Carbon sequestration by enhanced silicate weathering in agricultural soils

Overview: Amending agricultural soils with powdered silicate rocks (“rock dust”) is an emerging carbon dioxide (CO2) removal technology with reported co-benefits to farmers in the form of improved soil quality and increased crop yield. Rock dust increases the silicate mineral surface area available for chemical weathering, enhancing a well-known process for capturing CO2 that typically occurs on geologic timescales. This Gateway project will contribute research towards a three-year agricultural field trial beginning at Carleton College to test the efficacy of rock dust amendments with a focus on evaluating carbon sequestration, crop yields, and soil health.

Student projects will be focused into three project modules. In the first module, will focus on how CO2 is sequestered during silicate weathering. Students will learn the geologic foundations for this important negative feedback on climate, study how applications of enhanced weathering aim to increase rates of weathering, and contribute to the research of how weathering and carbon sequestration can be constrained by measurements in our field trials. For the second module, students will work to characterize basaltic rock powder using x-ray diffraction (XRD), scanning electron microscopy (SEM), and data for grain size distribution and elemental composition. Rock powders for this project are being sourced from nearby Dresser Quarry (~80 miles from Carleton), and this project will include a field trip to the quarry (pending COVID-19 restrictions) and surrounding areas to study the geologic context. The last research module will focus on carbon emissions and soil conservation in agriculture. In this module, students will learn about agricultural soils, land management practices, and ways to reduce carbon emissions associated with agriculture. This module will include field work and soil sampling/description along with meetings with local farmers to hear about their experiences in agriculture.

The research in this project will be augmented with programming to build students’ knowledge in foundational geology including meetings with current and former geology majors with a range of interdisciplinary interests. Together, this Gateway experience will provide students with a strong foundational understanding of geology and an appreciation for how geologists approach asking and addressing pressing questions.

When: July 5 – August 6, 2021

Where: The hope is that this project will take place on campus at Carleton College in Northfield, Minnesota. Given the current state of the COVID-19 pandemic, it is anticipated that at least some portions of this work will need to be conducted virtually via Zoom. Carleton College policy will dictate the exact details of student engagement online vs. in person. There will be sufficient data pre-collected and additional programming to support an entirely remote experience, if that is required.

Who: Four rising sophomores and project leader Dr. Daniel Maxbauer ([email protected]), Carleton College

Prerequisites and Recommended Courses:

  1. There are no prerequisite courses required. An introductory geology, biology, or chemistry course is recommended.
  2. Applicants should be interested and excited to learn about topics related to soil carbon sequestration, soil chemistry and mineralogy, and field and laboratory methods related to soil science.

Expectations and Obligations:

    1. Excitement and willingness to participate fully in an authentic research experience (either remotely or in person).
    2. Ability and interest to work collaboratively with peers and project leaders to foster community and connections with project team.


Silicate rock dusts were recognized decades ago as a natural source of slow-release nutrients with positive effects on soil rejuvenation, fertility, and health (Gillman, 1980; see review by van Straaten, 2006). Recently, attention has turned towards the value of “enhanced weathering” – applying powdered rock dust to soils to increase the rate of chemical weathering reactions that sequester atmospheric CO2 (see Hartmann et al., 2013; Moosdorf et al., 2014; Kantola et al., 2017; Beerling et al., 2018; Andrews and Taylor, 2019; Beerling et al., 2020). These studies are anchored in theory or models that emphasize the potential of enhanced weathering to help the world simultaneously feed a growing world population, conserve and improve soil health for future generations, and reduce CO2 emissions to limit climate change (Beerling et al., 2020). However, little data is available from field trials of enhanced weathering to test these claims (see Haque et al., 2020a for perhaps the only example in the literature) and most available data come from controlled experiments using soil microcosms (Haque et al., 2019; Haque et al., 2020b; Amann et al., 2020; Kellend et al., 2020).

Student modules, described below, are components of a larger project beginning at Carleton College in the spring of 2021 to initiate an agricultural field trial of enhanced silicate weathering (ESW) for carbon sequestration in agricultural systems. These field trials will be located on land owned by Carleton College located just a short 10-minute walk from the Geology Department in Anderson Hall where Gateway students will conduct their on-campus work (see Figure 1). Due to difficulty in field access with a standing corn crop during July, all soil sampling and instrument installation for this study will need to be complete prior to the Gateway project (see Figure 2 for description of data being collected for field trials). However, there remains a rich set of opportunities for contributions to this research. Below, relevant components of the larger project are described within each module, along with examples of student work in each area. It is anticipated that students will work in pairs or as a whole group in rotation on each module. Discussion and presentation for paired work will facilitate further community and development throughout the project. 

