Cruise Log: July 25, 2008
| The following overview of the scientific significance of our expedition was written by Nicholas Pester (graduate student at the University of Minnesota) and Eoghan Reeves (graduate student at the Woods Hole Oceanographic Institution). | |
Deep-sea hydrothermal springs are the result of incredibly complex geological/geochemical processes and, in turn, provide an oasis in which strange and unique life-forms are able to thrive. Now thirty years after the initial discovery of this phenomenon, extensive study of a variety of hydrothermal vent fields show that while these hot springs are a common occurrence along the global mid-ocean ridge system, the chemistry, petrology and biology may differ greatly in both time and space. These differences result from the dynamic nature of mid-ocean ridge spreading centers, where new ocean crust is created. Mid-ocean ridges are essentially extensive deep-sea mountain chains that represent diverging tectonic plates and are volcanically active to variable degrees. Here, as new crust is formed, older crust is moved away from the spreading center in a manner often likened to a conveyer belt. So the seafloor is actually moving at rates that vary from approximately 2 to 18 cm/year! This seafloor spreading rate is intimately linked to the degree of volcanic activity, both of which have profound effects on the local permeability of the crust and the heat driving hydrothermal circulation. The Rainbow hydrothermal vent field is located on a section of the Mid-Atlantic Ridge in what is considered “slow-spreading” crust (10 times slower than the maximum estimated rate of ~ 18 cm/yr) and consequently shows little evidence of regular volcanic activity. Although Rainbow is probably not unique in a global sense, it represents one of the few studied slow-spreading sites where high-temperature hot springs (as high as 370°C) has been observed for time periods greater than a decade. In addition, records of hydrothermally derived sediments suggest this site has been actively venting for as long as 10,000 years. On faster spreading ridges (e.g. the East Pacific Rise), magma persistently wells up into the shallow crust, providing a relatively constant heat source. For the Rainbow vent field, tectonic stress creates large faults and cracks that may go to great depths below the seafloor, therefore, the persistent heat source may involve deeper crustal and mantle rocks. While the complex interplay between magma supply and spreading rate is still not completely understood, it is clear that the difference in petrology and fluid chemistry between Rainbow and that of faster spreading locales can also be attributed to these two variables. Due to a process called fractional crystallization, the magma that wells up near the surface in faster spreading environments has a more evolved (basaltic) chemical composition. The oceanic crust at Rainbow is considered primitive or ultramafic, meaning the chemical composition is similar to that of the mantle. Subsequent hydrothermal alteration of these chemically distinct rocks is therefore the first step in explaining the unique fluid chemistry that has been consistently measured at Rainbow. For instance, the fluid concentrations of hydrogen sulfide, an important compound that drives microbial ecology in faster spreading vent sites, are appreciably lower at Rainbow, whereas hydrogen and methane concentrations are 5-10 times higher. This results in a vastly different biological structure and determining the role of hydrogen and methane at Rainbow is one of the primary objectives of this expedition. |
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| The high hydrogen content of the fluids is no doubt extremely important to the present day microbial ecosystems at Rainbow and has implications for the evolution of life in similar systems when the Earth was much younger. The high hydrogen creates very reducing (electron-rich) chemical conditions that are energetically favorable to converting carbon dioxide in the vent fluids, typically derived from underlying magma, to organic compounds such as methane and other hydrocarbons at high temperatures without the involvement of biology (abiotically). Fluids at Rainbow are noted for their high concentrations of hydrocarbons relative to basaltic systems, and methane-rich hydrothermal plumes have been observed at a number of locations in the Atlantic. These organic compounds provide energy to sustain large ecosystems, the most notable example of which are the extensive microbial communities living in the plumes above ultramafic hydrothermal systems that derive energy from converting hydrothermal methane back to carbon dioxide. |
Jason's robotic arm |
| Rainbow is also interesting because it represents an analog for hydrothermal systems that may have existed on the early Earth, when ultramafic rocks are thought to have been more common. If organic compounds can form abiotically in fluids emanating from ultramafic environments they may well have contributed to the onset of early life on earth, not only by providing precursors to the first primitive biomolecules, but also by providing a source of chemical energy that early microbial life could exploit. We study systems like Rainbow to increase our understanding of both the processes responsible for the formation of organic compounds in hydrothermal systems and the significance of these processes for the onset of life on Earth. | |
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