CAS ES 107 Chapter Notes - Chapter 8: Continental Shelf, Thermocline, Lithification

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Chapter 8: Part 2
Nutrient Limitation
-some elements in the water are poisonous to organisms because their concentrations are above
the optimum for growth
-for each element, there is some optimum concentration that favors biological productivity
-Redfield ratios marine phytoplankton incorporate many nutrient elements into their tissues in
ratios that appear to be nearly identical in all species, and identical to the ratios in seawater
-more productive waters tend to have high chlorophyll concentrations
-waters with the highest productivities are the cold waters of the high latitude Atlantic, Pacific,
and Southern Oceans because the thermocline doesn’t exist since surface waters are just as cold
as the deep waters. So wind-driven mixing of nutrient-rich deep waters up to the surface occur
much more rapidly at high latitudes
-at low latitudes, the strong temps and density gradients make it less productive
-regions of upwelling also have high productivity because wind-driven surface currents cause
offshore transport that allows deep nutrient-rich waters to well up to the surface
-the coastal regions of the continental shelves also are highly productive because of nutrient
inputs from rivers and localized upwelling driven by the topography of the seafloor
-the nutrient composition of the oceans is dominated by the production and decomposition of
organic matter
-so the composition of marine organisms determines the composition of the seawater
The Long-Term Organic Carbon Cycle
-geological processes become the important controls on atmospheric CO2 on longer time scales
-the fluxes of carbon are small, and the reservoirs involved are large
-land-derived and marine sediments to the seafloor fills sedimentary basins that become buried
and become lithified
-the organic carbon associated with these sediments is then entombed in sedimentary rock until
weathering liberates the material to the biosphere
-this organic carbon burial represents a leak of material from the short-term carbon cycle, and it
is this that maintains the O2 content of the atmosphere
-for every atom that enters this reservoir as organic carbon, one oxygen is left behind because the
O2 liberated during the photosynthesis of that carbon was not utilized during respiration or
decomposition and so the gas remains in the atmosphere
-when organic material is buried, it undergoes changes in structure and chemistry, so fossil fuels
may form
-if concentrations of terrestrial organic matter are high in buried basins, burial processes under
high pressures and temps can lead to the formation of coal
-as well, high concentrations of organic material in marine sediments can produce sedimentary
rocks that under high pressures and temps can serve as sources of petroleum
-sedimentary rocks contain the most organic carbon on Earth
-most of the organic carbon is found in shales fine-grained sedimentary rocks formed by the
lithification of muds
-sedimentary organic carbon residence time is 200 million years
-weathering of organic carbon in sedimentary rocks is an oxidation process requiring O2, either
by direct exposure or exposure to groundwaters containing dissolved O2
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Document Summary

Some elements in the water are poisonous to organisms because their concentrations are above the optimum for growth. For each element, there is some optimum concentration that favors biological productivity. Redfield ratios marine phytoplankton incorporate many nutrient elements into their tissues in ratios that appear to be nearly identical in all species, and identical to the ratios in seawater. More productive waters tend to have high chlorophyll concentrations. Waters with the highest productivities are the cold waters of the high latitude atlantic, pacific, and southern oceans because the thermocline doesn"t exist since surface waters are just as cold as the deep waters. So wind-driven mixing of nutrient-rich deep waters up to the surface occur much more rapidly at high latitudes. At low latitudes, the strong temps and density gradients make it less productive. Regions of upwelling also have high productivity because wind-driven surface currents cause offshore transport that allows deep nutrient-rich waters to well up to the surface.

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