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. 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 for some elements.
• Diffuse flow: many hydrothermal vents are less spectacular, and only look like warm shimmering water from a submarine – but globally this 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 up by 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.

Photosynethesis

Respiration

I. Horizontal and Vertical Patterns in the Ocean

• 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 organic matter (POC). In the open ocean, phytoplankton are nutrient-starved BECAUSE they sink and take nutrients with them.

II. General Patterns of different kinds of species found in the oceans

1. nutrients
2. oxygen
3. CO₂
4. DIC – "dissolved inorganic carbon" = CO₂ + H₂CO₃+HCO₃^-+CO₃^2-
5. Metals
6. Particles
7. 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 two-dimentional graph of depth vs. distance along this line, then represent the concentrations with contour lines and/or a color spectrum.

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