Leg 2  

(Click here for more on precruise efforts.)

Leg Summary:

Leg 2 will take place in the southern trough of the Guaymas Basin to examine and compare the characteristics of focused, high-temperature hydrothermal vents, and diffuse, low-temperature vents. The research will address: 1) microbial distributions and the growth of vent chimneys at the high-temperature sites; 2) metal and microbial distributions within the sediments; and 3) the relationships between microbial mats, diffusive venting, and petroleum products. The ROV will be used to survey the dive sites, and to collect samples of sediments, vent chimneys, and water from the bottom. The ROV will also deploy a variety of instruments to measure the temperatures and chemical characteristics of the different thermal regions (the Guaymas Instrumented Chimney Experiment). Instrument packages will be left on the seafloor during leg 2 to monitor chimney growth, and will be recovered during leg 6. The leg 2 coordinator is Dr. Debra Stakes from MBARI, and the Mexican collaborator is Alejandro Ortega Osorio from the Instituto Mexicano del Petroleo.

History & Purpose:

The presence of sediment cover has a profound effect on hydrothermal circulation at oceanic rift zones. The interaction of high-temperature fluids and sediment results in extensive subsurface alteration, vent fluids and mineral deposits with distinctive mineralogy and chemistry, and the rapid pyrolysis of organic matter to form petroleum. In addition, extensive bacterial mats mark areas of diffuse, low-temperature fluid discharge. Guaymas Basin is the premier site for process-oriented, integrated investigations of hydrothermal systems within a sedimented ridge environment.  

The goal of this project is to systematically characterize hydrothermal and microbial processes and derived products in the Southern Trough of the Guaymas Basin, Gulf of California. Studies will be conducted at multiple scales and encompass both high- and low-temperature vent sites. Site-specific investigations will focus on chimney growth. The Guaymas Instrumented Chimney Experiment will trace the evolution of the thermal/chemical/physical environment within newly formed chimney walls over time-scales of minutes to months, and determine the distribution of microorganisms within this temporal/chemical/thermal/spatial framework. For this experiment, three high-temperature vent sites will be selected in an area called “Rebecca’s Roost” (see below).

This is a 53-image color photomosaic of the hydrothermal structure called Rebecca's Roost in the Southern Trough of the Guaymas Basin. This structure is the primary dive target for Leg 2. Mosaic constructed by images collected in 1998 and compiled by Hanumant Singh, Dana Yoerger of Woods Hole Oceanographic Institution. (Click image for larger file.) 

The small summit chimneys will be removed and a ring containing an array of thermocouples will be placed over the vent orifice. Because the Guaymas chimneys are composed of rapidly growing sulfates and carbonates, we anticipate that these summit chimneys will grow through and engulf the thermocouples within 24-48 hours. The changing temperatures within the growing chimney wall will be constantly recorded on a datalogger that will be downloaded on the seafloor through an inductively coupled link from the ROV.

The chimneys will be subsequently removed and divided between detailed microbial, mineralogical and geochemical analyses. All three arrays will be left at the end of Leg 2 to be collected during the last dive of Leg 6. In addition to the three thermocouple arrays, we will also leave four “HOBOS” which will continuously measure fluid temperatures at other chimney sites. Three Osmosamplers will also be deployed with the HOBOS. The Osmosamplers use an osmotic pump to continuously “sip” hydrothermal fluid into a coil of tubing to provide a time series of fluid chemistry over the two-month period.

Additional work will involve mapping, surveying, and coring the sea floor between high-temperature discharge points. The scientists hope to determine the distribution of microorganisms, especially biofilms and bacterial mats on mineral and sediment substrates, surface and subsurface hydrothermal mineralization, mineral-deposit degradation (oxidation), and distribution of organic matter and hydrothermal petroleum.

Following the cruise, the scientists will conduct additional analyses of hydrothermal deposits to evaluate (1) the distribution and characterization of trace and precious metals (e.g., As, Hg, Au) in sulfide- carbonate-, and sulfate-rich facies of mounds and chimneys, and (2) the nature of hydrothermal deposits and related alteration in the shallow subsurface.

Fluid chemistry data on the abundance of H2, CO2, CH4, and aqueous organic compounds will allow scientists to assess subsurface geochemical processes responsible for the delivery of organic and inorganic metabolic energy sources and nutrients from deep-seated reaction zones to near seafloor environments and vent ecosystems.  The Guaymas vent fluids are the highest in organic content of any hydrothermal system sampled to date.

Solid phases (carbonates and sulfides) will be analyzed for mineralogy and stable isotopic composition (C, O, S, Sr). This data will be compared to the measured temperatures from the thermocouple arrays. This comparison of measured temperatures versus empirical temperatures will be a critical test of existing isotopic equilibrium constraints in ore-forming processes.