GENESIS OF THE AUTHIGENIC DEPOSITS ASSOCIATED WITH COLD SEEPS SITES IN GUAYMAS BASIN (GULF OF CALIFORNIA-MEXICO)

In the Guaymas Basin, located in the central part of the Gulf of California, hydrothermal activity linked to the generation of oceanic crust between the Pacific and North America plates causes venting and seepage of fluids at different temperatures through a thick layer of organic-rich sediments. This resulted in the formation of cold seeps and hot vents on the ocean floor. In this research, sedimentological, petrographic and geochemical data are integrated used to decipher the origin and conditions of precipitation of authigenic deposits from different active methane seepage sites. Authigenic deposits can be divided into three main groups: (a) rich in silica (biosiliceous packstone/wackestone rich in diatoms and composed of a fine matrix with opal-A nanospheres), (b) aragonite-dominated (composed of fibrous aragonite crystals), and (c) rich in barite (composed of platy barite crystals). They originated from variable methane advection affecting a diatomaceous ooze rich in marine organic matter (TOC: 0.38-7.19%, kerogen type II) causing pH variations and resulting in different episodes of dissolution/precipitation of silica and carbonate. The highly depleted δ¹³C values of aragonite crystals (from –48.0 to –36.5‰ V-PDB) and light δ³⁴S signature of pyrite (down to -26.7‰ V-CDT) suggest their formation via anaerobic oxidation of methane and bacterial sulfate reduction, respectively. Moreover, the size distribution of pyrite framboids (down to 2.1 µm) indicates dysoxic/anoxic conditions at seafloor during their precipitation. Metal/Ca ratios and strontium isotope values (⁸⁷Sr/⁸⁶Sr: 0.709156-0.709165) of aragonite crystals suggest they precipitated from slightly modified seawater.