Design and Construction of an Experimental Setup to Enhance Mineral Weathering through the Activity of Soil Organisms

Enhanced weathering (EW) is an emerging carbon dioxide (CO2) removal technology that can contribute to climate change mitigation. This technology relies ...

University of Hamburg

Terrestrial enhanced weathering, the spreading of ultramafic silicate rock flour to enhance natural weathering rates, has been suggested as part of a strategy to reduce global atmospheric CO2 levels. We budget potential CO2 sequestration against associated CO2 emissions to assess the net CO2 removal of terrestrial enhanced weathering. We combine global spatial data sets of potential source rocks, transport networks, and application areas with associated CO2 emissions in optimistic and pessimistic scenarios. The results show that the choice of source rocks and material comminution technique dominate the CO2 efficiency of enhanced weathering. CO2 emissions from transport amount to on average 0.5-3% of potentially sequestered CO2. The emissions of material mining and application are negligible. After accounting for all emissions, 0.5-1.0 t CO2 can be sequestered on average per tonne of rock, translating into a unit cost from 1.6 to 9.9 GJ per tonne CO2 sequestered by enhanced weathering. However, to control or reduce atmospheric CO2 concentrations substantially with enhanced weathering would require very large amounts of rock. Before enhanced weathering could be applied on large scales, more research is needed to assess weathering rates, potential side effects, social acceptability, and mechanisms of governance.

Carbon dioxide efficiency of terrestrial enhanced weathering.

Plate tectonics, carbon, and climate.

Calcium (Ca) is an essential element for almost all living organisms. Here, we examined global variation and controls of freshwater Ca concentrations, using 440 599 water samples from 43 184 inland water sites in 57 countries. We found that the global median Ca concentration was 4.0 mg L with 20.7% of the water samples showing Ca concentrations ≤ 1.5 mg L, a threshold considered critical for the survival of many Ca-demanding organisms. Spatially, freshwater Ca concentrations were strongly and proportionally linked to carbonate alkalinity, with the highest Ca and carbonate alkalinity in waters with a pH around 8.0 and decreasing in concentrations towards lower pH. However, on a temporal scale, by analyzing decadal trends in >200 water bodies since the 1980s, we observed a frequent decoupling between carbonate alkalinity and Ca concentrations, which we attributed mainly to the influence of anthropogenic acid deposition. As acid deposition has been ameliorated, in many freshwaters carbonate alkalinity concentrations have increased or remained constant, while Ca concentrations have rapidly declined towards or even below pre-industrial conditions as a consequence of recovery from anthropogenic acidification. Thus, a paradoxical outcome of the successful remediation of acid deposition is a globally widespread freshwater Ca concentration decline towards critically low levels for many aquatic organisms.

Widespread diminishing anthropogenic effects on calcium in freshwaters.

Resolving regional carbon budgets is critical for informing land-based mitigation policy. For nine regions covering nearly the whole globe, we collected inventory estimates of carbon-stock changes complemented by satellite estimates of biomass changes where inventory data are missing. The net land-atmospheric carbon exchange (NEE) was calculated by taking the sum of the carbon-stock change and lateral carbon fluxes from crop and wood trade, and riverine-carbon export to the ocean. Summing up NEE from all regions, we obtained a global 'bottom-up' NEE for net land anthropogenic CO uptake of -2.2 ± 0.6 PgC yr consistent with the independent top-down NEE from the global atmospheric carbon budget during 2000-2009. This estimate is so far the most comprehensive global bottom-up carbon budget accounting, which set up an important milestone for global carbon-cycle studies. By decomposing NEE into component fluxes, we found that global soil heterotrophic respiration amounts to a source of CO of 39 PgC yr with an interquartile of 33-46 PgC yr-a much smaller portion of net primary productivity than previously reported.

Empirical estimates of regional carbon budgets imply reduced global soil heterotrophic respiration.

A number of negative emission technologies (NETs) have been proposed to actively remove CO from the atmosphere, with enhanced silicate weathering (ESW) as a relatively new NET with considerable climate change mitigation potential. Models calibrated to ESW rates in lab experiments estimate the global potential for inorganic carbon sequestration by ESW at about 0.5-5 Gt CO  year , suggesting ESW could be an important component of the future NETs mix. In real soils, however, weathering rates may differ strongly from lab conditions. Research on natural weathering has shown that biota such as plants, microbes, and macro-invertebrates can strongly affect weathering rates, but biotic effects were excluded from most ESW lab assessments. Moreover, ESW may alter soil organic carbon sequestration and greenhouse gas emissions by influencing physicochemical and biological processes, which holds the potential to perpetuate even larger negative emissions. Here, we argue that it is likely that the climate change mitigation effect of ESW will be governed by biological processes, emphasizing the need to put these processes on the agenda of this emerging research field.

Is the climate change mitigation effect of enhanced silicate weathering governed by biological processes?

Anthropogenic perturbations are increasing uncertainties in estimating CO emissions via air-water CO flux (F) from large rivers of the Indian subcontinent. This study aimed to provide an improved estimate of the total F from the subcontinental rivers by combining calculations of the partial pressure of CO (pCO) in eight major rivers with new measurements in the Ganges and Godavari. The average pCO in the two newly surveyed rivers, including tributaries, wastewater drains, and impoundments, were 3-6 times greater than the previously reported values. In some highly polluted urban tributaries and middle reaches of the Ganges that drain metropolitan areas, the measured pCO exceeded 20,000 μatm, ~40 times the background levels of the headwaters originating in the carbonate-rich Himalayas. The high pCO above 28,000 μatm in the lower reach of the Godavari was seven times the moderate levels of pCO in the headwaters of the volcanic Deccan Traps, indicating enhanced CO production in soils and anthropogenic sources under favorable conditions for organic matter degradation. Across the northern rivers, pCO exhibited a significant negative relationship with dissolved oxygen, but a positive relationship with inorganic N or P concentrations. The strong influence of water pollution on riverine pCO suggests that CO emissions from hypoxic, eutrophic reaches can greatly exceed phytoplanktonic CO uptake. Spatially resolved pCO data, combined with three gas transfer velocity estimates, provided a higher range of F from the subcontinental rivers (100.9-130.2 Tg CO yr) than the previous estimates (7.5-61.2 Tg CO yr). The revised estimates representing 2-5% of the global riverine F illustrate the importance of the Indian subcontinental rivers under increasing anthropogenic pressures in constraining global inland waters F.

Jens Hartmann

Institute for Geology,
Center for Earth System Research and Sustainability,
Institute for Geology, Center for Earth System Research and Sustainability

University of Hamburg

Design and Construction of an Experimental Setup to Enhance Mineral Weathering through the Activity of Soil Organisms

Jens Hartmann

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Institute for Geology,Center for Earth System Research and Sustainability,Institute for Geology, Center for Earth System Research and Sustainability

University of Hamburg

Design and Construction of an Experimental Setup to Enhance Mineral Weathering through the Activity of Soil Organisms

Institute for Geology,
Center for Earth System Research and Sustainability,
Institute for Geology, Center for Earth System Research and Sustainability