Spatial Decoupling of Pyrite Maturation From Seepage Intensity: Synergistic Control by Benthic Bio‐Irrigation in Cold Seeps

Abstract

Authigenic pyrite, a characteristic mineral product of the anaerobic oxidation of methane coupled with sulfate reduction (AOM‐SR), serves as a key indicator for tracing cold seep dynamics. However, the traditional “flux‐only” paradigm largely overlooks the profound impact of benthic macrofauna on mineralogical maturation. Here, we integrated gas isotope geochemistry, micro‐mineralogy, and AMS ¹⁴ C dating to investigate the Wenhai I cold seep in the South China Sea across distinct benthic ecological zones. Results revealed a stable, microbial‐dominated methane seepage history exceeding 22 kyr, establishing a pronounced spatial concentration gradient from the active vent to the distal background. We identified a complete, kinetically driven pyrite structural sequence evolving from rapid‐nucleating framboids to mature euhedral crystals. Crucially, pyrite abundance and morphological maturation are spatially decoupled from the absolute methane concentration gradient. In dense chemosynthetic zones, intense bio‐irrigation by benthic “ecosystem engineers” (e.g., mussels and tubeworms) induces high‐frequency redox instability, effectively arresting pyrite growth at the early framboidal stage. Conversely, the physically undisturbed background zone acts as a stable “diagenetic incubator,” fostering millennial‐scale continuous secondary overgrowths into massive euhedral pyrite. Ultimately, we establish a tripartite coupling model of fluid seepage, benthic communities, and pyritization. These findings highlight authigenic pyrite morphology as a high‐fidelity biogeochemical proxy for reconstructing both paleo‐seepage intensity and the long‐term ecological stability of global cold seep systems.