Bathymetry

• Bathymetry is the measurement of ocean depths and the charting of the shape or topography of the ocean floor

• The ocean floor is a highly varied terrain that contains many interesting features

• Early methods used a long weighted line (called a sounding line)

Bathymetric techniques

• Echosounders send sound through water to determine water depth and sea floor features

Bathymetric techniques

• Side-scan sonar uses a “fish” towed behind a ship to give a more detailed picture of the sea floor

Bathymetric techniques

• Low frequency sound is used to determine structure beneath the sea floor

Bathymetric techniques• Satellites measure sea

surface elevation, which mimics sea floor bathymetry

Continental Margin- submerged outer edge of the continent. Includes the continental shelf and slope.

Features of the Underwater World

Continental shelf

Continental slope

Continental rise

Abyssal plain

Abyssal hills

Submarine canyon

Seamount

Guyot

Ridge

Trench

continental shelfslope

rise

abyssal plain

abyssal hills

ridge

transform fault

Continental Shelf:• shallow submerged extension of the

continent• more like the continent than the ocean

basin• important for mineral and oil deposits• 7.4% of Earth’s ocean area

Continental slope:

• the transition between the gently descending cont. shelf and deep-ocean basin

• generally, 4 - 25o slope, 12 miles wide, 12,000ft deep

Continental Rise:

• accumulation of sediment from turbidity currents

• like an avalanche

Shelf width depends on:

1. proximity to plate boundary (passive vs active margin)

2. sea level

3. fast moving ocean currents

Proximity to Plate Boundary

Passive versus active continental margins

Passive margin

– No plate boundary

Shelf

Slope

Rise

– E.g., eastern coasts of N. and S. America

– No volcanism

Passive versus active continental margins

Active margin

Plate boundary

Convergence zone

volcanism, many earthquakes, and active mountain building

E.g., California, Chilean (e.g., Peru, Chile), Marianas (e.g., Japan, Marianas)

Sea Level Change

Fast Moving Ocean Currents

Abyssal plain:

flat featureless expanses of sediment formed by suspension settling

common in Atlantic

Abyssal hills:

small sediment covered extinct volcanoes

< 650 ft high

Sea mount: volcanic projections that don’t rise above the surface of the sea

circular or elliptically shaped

many form at hot spots

Guyot: flat topped sea mounts

Submarine canyons:

Canyons that cut into continental shelf

Turbidity currents carve submarine canyons into the slope and shelf

Submarine canyons and deep-sea fans

• Moves sediment movement from the continents into the deep-sea via turbidity currents

• Debris from turbidity currents creates graded bedding deposits and deep-sea fans

Diver in the La Jolla Submarine Canyon

Ridges: mountainous chain of young basaltic rock at the active spreading center of the ocean

The Mid-Atlantic Ridge

• Traverses the center of the Atlantic Ocean

• Contains a central down-dropped rift valley

• Surfaces in Iceland

Features of the mid-ocean ridge

• Rift valleys– Form when

plates split apart

– Down-dropped areas associated with faults and earthquakes

Trenches: arc-shaped depression in the deep ocean floor

• Formed by plate convergence• Associated with volcanic arcs

– Island arc– Continental arc

• Most active geologic feature on earth

Ocean trenches

• Most trenches are in the Pacific Ocean

Location of Marianas Trench

Location of Mariana Trench

Ocean sediment

• Particles of organic or inorganic matter that settle through the water column and accumulate in a loose, unconsolidated form on the ocean floor

• Layers represent a record of Earth history, including:– Movement of tectonic plates– Past changes in climate– Ancient ocean circulation patterns– Cataclysmic events

Collecting ocean sediment

• Specially designed ships collect cores by rotary drilling

• Cores allow scientists to analyze ocean sediment

Sediment origin & compositionSediment origin & composition

Origin:1. Weathering and erosion2. Activity of living organisms3. Accumulation of dead organisms- oozes4. Volcanic eruptions5. Chemical processes within the water itself6. Space debris

Composition:Boulder >256mm Sand 0.062-2mmCobble 64-256mm Silt 0.004-

0.062mmPebble 4-64mm Clay <0.004Granule 2-4mm

Sources of SedimentSources of SedimentTerrigenous:

1. Continental runoff- desert sand blows off continent to ocean

2. Volcanic eruptions- dust and magma3. Rivers- washout of sediment

Mt. St. Helens

Biogenous:• Mostly calcareous and siliceous

• Abundant where ample nutrients encourage high biological productivity

• Parrotfish

• Oozes: pelagic sediment

containing >30% by volume

microorganism shells

Biogenous sediment composition

• Microscopic biogenous tests are composed of 2 main chemical compounds:

