Endeavour Ridge
48 ° 00
47 ° 57
47 ° 54
129 ° 05
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Endeavour Ridge |
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48 ° 00 |
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47 ° 57 |
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129 ° 05 |
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Introduction
The big picture
Motivation for this project stems from recent interest in the global geochemical cycle of iron,
due to revelations that fertilization of the surface ocean with iron may affect global climate
(i.e. Peng and Broecker, 1991). While iron fertilization of high-nutrient, low-chlorophyll (HNLC)
regions of the world's oceans such as the Southern Ocean and the equatorial Pacific Ocean may
drawdown atmospheric carbon dioxide on short timescales (Cooper et al., 1996), it is unknown how
fertilization will affect global climate over long timescales (Charette and Buesseler, 2000;
Ridgwell, 2000). Of primary importance to this debate is the question of which iron sources
(dust, slope sediments, hydrothermal fluids, and/or rivers) are most important to open ocean
ecosystems and which would be primarily responsible for natural iron fertilization.
Black smokers, chimney morphology, and growth of deposits
Within the context described above, this project examines iron in sulfide minerals in a
deep-sea hydrothermal vent chimney, or "black smoker". The chimney under consideration,
called "Finn", is from the Mothra vent field along the Endevour segment of the Juan de Fuca
ridge, Pacific Ocean.
Hannington et al. (1995) describe growth rates for black smokers as high as ten centimeters per
day, with individual chimney lifetimes anywhere from days to decades. It is noted, though, that
hydrothermal activity in any particular field can persist for several thousands of years.
The metals, including iron, found in chimney structures represent a small proportion of the total
metals released from the vent. According to Hannington et al. (1995), at least 90% of the metals
in the hydrothermal vent solution is lost in the buoyant vent plume. In addition, since particles
less than fifty microns comprise as much as 99% of the plume, these metals are normally dispersed
more than one kilometer from the vent source. This distance is usually much greater than the one
hundred meter radial extent characteristic of a depositıs debris apron.
As a result of these processes, the iron analyzed in Finn represents a small fraction of the total
iron that is released into the black smoker complex from sub-crustal levels. In addition, it should
be noted that the bulk of the iron in the fluid derives from alteration of oceanic crust or from
magmatic sources. The concentration of iron in seawater (around the picomolar range) is extremely
low compared to rocks (ppm) and is assumed to be a minor component in these systems.
Based on figure from Hannington et al. (1995) Physical and chemical processes of seafloor
mineralization at mid-ocean ridges. In Seafloor hydrothermal systems: physical, chemical,
biological, and geological interactions. Humphris, S. et al. [eds]. Geophysical Monograph
91 (Geophysical Monograph Series), American Geophysical Union: 115-157.