A groundbreaking study recently published in Nature Geoscience challenges longstanding assumptions about pyrite burial in modern marine sediments, revealing that sedimentary conditions drive current pyrite burial fluxes to surpass pyrite oxidation rates. This innovative research from Mertens, Paradis, and Hemingway pioneers the modeling of pyrite content and isotopic compositions within sedimentary environments, employing a comprehensive advection–diffusion–reaction framework that incorporates key chemical and physical processes governing sulfur cycling.
By developing a one-dimensional reaction-transport model, the researchers meticulously characterize the interactive dynamics of organic carbon degradation, reactive iron availability, sulfate reduction, and sulfide oxidation as they relate to pyrite formation. The model uniquely assumes steady state conditions, negligible sediment compaction, and the predominance of microbial sulfate reduction (MSR) as the principal isotope-fractionating process. Importantly, this modeling effort integrates Monod kinetics to capture the non-linear dependencies of sulfate reduction and sulfide oxidation rates on substrate concentrations, enriching the model’s predictive capability.
The modeling approach begins by defining organic carbon and reactive iron content depth profiles through empirically derived power-law decay functions, which describe the attenuation of these reactive substrates within the sulfidic anoxic zone of sediments. These expressions incorporate parameters such as sedimentation rate and initial content, offering a flexible yet robust description of sediment geochemistry. The team’s formulation underlines the significance of reactive continuum theory in accurately simulating the temporal and spatial variability of reactive substrate pools essential for sulfur cycling.
Sulfate and sulfide transport and transformations within the sediment are modeled using coupled partial differential equations that incorporate diffusion, advection by sediment burial, and biogeochemical reactions. Sulfate consumption via microbial sulfate reduction is balanced against its replenishment through diffusion, whereas sulfide dynamics involve production, consumption via reaction with reactive iron to form pyrite precursors, and transport. The application of Monod kinetics to both sulfate reduction and sulfide oxidation captures the saturable nature of microbial metabolism and chemical reactions, which profoundly influences sulfur speciation and isotopic signatures.
A novel aspect of this work is the explicit tracking of sulfur isotopologues (32S and 34S) throughout the modeled processes, enabling depth-resolved predictions of sulfur isotope fractionation between sulfate, sulfide, and pyrite. The model accounts for thermodynamically constrained isotope equilibrium during microbial sulfate reduction and assumes negligible isotope effects from intermediate sulfur species or alternative reduction pathways. This isotopic fidelity allows for robust comparisons with sedimentary pyrite δ34S measurements, shedding light on the sulfur cycle’s isotopic imprint in contemporary sediments.
Transitioning from dimensional to non-dimensional formulations, the researchers streamline the governing equations to reveal critical scaling relationships and dimensionless groups controlling pyrite formation. By introducing non-dimensional depth variables and concentration scales, the model elegantly captures the interplay of reaction rates and transport phenomena across sediment profiles, facilitating exhaustive parameter sensitivity analyses and global predictions. This mathematical distillation advances the theoretical understanding of sedimentary sulfur chemistry while maintaining applicability to diverse marine environments.
Validation of the model’s predictive prowess was established through an extensive compilation of 216 sedimentary profiles worldwide, encompassing sulfur cycle content and isotopic data. The model demonstrated remarkable agreement with observed organic carbon and reactive iron contents, sulfate and sulfide concentration gradients, and pyrite isotopic compositions across shelf, slope, and abyssal settings. Such global calibration underlines the model’s capacity to capture spatial heterogeneity in sedimentary sulfur cycling induced by variable organic matter supply and sedimentation regimes.
One of the study’s most striking findings is that pyrite burial fluxes in modern marine sediments frequently exceed the rates of pyrite oxidation, overturning the conventional paradigm that oxidation dominantly limits pyrite preservation. This disparity emerges from sedimentary conditions characterized by abundant reactive iron and organic carbon, coupled with efficient burial processes that mitigate exposure to oxic conditions. The model predicts that these factors synergistically foster enhanced pyrite accumulation, with implications for sulfur cycling, sediment diagenesis, and global biogeochemical feedbacks.
