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Description
In the southern trough of the Guaymas Basin, Gulf of California, intrusion of magma occurs within thick blankets of rapidly deposited mixtures of terrigenous and biogenous sediment. Evidence from deep sea drilling cores, piston cores, and small "Alvin" push cores indicate that the circulation of hot fluids results in a very complex hydrothermal system. In this paper, special attention is given to short (20 cm) surface cores collected in areas characterized by upward advection of hydrothermal fluids, both in areas of strong advective flow as well as diffuse seepage through large bacterial mats of Beggiatoa. A wet chemical method of sediment analysis was modified in order that viable solid phase AI, Ca, Fe, K, Mg, P, Si, and Ti concentrations were generated. Visual, smear slide, coulometric, X-ray diffraction (XRD), and scanning electron microscope (SEM) observations of sediment material were used to supplement the analytical results, and comparison was made to previously obtained interstitial water data. The results of these analyses indicate that a rapid decrease in organic carbon is associated with enhanced thermal activity, in conjunction with sulfate reduction in the upper portion of these sediments. This has resulted in petroleum generation. The development of sulfide in interstitial waters, derived from sulfate reduction, creates a steady source from which the Beggiatoa mats can survive at the surface of these sediments. It also results in the presence of pyrite and greigite in the sediments. A dissolution of biogenic silica is noted at depth in most of these sediments. As the upward migration of dissolved silica encounters colder sea water near the sediment surface, cementation of diatoms by amorphous silica results. Evidence that such silica cemented areas can restrict the hydrothermal circulation of fluids through the sediments is indicated by consistent changes in interstitial water concentrations of calcium, magnesium, potassium, lithium, strontium, sulfate, and ammonia at these depths. Where the two solutions meet at 6 cm depth in one core there is mobilization of iron into adjacent sedimentary strata. Few changes with depth were evident in elemental concentrations. However, overall interpretation of the sediment data suggests that higher silica, iron, aluminum, and potassium concentrations in the sediments are present as a consequence of ongoing hydrothermal activity. Higher inorganic carbon, calcium, phosphorus, magnesium, and apatite concentrations at depth in one core indicate that hydrothermal activity in that core is of relatively recent origin, thus not allowing enough time for these concentrations to have diminished.
