Exam 2 - Geological Oceanography

Bottom Boundary Layers

The layer (of thickness D) in which velocities change from zero at the boundary to a velocity that is unaffected by the boundary

• Outer region

• Intermediate layer

• Inner region

Turbulence

can keep sediments afloat

Suspended Load

particles that are kept floating during turbulence

Bed load

particles that are moving (sliding and rolling) on the sea floor during turbulence

Using sediments to measure currents

where currents meet obstacles, currents speed up and reduce sedimentation or start eroding

moat

forms around obstacles with an increased deposition in the lee of the obstacle

ripples

perpendicular to direction of flow and is effective at sorting sediments by density, grain size (peaks prominent close to time of formation from a storm)

current generated ripples

asymmetrical gentle slope upcurrent side, steep on downcurrent side

Wave generated ripples

oscillatory motion creates symmetrical ripples wavelength increases with wave period

Beach composition affects slope

fine grain → Coase grain

Mud → Sand → Shingles (gravel size to cobble size)

Angle of repose

• steepest angle dry sediment can stand without sliding

• steep (11 degree) vs. shallow (0.5 degree)

Permeability

measure of the ease of fluid flow through sediments (Grain size and sorting)

• smaller grain size is more compact leading to less permeability

• larger grain size is less compact leading to more permeability

infiltration

movement of water down into the water table due to runup

drawback

smaller grain size reduces permeability thus reducing infiltration, as well as steeper beach, leading to more ______.

larger saturated zones

tide drops faster than the water table can dry out

outer region of the bottom boundary layers

affected by the outer flow (or free surface) (1-10 m)

intermediate layer of the bottom boundary layers

both far from outer edge and wall (log layer) (~0.1 m)

erosion

increases angle relative to angle of repose

deposition

degreases angle relative to angle of repose

inner region of bottom boundary layers

is dominated

Conditions favorable to erosion

• saturated sand tends to erode

  • in water, effective density reduced

• Waves: Moderate to large height, short period (8 secs)

• sand: low permeability

Conditions favorable to deposition

• Waves: Moderate height, long period

• Sand: High permeability

Sources of Sediments

• Erosion of rocks at shore

• Biogenous sediments

  • Bioerosion (fish)

  • Erosion (waves smacking into coral)

• Runoff

• Cliffs

Low sediment supply beaches

• Lag beach

• Beach rock

High energy beaches

• gravel beach

• storm beach

Dissipative

beach state which multiple sand bars that create multiple wave crests

reflective

beach state which waves break right on the shoreline and slope is much steeper

Intermediate

beach state with moderate waves and fine to medium sand. They are characterized by a surf zone with one or two sand bars. The sand bar is usually cut by rip channels and currents

Surf zone circulation

complex circulation cells develop in the presence of waves at the shoreline

resuspension

the process of moving previously deposited sediment particles from the bottom of a body of water into the water column

silt

wave motion too strong for the particle and gets moved back out to sea by riptides

High supply

• deposition>erosion (Bays)

  • wave activity sorts sediments

Low supply

• Headlands

• lag or rocky beach

intermediate supply

• deposition = erosion

• grain size may fluctuate seasonally

wave refraction

causes increased erosion at headlands, deposition in bays

Littoral cell

sediments are transported alongshore, then move offshore @ submarine canyons

Submerged specific gravity

• Vertically downwards of Pp > Pf

• Upwards if Pp < Pf

coal/flocs

submerged specific gravity = <0

Magnetite

submerged specific gravity = 4.1

Olivine

submerged specific gravity = 2.3

Quartz and kaolinite

submerged specific gravity = 1.6

Stokes settling

two forces act on a sinking particle:

• Gravitational (Fg)

• Drag (Fd)

Limits of stokes

• assumes smooth spherical particles - rough particles settle more slowly

• No grain-grain interference

  • dense concentrations settle more slowly

  • Flocculation

• Assumes laminar flow

  • ignores turbulence

  • important for coarse sand and larger

Flocculation

joining of small particles (especially clays) as a result of chemical and/or biological processes

Marine snow

• Larvacean houses

• fecal aggregates from zooplankton

• diatom floes formed at the end of blooms

• aggregates formed in aging systems from unidentifiable debris

laminar vs. turbulent flow

most flows generate turbulence

Laminar flow

a type of fluid flow where fluid particles move in parallel layers without fluctuations or mixing

turbulent flow

a type of fluid flow where the speed of the fluid at a point in continuously undergoing changes in both magnitude and direction

sediment waves

vary from small to large

• wave height: few cm - 200 m

• wave length: few cm - <10 km

eustatic sea-level

global sea level

relative sea-level

local sea level

magnitude of sea level change

Max: ~100-<200 m

Transgressions

• Landward migration of the shoreline

• sea level rising

Regressions

• Oceanic migration of the shoreline

• sea level dropping

• high sedimentation rates @ river deltas (local regression)

• tend to be thinner because of erosion/disconformity

Disconformity

• a period of time with no sedimentation, likely erosion

• Break in the geologic record above regressions

sea level change in geologic record

• Transgressions

• Regressions

• Break in record above regressions

Seismic reflection (sea level changes)

