Showing papers by "Noah J. Planavsky published in 2023"

Ocean alkalinity enhancement through restoration of blue carbon ecosystems

Abstract: Abstract Blue carbon ecosystems provide a wide range of ecosystem services, are critical for maintaining marine biodiversity and may potentially serve as sites of economically viable carbon dioxide removal through enhanced organic carbon storage. Here we use biogeochemical simulations to show that restoration of these marine ecosystems can also lead to permanent carbon dioxide removal by driving ocean alkalinity enhancement and atmosphere-to-ocean CO 2 fluxes. Most notably, our findings suggest that restoring mangroves, which are common in tropical shallow marine settings, will lead to notable local ocean alkalinity enhancement across a wide range of scenarios. Enhanced alkalinity production is linked to increased rates of anaerobic respiration and to increased dissolution of calcium carbonate within sediments. This work provides further motivation to pursue feasible blue carbon restoration projects and a basis for incorporating inorganic carbon removal in regulatory and economic incentivization of blue carbon ecosystem restoration.

New estimates of the storage permanence and ocean co-benefits of enhanced rock weathering

Abstract: Abstract Avoiding many of the most severe consequences of anthropogenic climate change in the coming century will very likely require the development of “negative emissions technologies”—practices that lead to net carbon dioxide removal (CDR) from Earth's atmosphere. However, feedbacks within the carbon cycle place intrinsic limits on the long-term impact of CDR on atmospheric CO2 that are likely to vary across CDR technologies in ways that are poorly constrained. Here, we use an ensemble of Earth system models to provide new insights into the efficiency of CDR through enhanced rock weathering (ERW) by explicitly quantifying long-term storage of carbon in the ocean during ERW relative to an equivalent modulated emissions scenario. We find that although the backflux of CO2 to the atmosphere in the face of CDR is in all cases significant and time-varying, even for direct removal and underground storage, the leakage of initially captured carbon associated with ERW is well below that currently assumed. In addition, net alkalinity addition to the surface ocean from ERW leads to significant increases in seawater carbonate mineral saturation state relative to an equivalent emissions trajectory, a co-benefit for calcifying marine organisms. These results suggest that potential carbon leakage from the oceans during ERW is a small component of the overall ERW life cycle and that it can be rigorously quantified and incorporated into technoeconomic assessments of ERW at scale.

Mitigation of soil nitrous oxide emissions during maize production with basalt amendments

Abstract: Nitrous oxide (N2O) is a potent and long-lived greenhouse gas that accounts for roughly 6% of global anthropogenic greenhouse gas emissions, and it has risen from its preindustrial concentration of 270 ppb N2O to 332 ppb N2O as a result of human activities. The majority of anthropogenic N2O emissions (52–80%) come from agricultural settings due to high rates of reactive nitrogen fertilizer application. Amending soils with fine-grained basalt is gaining traction as a carbon dioxide removal (CDR) pathway, and model simulations suggest that this process may also significantly decrease soil N2O emissions. Here, we continuously measure N2O fluxes from large-scale maize mesocosms in a greenhouse setting and use a machine learning framework to assess the relative importance of the levers on N2O fluxes. We observe significant decreases in cumulative N2O emissions (between 29–32%) from mesocosm systems with basalt addition. We find that basalt application rate, soil pH, and surface soil moisture are the strongest levers on N2O emissions depending on the system settings. These results provide empirical support for a potentially significant co-benefit of deploying enhanced rock weathering of silicates (ERW) on managed lands, particularly those subject to elevated rates of reactive nitrogen input.

Technical comment on “Reexamination of 2.5-Ga ‘whiff’ of oxygen interval points to anoxic ocean before GOE”

Abstract: Many lines of inorganic geochemical evidence suggest transient “whiffs” of environmental oxygenation before the Great Oxidation Event (GOE). Slotznick et al. assert that analyses of paleoredox proxies in the Mount McRae Shale, Western Australia, were misinterpreted and hence that environmental O2 levels were persistently negligible before the GOE. We find these arguments logically flawed and factually incomplete.

