Thrombolite Genesis Under Dynamic Hydrocarbon Fluid Flux: Microbial Records From New Zealand Paleo-Seeps

Distinctive associations of microcrystalline thrombolites and fibrous aragonite cements in deep-marine Miocene New Zealand hydrocarbon seep carbonates capture snapshots of microbial resilience and adaptation to changing seepage intensity driven by hydrocarbon-rich fluid fluxes. These successions record repeated fracturing, sealing, and seepage recurrences, with thrombolites occurring within fractures and vugs and as coatings on worm tubes, intergrown with multiple generations of fibrous aragonite cements. Carbonate fabrics exhibit a wide range of carbon stable isotope values, indicating mixed thermogenic–biogenic hydrocarbon sources that sustained chemosynthesis-based communities in which sulfate-driven anaerobic oxidation of methane (SD-AOM) stimulated authigenic carbonate precipitation, whereas the oxygen isotope spread is consistent with kinetically controlled precipitation. These primary isotopic values were subsequently modified by burial diagenesis. Lipid biomarker assemblages dominated by strongly 13 C-depleted archaeal and bacterial compounds, including diagnostic lipids of anaerobic methane-oxidizing archaea (ANME) and associated sulfate-reducing bacteria (SRB), together with additional 13 C-depleted lipids derived from aerobic methanotrophs, demonstrate the operation of both anaerobic and aerobic methane oxidation. Rare earth element (REE) patterns in thrombolites and associated fibrous aragonite, characterized by fabric-dependent variations in REE contents, reveal a dynamic interplay among seawater ingress, microbial elemental scavenging and organic complexation, and diagenetic modification during late-stage diagenesis. Microbial growth and mineralization produced clotted, microcrystalline carbonate with thrombolite fabrics, expressed as both stratiform mat-like structures and digitate to bulbous bodies. These fabrics likely reflect adaptation to steep redox and fluid-chemical gradients and were subsequently overprinted during burial by neomorphic recrystallization and interaction with calcium-saturated fluids, producing fabric-modifying textures. This study provides a refined record of the interplay between microbial communities, advective–diffusive fluid systems, mineral precipitation, and diagenetic overprinting in deep-sea chemosynthesis-based ecosystems.