modeling the global carbon cycle
balance and imbalance in biogeochemical cycles
Models of global element cycles seek to understand how biogeochemical processes and environmental conditions interact to sustain planetary habitability. However, outcomes from such models often reflect specific interpretations of geochemical archives. In this project we performed a new calculation, based in linear algebra, that identified fundamental modes of biogeochemical variability. By remaining agnostic to the relationships between environmental conditions and the intensity of biogeochemical processes, we sought to recognize and systematize patterns that underly the stability of major element cycles. We then applied the framework to synthesize existing hypotheses for Cenozoic climate change. A manuscript describing this research was recently published in PNAS (pdf).
Matrix representation of biogeochemical reactions (Ax=b). Columns of the matrix A correspond to individual reactions and rows correspond to chemical species. Entries in A reflect stoichiometric coefficients (moles/reaction), entries in x describe reaction rates (reaction/time), and entries of b are time derivatives of chemical reservoirs (moles/time).
MEANDIR: a new model for inversion of river chemistry
The dissolved chemistry of rivers has been extensively studied to elucidate physical and climatic controls of chemical weathering. Within this effort, Monte Carlo mixing models are a common tool for inverting measurements of dissolved river chemistry to distinguish contributions of weathering lithologies from other end-members. However, choices in model construction can result in unphysical or inaccurate results, and the methods underlying river inversion models are typically opaque. In this project, I constructed a MATLAB-based model that enables highly customizable Monte Carlo inversion of dissolved river chemistry. The package is called Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), and it extends former efforts by representing chemical processes as independent end-members in the inversion, including pyrite oxidation, petrogenic organic carbon oxidation, and carbon dioxide exchange between the river and atmosphere. A manuscript based on this work was published in The American Journal of Science (pdf), and you can download the code from my GitHub site. Feel free to e-mail me with any questions or problems - and remember that not all who MEANDIR are lost.
Results from MEANDIR inversion of river observations from the Mackenzie River (Horan et al., 2019). R is the fraction of carbonate weathering, Z is the fraction of weathering with sulfuric acid, and C tracks organic carbon oxidation relative to silicate and carbonate weathering. Color is the median ALK/DIC of inversion results. All samples have ALK/DIC ratios less than 2, implying that weathering upstream of the sampling site is associated with long-term increases in the atmospheric concentration of carbon dioxide.
modeling the proton and electron cycles
Some of the inputs and outputs to ocean-atmosphere dissolved inorganic carbon and alkalinity.
Geochemical processes alter the concentration of atmospheric carbon dioxide by altering ocean-atmosphere dissolved inorganic carbon and alkalinity. Note that, near modern conditions, contours of pCO2 have slopes approximately equal to 1.
A traditional view of biogeochemical cycles focuses on the movement of individual elements; for example, we refer to “the C cycle” or “the N cycle”. However, when considering the evolution of novel microbial metabolisms or the influence of seawater alkalinity on atmospheric carbon dioxide, the dominant controls are fluxes of electrons and protons. In this project, I am thus constructing a model of the major chemical reactions at earth’s surface and during early diagenesis from the perspective of protons and electrons. Through the fusion of fluvial geochemistry and chemical oceanography, this work is unraveling the impacts of weathering and sedimentary processes on planetary habitability.