A new soil-based approach for empirical monitoring of enhanced rock weathering rates

Abstract: Enhanced Rock Weathering (ERW) is a promising scalable and cost-effective Carbon Dioxide Removal (CDR) strategy with significant environmental and agronomic co-benefits. A major barrier to the widescale implementation of ERW is a robust Monitoring, Reporting, and Verification (MRV) framework. To successfully quantify the amount of carbon dioxide removed by ERW at scale, MRV must be accurate, precise, and cost-effective. Here, we outline a new mass-balance-based method where metal analysis on soil samples is used to track the extent of in-situ alkaline mineral weathering. We show that signal-to-noise issues of in-situ soil analysis can be mitigated by using isotope-dilution mass spectrometry to reduce analytical error. We implement a proof of concept experiment demonstrating the method in controlled mesocosms. In our experiment, basalt feedstock is added to soil columns containing the cereal crop Sorghum bicolor at a rate equivalent to 50 t ha-1. Using our approach, we calculate an average initial CDR value of 1.47 +- 1.09 tCO2eq ha-1 from our experiments after 235 days, within error of an independent estimate calculated using conventional elemental budgeting of reaction products. Our method provides a robust time-integrated estimate of initial CDR, to feed into models that track and validate large-scale carbon removal through ERW.

Revisiting marine redox conditions during the Ediacaran Shuram carbon isotope excursion

Abstract: The Neoproterozoic carbonate record contains multiple carbon isotope anomalies, which are the subject of intense debate. The largest of these anomalies, the Shuram excursion (SE), occurred in the mid‐Ediacaran (~574–567 Ma). Accurately reconstructing marine redox landscape is a clear path toward making sense of the mechanism that drives this δ13C anomaly. Here, we report new uranium isotopic data from the shallow‐marine carbonates of the Wonoka Formation, Flinders Ranges, South Australia, where the SE is well preserved. Our data indicate that the δ238U trend during the SE is highly reproducible across globally disparate sections from different depositional settings. Previously, it was proposed that the positive shift of δ238U values during the SE suggests an extensive, near‐modern level of marine oxygenation. However, recent publications suggest that the fractionation of uranium isotopes in ferruginous and anoxic conditions is comparable, opening up the possibility of non‐unique interpretations of the carbonate uranium isotopic record. Here, we build on this idea by investigating the SE in conjunction with additional geochemical proxies. Using a revised uranium isotope mass balance model and an inverse stochastic carbon cycle model, we reevaluate models for δ13C and δ238U trends during the SE. We suggest that global seawater δ238U values during the SE could be explained by an expansion of ferruginous conditions and do not require a near‐modern level of oxygenation during the mid‐Ediacaran.

A biogeochemical model of mineral-based ocean alkalinity enhancement: impacts on the biological pump and ocean carbon uptake

Abstract: Minimizing anthropogenic climate disruption in the coming century will likely require carbon dioxide removal (CDR) from Earth’s atmosphere in addition to deep and rapid cuts to greenhouse gas emissions. Ocean alkalinity enhancement—the modification of surface ocean chemistry to drive marine uptake of atmospheric CO2—is seen as a potentially significant component of ocean-based CDR portfolios. However, there has been limited mechanistic exploration of the large-scale CDR potential of mineral-based ocean alkalinity enhancement, potential bottlenecks in alkalinity release, and the biophysical impacts of alkaline mineral feedstocks on marine ecology and the marine biological carbon pump. Here we a series of biogeochemical models to evaluate the gross CDR potential and environmental impacts of ocean alkalinity enhancement using solid mineral feedstocks. We find that natural alkalinity sources—basalt and olivine—lead to very low CDR efficiency while strongly perturbing marine food quality and fecal pellet production by marine zooplankton. Artificial alkalinity sources—the synthetic metal oxides MgO and CaO—are potentially capable of significant CDR with reduced environmental impact, but their deployment at scale faces major challenges associated with substrate limitation and process CO2 emissions during feedstock production. Taken together, our results highlight distinct challenges for ocean alkalinity enhancement as a CDR strategy and indicate that mineral-based ocean alkalinity enhancement should be pursued with caution.

Enhanced weathering in the U.S. Corn Belt delivers carbon removal with agronomic benefits

Abstract: Enhanced weathering (EW) with crushed basalt on farmlands is a promising scalable atmospheric carbon dioxide removal strategy that urgently requires performance assessment with commercial farming practices. Our large-scale replicated EW field trial in the heart of the U.S. Corn Belt shows cumulative time-integrated carbon sequestration of 15.4 +/- 4.1 t CO2 ha-1 over four years, with additional emissions mitigation of ~0.1 - 0.4 t CO2,e ha-1 yr-1 for soil nitrous oxide, a potent long-lived greenhouse gas. Maize and soybean yields increased 12-16% with EW following improved soil fertility, decreased soil acidification, and upregulation of root nutrient transport genes. Our findings suggest that widespread adoption of EW across farming sectors has the potential to contribute significantly to net-zero greenhouse gas emissions goals and global food and soil security.