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.
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
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.)
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.
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.
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
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.