Proposal
Summary:
The key hypothesis is that the pH dependence of B speciation allows B in paleosol carbonates to be used as a proxy for pore space CO2 concentrations. The greater objective of this research is to better constrain paleosol-based determinations of atmospheric CO2 using this proxy and improve understanding of climates in Earth’s past.
The method objective is to select pixels from LA-ICP-MS maps of carbonate nodules and sort them according to their luminosity under cathodoluminescence. The method presented by Drost et al. (2018) will be applied in a novel way to these nodules. The expected outcome is several sets of linear regression graphs for different luminosities in which trace element concentrations are plotted against one another. Ultimately, the objective of this method project is to be able to estimate the linear relationship between trace element concentrations based on luminosity alone.
Research Objective:
The motivation to study B in soils is that the pH dependence of B speciation likely allows B in paleosol carbonates to be used as a proxy for pore space CO2 concentrations. Such a proxy would allow the investigation of respiration rate changes across landscapes through geologic time. Most importantly, a soil CO2 proxy would greatly improve determinations of atmospheric CO2 concentrations through geologic time (as pore space CO2 is an endmember mixture of respired and atmospheric CO2), reducing the uncertainty associated with paleosol-based determinations of atmospheric CO2 to 10% of their current levels.
Drost et al. (2018) present a new approach to LA-ICP-MS U-Pb dating of carbonates based on selection and pooling of pixels from 2-D elemental and isotopic ratio maps. The authors emphasize that this approach, which uses the Monocle add-on for Iolite software, is especially useful for targeting subdomains in geologically complex samples. This method project proposes to select and pool pixels from 2-D elemental maps of carbonate nodules according to their luminosity in cathodoluminescence images of the same nodules. The luminosities of these carbonate nodules are layered and complex and the method developed by Drost may be the best way to group similar luminosities together. Once nodules have been sorted by luminosity then trace element concentrations will be regressed against one another (e.g. B vs Al, B vs Si) to plot their relationships as a function of luminosity.
This LA-ICP-Q-MS method project is interested in the following analytes: Mg, Sr, Ba, B, Mn, Al, and Si.
Significance of Project:
This method is potentially significant because it may allow pixel selection and grouping from 2-D elemental and isotopic ratio maps of carbonate nodules using luminosity data values from a separate cathodoluminescence maps of the same nodules. This method is also potentially significant because it may reveal that linear regressions of trace element compositions as a function of luminosity are the same or similar across different soil carbonate nodules in the area that they were collected from (Dance Bayou, Texas).
The analytes listed above are important because they represent the array of trace elements present in the carbonate nodules. The concentration of some of these trace elements is low (B is 1-15 ppm) so a high degree of accuracy and precision are needed to distinguish from background levels.
Literature Review:
Drost et al. (2018) present a new approach to LA-ICP-MS U-Pb dating of carbonates that is based on the selection and pooling of pixels from 2-D elemental and isotopic ratio maps. The authors use element, element ratio, and isotope ratio maps produced by LA-ICP-MS and overlay them with photomicrographs or scanning electron microscopy images to spatially link compositional data to textural and structural features. The authors sort pixels by applying appropriate selection criteria that allows them to pool the pixels into “pseudo-analyses.” Roberts et al. (2020) also discuss the growing popularity of LA-ICP-MS U-Pb geochronology as an absolute dating method for carbonate minerals and the use of spot analyses guided by petrographic and elemental imaging as a data collection method.
Chew et al. (2021) is a review paper that explores the LA-ICP-MS techniques many applications in the geosciences. In particular, the authors discuss how fine-scale processes that affect U-Pb dating of carbonates can be better constrained when combined with other textural imaging techniques such cathodoluminescence.
Sheldon and Tabor (2009) explore the rise of quantitative analysis in paleosol study as opposed to traditional, qualitative methods. In one section of the article they explore how the pH dependence of B speciation may allow B in paleosol carbonates to act as a proxy for pore space CO2. Wei et al. (2015) is an applied example of this hypothesis—their results suggest that boron isotopic compositions in soils can be a potential geochemical proxy to reconstruct the paleoenvironmental changes in loess-paleosol sequences in the Chinese Loess Plateau.
Materials and Methods:
This project proposes to use four soil carbonate nodules, which have already been prepared by setting them in indium-doped mounts and then polished. Some minor polishing work using a fine grain may be necessary to touch up the surfaces for LA-ICP-MS. Cathodoluminescence imaging of these same mounts has already been completed at the University of Oklahoma. The LA-ICP-MS time and cost of this project is based on a soil carbonate nodule thin section that was mapped in Nathan Miller’s lab. Due to the low concentration of trace elements of interest, we will probably have to use a small spot size and slow scan speed during the mapping process. It will be important to carefully process the data afterwards so that the concentrations of elements such as B can be distinguished from background levels.
To develop and test the proposed method, I will follow Drost et al. (2018) and use an existing LA-ICP-MS map of a soil carbonate nodule and its cathodoluminescence image. Once the method is refined it can then be applied to the four LA-ICP-MS maps created as part of this project.
Possible Outcomes:
This method may show that different cathodoluminescence intensities across the four carbonate nodules have similar linear regressions between trace element concentrations. The results may suggest that we could approximate B concentrations according to the luminosities alone within the Dance Bayou, simplifying and streamlining the investigation of respiration rate changes across paleo-landscapes. The results may also indicate that it is not reasonable to approximate B concentrations this way, which will still be a useful methodological outcome as we will know not to use that approach in the future.
Timeframe and Budget:
There are four small calcium carbonate nodules in indium-doped mounts that I would like to perform LA-ICP-MS on. These nodules have had cathodoluminescence imaging done to them, and I want to do LA-ICP-MS on a range of luminosities for each nodule. In a previous experience in Nathan Miller’s lab doing LA-ICP-MS with a calcium carbonate nodule, it took about one hour per square millimeter of surface ablated. We ablated four square millimeters of surface to capture the full range of luminosities. We also had to map the ablation area and pre-ablate the surface of the nodule and standards, which I will approximate to be two hours per nodule. Therefore, for this method project, I calculate the following:
Time: (1hr per square millimeter * 4 square millimeters per nodule * 4 nodules) + (2 hours prep time * 4 nodules) = 24 hours
Budget: $73 per hour * 24 hours = $1752