Download class handout

Marine Sciences, Fall 2004
Hydrothermal Vents/Open Ocean Chemistry
Oct. 21, 2004
R. Sherrell
1
Reading:
Tivey, H.K. The Remarkable Diversity of Seafloor Vents, Oceanus (2004) 42(2)
(Available at http://oceanusmag.whoi.edu/v42n2/megtivey.html)
Chapter 3: Plate Tectonics, pp. 86-91
Chapter18: The Benthos, pp. 470-473
1. Hydrothermal vents
-
-
-
Circulation of seawater through the oceanic crust at mid-ocean ridges (spreading centers at
edges of tectonic plates). Seawater enters through cracks, comes out fast through “pipes”. Its
chemistry is radically altered by interactions with hot rocks inside the crust.
First discovered in the late 1970’s, though existence was inferred from various indirect
evidences – like 3He anomalies. (see fig.)
Plumes: “hot smokers” are actually not smoke but particles of iron oxides that form in seconds
as hot hydrothermal “fluid” (it’s not seawater anymore) mixes with ambient deep-sea water
(change in temperature, pH, and oxygen content). Particles adsorb many elements from
seawater, then sink to sediments, causing hydrothermal systems to be a net sink (removal) for
some elements.
Diffuse flow: many hydrothermal vents are less spectacular, and only look like warm
shimmering water from a submarine – but globally this diffuse flow is an important input!
Impact on geochemical budgets: vents are a source for many elements (e.g. Ca2+), but a sink
for some. The most important sink is for Mg2+ , which is completely removed inside the crust
(taken out of fluid by reaction with rocks). This is a major sink for Mg in the whole ocean
budget, and helped to solve a “missing sink” in Mg budgets constructed before vents were
discovered.
2. Open Ocean Geochemistry
Let’s review…most marine chemistry is driven by biological processes.
Photosynthesis
Respiration
I. Horizontal and Vertical Patterns in the Ocean
Geographical differences in depth profiles:
Nutrients—differences in vertical profile shapes are driven mainly by differences in deep-water
concentrations. Surface concentrations are uniformly low due to uptake by phytoplankton. Deep
concentrations are considerably more variable.
-
Differences in the intensity of the vertical segregation are the result of advective
transport in the deep ocean.
-
Note that light and nutrients are destined to be separated, due to uptake and
gravitational removal of nutrients in the form of sinking particulate organic matter
(POC). In the open ocean, phytoplankton are nutrient-starved BECAUSE they sink
and take nutrients with them.
Marine Sciences, Fall 2004
Hydrothermal Vents/Open Ocean Chemistry
Oct. 21, 2004
R. Sherrell
2
Oxygen—opposite pattern from nutrients – lowest in N. Pacific, near bottom of main thermocline.
II. General Patterns of different kinds of species found in the oceans
A. nutrients
B. oxygen
C. CO2
D. DIC – “Dissolved Inorganic Carbon” = CO2 + H2CO3+HCO3-+CO32E. Metals
F. Particles
G. Dissolved organic matter
Ocean Conveyor Belt – slow deep water movement from the N. Atlantic, to the Southern Ocean,
then north into the Indian and Pacific Oceans. “Oldest” deepwater (longest time since formation
of deepwater from surface water) is in the North Pacific. Constant remineralization (redissolution through respiration) of raining particulate matter from surface waters everywhere
causes nutrients to accumulate in deepwater along the flow path, and oxygen to be depleted.
REMEMBER: Every individual concentration profile reflects both vertical processes (like
sinking and respiration of POC) and horizontal advection (flow) of water masses at various
depth intervals in the deep sea.
Geochemical “sections”: This refers to a way of representing both the vertical and horizontal
distribution of chemical properties in the ocean. If you have a line of “stations”, and a vertical
concentration profile at each, you can make a two-dimensional graph of depth vs. distance along
this line, then represent the concentrations with contour lines and/or a color spectrum. This is
like a vertical slice through the ocean, and is called a “section”.
GEOSECS: 1970’s, the first major attempt to describe the distribution of major chemical
species (elements, compounds, isotopes) in the ocean. Organized by the famous geochemist
Wally Broecker (Columbia Univ.) and a few colleagues, it was the first study to systematically
generate “sections” of chemical distributions in all the major ocean basins. Since then, a much
larger program, combining chemistry and physics, called WOCE, has been completed. Data are
still being analyzed today.
III. The Biological Carbon Pump
A. What is it? The “carbon pump” refers to the biologically produced flux of carbon (and other
elements in organic matter) out of the euphotic zone of the ocean. Organic carbon can go through
many steps before it leaves the euphotic zone (see figure).
B. Why do we spend our time studying this?
-It regulates to some extent the CO2 content (partial pressure) of the atmosphere
-It determines the O2 and nutrient content of the deep sea
-If it changes in response to global warming, we should know it
Marine Sciences, Fall 2004
Hydrothermal Vents/Open Ocean Chemistry
Oct. 21, 2004
R. Sherrell
3
Marine Sciences, Fall 2004
Hydrothermal Vents/Open Ocean Chemistry
Oct. 21, 2004
R. Sherrell
4