Figure 1. Map of study area and experimental design. Field trial location outlined in yellow, with respect to Carleton College’s main academic campus and Peterson Farms, the cooperating farmer for the field trials. Lower panel describes the proposed experimental design for the study. A randomized block design will be utilized to distribute treatment and control plots (shown in variable shades of grey with application rates) across the study area. Approximate locations of lysimeters (shallow water sampling), soil collars (soil respiration monitoring sites), drain gauges (deep water drainage), and soil sensors (temperature, moisture, and electrical conductivity) are shown with colored symbols. An example soil sampling grid is provided in the high application treatment plot in Block 1. 

Figure 2. Overview of measurements for ESW field trials. Students in the Gateway program will be involved with and learn about portions of this larger project as described in the project modules.


Module 1: CO2 sequestration through silicate weathering

Silicate weathering is one of the most fundamental negative feedbacks on Earth’s climate via modulation of the amount of CO2 in Earth’s atmosphere (Kump et al., 2000; Penman et al., 2020). Silicate mineral dissolution consumes CO2 according to the general expression below using Wollastonite (a calcium silicate mineral; CaSiO3):

CaSiO3 + 2CO2 + H2O -> Ca2+ + 2HCO3 + SiO2

Bicarbonate (HCO3) and calcium (Ca2+) ions produced by this reaction on geologic times scales (>105 years) are transported to the oceans where they ultimately are precipitated as calcium carbonate minerals (CaCO3):

Ca2+ +2HCO3 -> CaCO3 + H2O + CO2

Notably, CO2 is produced during CaCO3 precipitation. However, the summation of the previous two reactions yields a net reaction that consumes one mole of CO2 for each mole of CaCO3 produced, as shown below:

CaSiO3 + CO2 -> CaCO3 + SiO2

Utilizing these same principles, applications of enhanced weathering aim to increase the rate of chemical weathering by applying fine-grained silicate rock powders to agricultural soils. On human timescales (<103 years) HCO3 drainage to local watersheds and CaCO3 precipitation in soil (as opposed to the oceans) represent long-term sinks for carbon sequestration (Hartmann et al., 2013; Taylor et al., 2016).

Student Projects and Activities:

  • Soil water sampling from field lysimeters (see Figure 1) and measuring total alkalinity (HCO3) via titration in the laboratory.
  • Conducting loss-on-ignition (LOI) analysis to measure total inorganic carbon (CaCO3) in soil core samples (previously collected) to establish important baseline data for field trials.
  • Student manipulation and analysis of data for HCO3 and CaCO3 abundances to quantify silicate mineral weathering.
  • Case study activities of silicate weathering examples in the geologic record (for example, studying carbon cycle recovery during the Paleocene-Eocene Thermal Maximum).
  • Note: data will be pre-collected for each of these activities in the case that this module needs to be conducted online.

Module 2: Characterization of basaltic rock dust for enhanced weathering

The chemistry and mineralogy of silicate rock powders are important considerations for applications of enhanced weathering. For instance, the efficiency of ESW to capture and sequester CO2 relies primarily on the rate of silicate weathering which varies with mineralogy. In addition, elemental composition of silicate rocks will dictate which mineral nutrients are available to crops (e.g., Ca, Mg, K, P, Zn) and whether potentially toxic elements (Cr, Ni) are a possible environmental hazard. To illustrate these trade-offs, consider that ultramafic rocks rich in olivine (readily dissolves in soil environments) have the greatest potential for carbon sequestration (via rapid rates of silicate weathering) but also pose the greatest environmental risk due to relatively high concentrations of Cr and Ni (Hartmann et al., 2013; Renforth et al., 2015; Beerling et al., 2018). Instead, mafic basaltic rocks (lower proportions of both olivine and toxic elements) are suggested as a preferred target for ESW applications (Olsson et al., 2012; Beerling et al., 2018).

This module will focus on material characterization of basaltic rock powders to be utilized in ESW field trials at Carleton. Material is being sourced from the Dresser Quarry in Dresser, Wisconsin (just 80 miles from Carleton College) where Mesoproterozoic basaltic lava flows of the Keweenawan Supergroup (Runkel and Boerboom, 2010) are actively quarried for aggregate production. The rock powders utilized in this study represent a waste product of existing operations.  We will collect mineralogical (XRD) and bulk elemental (ICP-OES) data on basaltic rock powders from Dresser Quarry that will serve as important baseline data for this study. In addition, students in this project will explore using scanning electron microscopy (SEM) to provide further constraints mineralogy, elemental composition, and grain size.

Student Projects and Activities:

  • Learn about different rock and mineral types along with their physical and chemical properties
  • Analyze basaltic rock powders using XRD and SEM analyses (both with instrumentation available at Carleton College)
  • Relate information from XRD and SEM analyses with elemental composition data (analysis to be conducted prior to the Gateway program) to determine how mineral and elemental composition relate to enhanced weathering (for example, differences in mineral weathering rates, nutrient availability as minerals break down, presence of possible toxic elements)
  • Field trip to Dresser Quarry and the nearby Interstate State Park at Taylor’s Falls, Minnesota to study the geology of the Keweenawan Supergroup basalts (COVID pending).
  • Note: some data will be pre-collected for each of these activities in the case that this module needs to be conducted online.

