EES215

Lecture 13

Oceans - general description: size; Large units (Pacific, Atlantic, Indian); seas: Mediterranean; North; Caribbean; White Sea; Black Sea; Baltic Sea. Seas are parts of the large units, but are separated by straits, sills etc. from the large units. Fig. 1

Ocean Floor:  ocean basins; trenches; mid-ocean ridges (MOR); continental slope, rise; continental shelf. Shelf is part of continental crust (currently flooded), transition between oceanic and continental crust occurs in continental rise and slope (passive margin; example east coast of North America). Fig. 2

Island arcs  are formed by active margins (subduction zones); marginal seas are landward from island arcs (example Japan and Sea of Japan)

Hot spots  form oceanic islands and seamount chains (e.g. Hawaii).

Sea level currently is constant within a few centimeters; major changes in sea level occurred to presence or absence of ice sheets as the sheet ice covering North America 18,000 yrs ago, at which time the sea level was lower by 100 m. Fig. 3

Presence of ice sheets also causes subsidence of continental crust; removal results in elastic rebound of center area and subsidence of marginal areas (e.g. Scandinavia is currently rising, coast of Holland and England is sinking as result of removal of ice sheets; current movement is used to estimate viscosity of underlying mantle). Fig. 4

Oceans have been around for most of the Earth’s history, similar in make-up and total mass. Distribution of ocean basins changed related to plate movement (e.g. Gondwana Land; presence of Tethys Sea; opening of Atlantic Ocean)

Make up of water: major constituents; typical element distribution; origin of elements in the oceans (volcanism - submarine and subaerial; erosion); residence time for elements depends on geochemical behavior (Fig. 5)
Presence of salts in solution lowers freezing/melting point and eliminates density anomaly for water: important for density stratification and ocean convection (Fig. 6)

Presence of oxygen: Derived from atmosphere; presence in most of the oceans indicates mixing of ocean waters (mixing time ~ 1000 yrs). (Fig. 7)
Some almost closed basins, such as Black Sea, do not turn over and are anoxic (i.e. without oxygen) at depth.

Forces on oceans:
Gravitational: gravitational potential derived from Earth: first order shape of sealevel distribution - standard for comparison - definition of geoid (see discussion of gravity)
Gravitation also important for density stratification of ocean water gravitational attraction of sun and moon – tides (Fig. 8)
tides are an ocean wide phenomenon: diurnal and semidiurnal cycles; influence of sun and moon; spring tide and neap tide; generally cause changes of less than 1 m between high and low tide in the open ocean
In coastal areas tides can get funneled into small areas and can cause large tidal differences (resonance phenomenon) - example North Sea; Bay of Fundy (up to 15 m tidal difference); potential for tidal power stations. (Fig. 9)
Seas with narrow and/or shallow connections to open ocean have small tidal differences (e.g. Black Sea; Baltic Sea).