Hydrothermal Vents / Open Ocean
Chemistry
Note: terms to understand are underlined
1. Hydrothermal vents
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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”. It’s
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 seawater
(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.
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Differences in the intensity of the vertical segregation
are the result of advective transport in the deep ocean.
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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.
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 (re-dissolution 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 twodimensional 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