suspended particulate material. As a silicate-rich material, also containing substantial amounts of micronutrients (e.g., iron and manganese), dissolution of GRF has the potential to both increase alkalinity and support phytoplankton growth. Therefore, it may be considered a source for large-scale marine CO₂ removal (mCDR). In this presentation we focus on its potential for supporting phytoplankton growth. We present results from incubation experiments in the field with natural phytoplankton communities and from climate-regulated laboratory experiments with a single-species phytoplankton culture. Field-incubations (6 days) with a subtropical phytoplankton community showed a significant increase in photosynthetic activity (Fv/Fm) in treatments with GRF. Similar field-experiments with natural communities from an Arctic fjord in Greenland, with a high natural background concentration of GRF, showed a modest or a neutral response to further addition of GRF. Long laboratory incubation experiments (3 weeks) with an Arctic green alga showed a significant increase in both growth rate and photosynthetic activity in treatments with GRF. The growth increased gradually with increasing concentrations of GRF until saturation was reached. This response was consistent with a simple model of trace-metal limited growth where micronutrients (e.g., iron) is biologically mobilized from GRF during the incubation period. These results show that substances in GRF, likely trace metals, can be biologically mobilized on timescales of days to weeks and thereby support growth of phytoplankton. Thus, GRF may be a source for large-scale mCDR due to its potential for increasing ocean productivity and strengthening the biological pump.