Benthic dunite and calcite weathering as a method for ocean alkalinity enhancement

Abstract

 

The natural dissolution of mafic silicate rocks (e.g. dunite) and carbonate minerals in the marine environment increases alkalinity and draws down CO₂. Consequently, large-scale manual dispersal of such minerals has been proposed as a potential measure to alleviate rising atmospheric CO₂ levels through ocean alkalinity enhancement (OAE). This study investigates the effects of biogeochemical processes on alkaline mineral dissolution in surface sediments in a controlled experimental environment. Dunite and calcite were added to the surface of organic rich sediments from the Baltic Sea in order to simulate mineral dissolution and OAE under oxic conditions. Eight sediment cores were incubated with ~20 cm of overlying Baltic Sea bottom water over a period of 4 months; three replicates were treated with calcite, three with dunite, and two served as unamended controls.

First results indicate that the addition of the two materials directly increased benthic fluxes of alkalinity (from 1.3 to 2.5 µmol/cm²/d) and other respective weathering products such as calcium and silicate compared to the control experiments. These enhanced fluxes vanished into the strong natural benthic background after ~4 weeks. The main driver for enhanced and natural weathering is undersaturation with respect to the dissolving minerals which appears to be governed by microbial activity.

As the experiment progressed, porewater pH profiles in sediment cores where the sulfur oxidizing bacteria Beggiatoa spp. were visible shifted towards profiles that were more characteristic of sediments displaying cable bacteria activity. Very low pH values (~5.6) produced by presumably cable bacteria at ~1-3 cm depth in the sediment led to strong calcium carbonate dissolution. Additionally, their metabolism provides alkalinity to the bottom water by the formation of water directly from oxygen and protons, hence without addition of corresponding cations. This microbial activity produced high pH values in the upper millimeters of the sediments (~8.5) leading to Ω_calcite values >15 that might promote CaCO₃ precipitation. Enhanced dunite weathering is indicated by slightly enhanced sedimentary Si fluxes, although this proved difficult to discern from the natural background flux arising from biogenic opal dissolution.

The overall natural complexity of the sediment chemistry combined with the alteration of the sediments during the incubation complicate a clear disentangling of natural and enhanced mineral weathering. Further investigation of these sedimentary systems along with field experiments will be necessary to provide estimates on the feasibility of benthic weathering as a realistic OAE and climate change mitigation measure.