• Transgressions

  • sediment strata wider, shallower

• Regressions

  • Erosion on the shelf (hiatus)

  • Deeper accumulation

Slow mid-ocean ridge spreading

• oceanic crust cools and contracts

• Sea level will be lower

• Atlantic ocean

fast mid-ocean ridge spreading

• More hot, buoyant oceanic crust occupies more space in the ocean basin

• Faster spreading when no continental collisions

• Sea water displaced onto continental shelf

• Sea level will be higher

• Pacific ocean

Plate tectonics control

• continent thickness determines how much is exposed

• Continental collisions lead to thickening of continental crust

Changes in shape of ocean basins

• Tectonic

• Million-year (Ma) Time scales

Land and shelves

continents shrink

lower sea level

when collision happens

higher sea level and sedimentation

continental area increases when continents break apart

Continental-scale glaciations

• Glaciers ~2km thick

• LGM: Enough ice to lower sea level by 125 m

fast sea level change

happening on ka time scales

eustatic sea level changes

• Continental-scale glaciers (form, melt)

• Changes in temp (colder water = higher density)

Regional sea level changes

• post-glacial rebound

  • regions are still adjusting to ice loss from the last glacial maximum (15 ky)

  • ex) Alaska and Scandinavia (subsidence of the northern sea)

Biological pump

organic matter produced by photosynthesis in the surface ocean dies and sinks, transferring (“pumping”) organic carbon and nutrients into the deep sea

bio pump removes

carbon nutrients from the surface

bio pump increases

carbon nutrients in the deep sea

bio pump provides

food to deep sea life

Redfield Ratio

C:N:P = 106:16:1

ocean productivity where?

• coastal-eastern boundary current

• Equatorial upwelling

• polar upwelling

ocean productivity why?

upwelling

how ocean productivy change

• El Niño (change of winds) potentially less upwelling

Upwelling

• Localized

• Driven by:

  • Winds and Coriolis effect

• Bring up water from depths of a few hundred meters

• Coastal upwelling important for oil, gas formation

Export C/Production

• Data from sediment traps deployed at different depths

• Carbon flux decreases with depth as organic matter is consumed by detritovores and bacteria

• Remineralization

  • Respiration

  • release nutrients

Transfer of Carbon to sediments

Open Ocean

• 10% sinks out of the “fertile zone”

  • Photic zone + region immediately below

• 1% reaches the seafloor

• 0.03% accumulates in sediments

Increase export

phytoplankton in a large population produce larger fecal pellets that get transported to deep ocean

Burial of carbon

anoxic water column turning off respiration which organisms can’t intake increasing burial

Diatoms and export production

• Require silica

• dominate in upwelling regions

• Large-shorter food chain to big fish

  • Rytherʻs Principle

• During blooms

  • export C/Production ~50%

• Greater preservation in sediments than organic carbon

What does it mean when we say a sediment has a high silica content?

diatom or radiolarian rich sediment (upwelling/nutrient rich areas)

What do carbonate-rich pelagic sediments tell us?

forams or nannofossils (coccolithophores) oozes

What do sediments with no fossils tell us?

anoxic conditions

Namibia Upwelling

• SW Africa, Atlantic Coast

• Upwelling identified by cold temperature anomaly

Matuyama Diatom Maximum

• Matuyama-Magnetic Chron

• Glaciation of Greenland

• Mats form with rapid sedimentation through the water column

Matuyama-Magnetic Chron

• 2.5-0.7 Ma

• Olduvai sub-chron: 2.0 Ma

Glaciation of Greenland

3-2.5 Ma

Mats form with rapid sedimentation through the water column

Export X/Production >50%

Namibia Coast Matuyama Diatom Maximum

• 316.3-325.9 mbsf

• olive (5Y 4/3)

  • Diatom-bearing nannofossil clay

  • Section 1, 90 cm, to section 3, 65 cm; section 4, 15 cm, to the end of the core

• dark olive gray (5Y 3/2)

  • foraminifer-bearing diatomaceous clayey nannofossil ooze

• Name:

  • Least Abundant first

    • “Bearing” = 5-10%

  • Most abundant last

    • ooze = >60% biogenic

We study modern reefs because…

• Current geological processes can be used to interpret the rock record

• Uniformitarianism: everything happens continuously through time

• exceptions: catastrophism is the theory that the Earth has largely been shaped by sudden, short-lived, violent events, possibly worldwide in scope.

Reefs Are…

• Constructed of biologically-produced material (framework)

• Rigid structure

  • interlocked and in place framework

  • reworked framework bound together by secondary encrustation or cementation

• Stands topographically above the surrounding seascape, exerting at least local control on oceanographic processes

• Majority of the framework were formed in an environment similar to thee one in which they were ultimately deposited

Coral reefs

secretion of calcium carbonate by living organisms, forming rigid structures that stand above the seafloor

types of reefs

• Oysters

• Coralline algae

Paleo-reefs

• Bivalves

• sponges

• stromatolites

Optimal Conditions coral reefs

• warm water (>20˚C)

• High salinity

• low nutrient

Pile of rubble

• Interlocking framework of broken organic hard parts

• Primarily coral

Bioherm

• Massive, mound-shaped biological accumulation

• single species or organism

% recognized coral

14-25%