1. Silica (SiO2) including opal (SiO2 · nH2O)

Diatoms (algae)

Radiolarians (protozoan)

2. Calcium carbonate or calcite (CaCO3)

Coccolithophores (algae)

Foraminifers (protozoan)

Calcareous Oozes

pteropod

CoccolithophoresForaminifers

Siliceous Oozes

radiolarian diatom

Biogenous ooze turns to rock

• When biogenous ooze hardens and lithifies, can form:– Diatomaceous

earth (if composed of diatom-rich ooze)

– Chalk (if composed of coccolith-rich ooze)

Chalk cliffs of southern England

Distribution of biogenous ooze

• Most biogenous ooze found as pelagic deposits

• Factors affecting the distribution of biogenous ooze:– Productivity (amount of organisms in

surface waters)– Destruction (dissolving at depth)– Dilution (mixing with lithogenous clays)

Hydrogenous sediment

• Hydrogenous sediment forms when dissolved materials come out of solution (precipitate)

• Precipitation is caused by a change in conditions including:– Changes in temperature– Changes in pressure– Addition of chemically active fluids

Types of hydrogenous sediment

Manganese nodules

Phosphates

Carbonates

Metal sulfides

Evaporite salts

Magnesium nodules:• 1st discovered by Challenger expedition (1873-76)• 16 million tons accumulate each year• Growth rate: 1-10 mm/million years• Need nuclei to form

Mining has not developed because:1. Low international market price for metals2. Unresolved legal ownership problems3. Technical costs and development4. Effect on hydrothermal vent communities unknown

Mining manganese

nodules

Cosmogenous sediments:

• Extraterrestrial in origin

• Two main types:

Microscopic space dust

Macroscopic meteor debris Diatoms (algae)

• Forms an insignificant proportion of ocean sediment

tektites

Studying sediments

• Learn about past climates• Ocean circulation• Plate tectonics

O2 isotope stratigraphy:O2

16 (most abundant): O218 (next abundant)

O216 has higher vapor pressure

At a given time when a shell forms in cold water, more O2

18 goes in relative to O216

Map of the World Ocean showing generalized distribution of the principal kinds of sediment on the ocean floor

Deep Ocean Characteristics

• Cold• Still• Stable• Dark• Essentially no productivity• Sparse Life• Extremely high pressure• Little food

Deep sea orgs are dependant upon surface production from:

Dead phytoplankton, zooplankton, fish, mammals

Fecal pellets and crustacean molts

Macrophyte detritus

Animal migrations

Potential food source for deep sea organisms

Scavengers can feed on carcass in less than 6 months

Whale Fall; i.e., marine snow

Factors affecting organic material reaching the sea floor:

Storms

Seasonal variation

100m

dysphotic

aphotic

photic

• Ballard & Grassle (1977)- Alvin to Galapagos • Sulfur-rich vents 660 oF, but quickly cool to 73 oF• Thermophilic bacteria associated with vent

communities

Hydrothermal vents found at mid-ocean ridge spreading centers.

Active hydrothermal vents were first discovered at Loihi in the late 1980's

Loihi

• 15 mi S.E. of Kilauea

• 3,178 ft below sea level

• Will reach sea level in ~10,000 yrs

Loihi

1. Cold seawater sinks into cracks deep into ocean floor

2. Water heated by magma rises and leaches out minerals from surrounding rocks

3. The water emerges from vents and precipitates out minerals

magma

Bottom current

Seawater seepage

basalt

660oF

FeS, FeS2, CuFeS2

H2SFe, Mg

magma

Bottom current

Seawater seepage

basalt

660oF

FeS, FeS2, CuFeS2

H2SFe, Mg

Tube worms- pogonophoransOther orgs: • Calyptogena- large white clam• Large crabs• Sea anemones• Shrimp

Support communities with high biomass, but low diversity

Primary producers are chemosynthetic bacteria

90% endemism

Communities are small (25-60m in diameter)

Chemosynthesis:Oxygen + hydrogen sulfide + water + carbon dioxidesugar + sulfuric acid

602 + 6H2S + 6H20 + 6C02 C6H12O6 + 6H2S04

                                                 

             

Photosynthesis:6H2O + 6CO2 + nutrients + light energy C6H12O6 + 6O2

Chemosynthesis:

602 + 6H2S + 6H20 + 6C02 C6H12O6 + 6H2S04

Inquiry

1. What 3 factors effect shelf width?

2. Where are maganese nodules

located?

3. Radiolarians found in sediment form

______ ooze?

4. What does endemism mean?

5. How does the tubeworm get its

energy?

6. How does a hydrothermal vent form?