Module 3: Carbon emissions and soil conservation in agriculture

ESW applications in agriculture represent an opportunity to improve soil quality and crop yields while increasing carbon sequestration (Beerling et al., 2020). However, there are a variety of strategies for improving soil conservation and managing carbon emissions in agriculture (Ciborowski, 2019). Farmers are faced with many competing interests in their decisions on land management. This module will focus on introducing students to ways that geologists and soil scientists study agricultural systems and how farmers utilize information from scientists to help guide their decision making process.

The field trials described in this project are being managed in cooperation with Mike Peterson, who has nearly 40 years of experience in agriculture and has a longstanding relationship with Carleton College. In addition, Peterson is an active member of the agricultural community in the Northield area and was recently awarded the Rice County Conservationist of the Year award in 2020. Peterson, along with at least two other local farmers, will serve as a valuable source of information for students in this project to learn form.

Student Projects and Activities:

  • Soil coring and description in agricultural fields. Activity intended to teach students field methods in soil science and expose them to some of the strategies used to collect soil samples in this study.
  • Conduct survey measurements of soil respiration in agricultural fields. This will be a routine measurement made as a component of the ESW field trial. Students will learn about soil respiration in a morning module that includes field work to measure soil respiration in control and treatment plots (soil collar locations in Figure 1).
  • Farm visits to local area farms. Pending COVID-19 restrictions, we aim to visit at least three area farm operations to provide students an opportunity to hear from local farmers about what they consider in their land management decisions. All three local farmers are conservation minded and have recently implemented complementary soil conservation practices including minimal-tillage and cover crops.

Note: soil respiration data will be available for analysis for students if this module needs to be remote. Best efforts will be made to conduct Zoom interviews with farmers for students in the case that field visits are not possible.

Additional opportunities to foster student learning

Within each project module, it is anticipated that Gateway students will have limited background information to support their research work in these areas. Each module will be associated with lecture, discussion, and classroom activity designed to build foundational understanding in each area prior to tackling more advanced work related to this research. For example, prior to the characterization of the basalt rock powders there will be lecture and hands-on activities building student understanding of different rock types, of the mineral composition of various igneous rocks, and an overview of the fundamentals behind methods researchers use to characterize the mineral and chemical compositions of rocks. The aim of this scaffolding will be to provide students skills and understanding to contribute towards project research – but also to provide some foundational geology for Gateway students that they can take with them beyond any project specific applications.

Additional programming during this Gateway project will be aimed to provide students with an introduction to the field of geology, broadly, and to the types of career paths you can take in the geosciences. If possible, collaborating on these aspects of the program with other Gateway projects through virtual presentation and meetings would broaden this experience and build community within the Keck program.

Considerations for a remote project

While this project works best if students are able to participate in person, it is understood that there is a very real possibility this work will need to be completed remotely via Zoom. Given this Gateway project is part of a larger field project, we anticipate being able to have data pre-collected for each module described here. This will allow for students to work with data for each module that is directly related to the project. In addition, we will provide much the same background and supporting content virtually as we would have done in person. Lessons learned from teaching online will help to guide a plan for this project to build community and encourage participation.

Learning Goals

  1. Build a foundational understanding in geology through hands-on experiential learning. By the end of the project student should feel comfortable making observations in the field or about a data set and generating testable ideas.
  2. Prepare a professional scientific abstract. Students in this project will be involved in organizing module information into an abstract and poster for submission to the American Geophysical Union (AGU) annual meeting
  3. Communicate scientific information in formal and informal presentations along, short writing assignments (abstracts), and through figures/graphics.
  4. Develop an awareness for geology as a field of study and as a pathway for future career options. Collaboration with other Gateway projects and guest-presentations via Zoom from recent alums and current geology majors will help augment this learning goal.

Project Logistics

Students will be offered room and board at Carleton College during in person work associated with this project. The field trials described by this research are in an agricultural field owned by the college located just East of the campus recreational center (see Figure 1). Details of in person activity and housing will be governed by the current college policy COVID-19 restrictions that are in place for summer 2021. Most field work and activities described in this proposal will occur at the field trial site or the campus Arboretum, both within easy walking distance of the main campus. Any off-campus field trips will utilize college vehicles, pending COVID-19 restrictions.

Safety: Students will be expected to follow all laboratory and field safety protocols that are in place for on-campus work at Carleton College. Maxbauer has extensive experience in leading field trips and managing safety protocols with students in the areas surrounding campus as a part of the curriculum in the Geology Department. These include, but are not limited to, wearing appropriate footwear, high-vis safety vests, hard hats, gloves, and safety glasses when in the field or lab. Daily safety procedures will be discussed each morning prior to work for the day.

 Additional health and safety guidelines related to the COVID-19 pandemic will be developed given current circumstances in July and will be consistent with Carleton College’s COVID-19 protocols (based on recommendations from the MNDH and CDC).


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