Geochemical controls on silicification and biofilm stabilization in an intertidal hydrothermal spring: evidence from Puertecitos, Baja California

Abstract

Intertidal hydrothermal systems provide natural laboratories for investigating near-surface geochemical and microbial interactions. Here, we integrate field measurements (temperature, pH, and electrical conductivity), PHREEQC-based saturation index (SI) modeling, and SEM/EDS analyses to quantify how temperature, redox (pe), and seawater mixing regulate mineral precipitation and microbial preservation in the intertidal hydrothermal spring at Puertecitos, Baja California. Major-ion chemistry was measured for the two endmembers (YAPU01, YAPU16). The intermediate stations are described using in situ T–pH–EC as mixing proxies. Modeling of vent-proximal water (YAPU01; 73 °C, pe ≈ 0 and pe = 8.41) and marine-influenced rim water (YAPU16; 32.7 °C, pe = 8.41) shows persistent silica supersaturation across the natural temperature window, which strengthens upon cooling. Oxidation stabilizes Fe-oxyhydroxides at lower temperatures, whereas carbonates and gypsum reach supersaturation primarily within the cooler, marine-influenced apron. SEM/EDS observations reveal 1) abundant filamentous and bacillary morphotypes at ∼60 °C that decline to near zero by ∼46-50 °C, 2) biofilm dominance at ≤41-33 °C, and 3) silica matrices with carbon co-localized to rods and bacilli, consistent with microbial templating and progressive silicification. In contrast, Ba-free, C/N poor elongated fabrics are interpreted as nondiagnostic sinter textures. Together, these results define a process framework in which abiotic silica precipitation is pervasive but microbially templated, and additional Fe-oxide-gypsum-carbonate pathways emerge with cooling and redox evolution. This integrated geochemical-textural approach clarifies shoreline diagenesis, biosignature preservation potential, and trace element scavenging at the hydrothermal-marine interface.