The researchers carefully delineate the pyrite formation zone within sediments by calculating the maximal depth wherein pyrite accumulates above a defined threshold, ensuring physiochemical relevance. Sensitivity tests verify that results remain consistent across plausible parameter ranges for organic carbon content, sedimentation rate, porosity, radiogenic isotope inputs, and diffusivities. Notably, pyrite burial flux and isotopic composition show robustness against variations in the depth limit used to define the formation zone, affirming the model’s reliability.
To quantify global pyrite burial, the team integrates modeled pyrite contents over sediment depth and spatial grids, converting non-dimensional outputs into dimensional mole fluxes. This synthesis, when combined with crust-wide sediment and ocean parameters, yields estimates of pyrite burial at unprecedented geographical resolution. Complementing this, weighted averages of pyrite δ34S isotopic signatures were computed, exposing coherent regional and global patterns reflective of prevailing sedimentary and microbial environments.
Beyond the modern ocean, the study offers valuable insights into Earth’s sulfur isotope history by compiling and analyzing sedimentary pyrite and seawater sulfate δ34S records spanning the Phanerozoic eon. Employing rigorous data smoothing and bootstrap resampling techniques to mitigate noise and sampling biases ensures temporal trends in Δpyrite (the isotopic offset between sulfate and pyrite) are robustly characterized. These historical perspectives contextualize modern observations within a broader geochemical framework, linking sedimentary processes to global biogeochemical cycles.
The meticulous inclusion of isotopic fractionation factors anchored in thermodynamic equilibrium and microbial metabolic rates lends confidence in the inferred sulfur isotope trends. By coupling isotopic and concentration data, this research advances understanding of how biological, chemical, and physical controls have evolved across geological timescales and how they manifest in sedimentary sulfur reservoirs today.
This investigation also critically evaluates the roles of sediment physical parameters such as porosity and sedimentation rate, demonstrating their interconnected influence on organic matter reactivity and reactive iron availability. Adjustments to these parameters within measured uncertainty bounds show limited impacts on pyrite burial rates and isotopic composition, emphasizing that key geochemical controls override sedimentological variability in regulating sulfur cycling dynamics.
Importantly, the assumptions underlying the model, including the negligible role of bioturbation in anoxic sulfidic zones and the steady-state approximation, are well justified through previous empirical studies and sensitivity analyses. These assumptions enhance model tractability without compromising its ability to reproduce observed sedimentary sulfur systematics and offer a foundation for future investigations incorporating transient dynamics or additional sulfur cycling pathways.
The implications of this research are profound, redefining our understanding of pyrite burial’s role as a key sink in the marine sulfur cycle and its feedbacks to ocean redox conditions, primary productivity, and atmospheric oxygen regulation. By elucidating the conditions that enable pyrite accumulation to outpace oxidation, this work provides valuable constraints for paleoenvironmental reconstructions and models of Earth’s redox history.
In summary, the model crafted by Mertens, Paradis, and Hemingway represents a significant leap forward in marine geochemistry, bridging detailed sediment porewater chemistry, isotopic fractionation theory, and global scale environmental modeling. Its insights into the coupling between sedimentary conditions and sulfur cycling herald a new era of nuanced understanding of pyrite burial, sediment diagenesis, and their broader Earth system implications.
As sediment geochemistry continues to evolve with integrated observational and modeling approaches, studies like this will be critical in untangling the complex interplay of microbial, chemical, and physical processes that regulate elemental cycles. The availability of fully documented MATLAB code and extensive datasets further facilitates replication and extension by the broader scientific community, ensuring the enduring impact of this pioneering work.
Subject of Research:
Marine sediment biogeochemistry and sulfur cycling, focusing on pyrite formation and isotopic fractionation.
Article Title:
Sedimentary conditions drive modern pyrite burial flux to exceed oxidation
Article References:
Mertens, C., Paradis, S. & Hemingway, J.D. Sedimentary conditions drive modern pyrite burial flux to exceed oxidation. Nature Geoscience (2025). https://doi.org/10.1038/s41561-025-01855-5
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