MARS1001

Why is the ocean floor so complex?

Geological history and plate tectonics

Modern calculation of ocean depth…

Uses multibeam echosounders from ships. D = ½ x velocity (~1500m/s in water) x travel time for return. Intensity = firmness of ocean floor.

Shallow Water Mapping

Can be done through satellites or LiDAR technology (laser mapping, 3D models)

Future potentials for seafloor mapping

Remotely operated vehicles and autonomous underwater vehicles

Recognising Major Seafloor Features

Seamounts (underwater mountains), plateaus (large, shallow, flat plains), deep-sea trenches (formed through subduction at plate convergence zones), submarine canyons

Submarine Canyons

Channels for sediment and nutrients to deep ocean. Formed from ancient river channels, mass wasting and strong current movement through time. Cut into the seabed of continental shelf and slope.

Reasons to map the seafloor

Predict weather events, find resources, design shipping routes, build infrastructure (e.g. internet and communication cables), navigate the seas, and recognise, protect and manage ocean habitats.

Significance of Water Density

Drives deep ocean currents. Influenced by temperature and salinity, which is the basis of thermohaline circulation (density driven circulation of water).

Density and Temperature Across Ocean Layers

Dense, cold water sinks and light, warm water rises. Sea surface density is highest at high latitudes, primarily controlled by sea surface temperature. Also influenced by fresh water flux in some regions.

Salinity

Measure as the conductivity of water, by total inorganic solids dissolved. Can be expressed in PSU, parts per thousand or absolute salinity.

What changes the salinity of the ocean?

Evaporation and sea ice formation (rejects salt) increases salinity. Fresh water input dilutes water (glacial run-off, precipitation).

Salinity Across The Ocean Layers

In the tropic and subtropical regions, intense solar radiation drives mass evaporation of pure water, leaving very salty surface waters. In polar regions salty water sinks to the deep ocean and glacial run off and high precipitation continuously dilutes surface water with fresh water.

Mixing of Different Density Water

Segregate based on their differing properties, creating vertical stratification. Water in oceans will eventually be mixed by wind, waves and currents. In oceans - boundary of ocean basins where fresh meets salt water e.g. from run-off, rivers and precipitation.

Thermohaline Circulation in Climate

Connects all ocean basin circulation together, driving global climate change by redistributing heat, salt and carbon around the oceans.

Role of Southern Ocean in Thermohaline Circulation

Mixing hub of global waters, physically connecting and exchanging water between the Atlantic, Pacific and Indian Oceans, primarily through ACC, driven by strong, uninterrupted Westerlies. Formation of sea ice continuously generates cold, dense and salty water which spreads from the pole across the ocean floor into the other ocean basins driving the global conveyer belt.

General Movement of Ocean Water

Winds flowing from E → W at the Equator pushes water north and south due to Ekman Transport (equatorial divergence). When water meets the Westerlies (W → E), it is pushed opposite (SH - north, NH - south), creating circular currents (subtropical gyres). Water is warmed at equator and cooled down at poles creating a flow of warm water towards poles and cold water back from.

Boundary Currents

Outer edges of gyres. Due to respective directions of water movement in both hemispheres western currents are fast and narrow (towards poles) and eastern currents are wide and slow (towards equator).

Upwelling

When wind pushes surface water away from an area (i.e. coast), and cold, nutrient rich water rises to take its place.

Downwelling

When surface water is piled up in an area (e.g. against coast) and sinks into the deeper ocean layers.

Welling on Coasts

NH wind N, SH wind S = downwelling West, upwelling East.
NH wind S, SH wind N = upwelling West, downwelling East.

Southern Oscillation

Climate phenomenon of a fluctuation in wind strength which changes SST. Neutral phase - warm water carried across the Equatorial Pacific all the way to Indonesia. Evaporation high, rainfall.

El Nino

Trade winds weaken or reverse and warm water is not carried as far across Equatorial Pacific, shifting warmth to the central and Eastern Pacific. Less evaporation near Australia = less clouds, more direct heat and less rain.

El Nina

Trade winds strengthen, pushing warm water towards Asia and Australia. More evaporation near Australia = increased cloudiness, cooler days and more rain. Warmer water also increases cyclone formation.

How does surface ocean circulation influence biological productivity?

Redistributes nutrients produced by phytoplankton and that sink into the water column, including particulate matter. Upwelling returns nutrients from deep ocean back to the photic zone.