Sealevel Change and Coastal Environments

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Sea-level Change and Coastal Environments
Lectures 4 5
OEAS-306
January 29, 2009

• Outline
• Lecture 4Sea-Level Change
• Cause of Sea-level change
• Long-term Sea-level variations
• Oxygen Isotope Data
• Holocene Sea-Level Rise
• Lecture 5Coastal Environments
• Beaches and Barrier Islands
• Deltas
• Estuaries

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What Causes Sea-Level Change?
Sea-Level change can occur globally (eustatic)
and locally.

• Volume of water in Ocean
• -- Inputs Rainfall, snowfall, river discharge,
  groundwater and volcanism.
• -- Outputs Evaporation, freezing, subduction.
• -- Temperature (Water expands and contracts as a
  function of temperature).
• Shape of the Ocean Basins
• -- Tectonics (rate of sea-floor spreading,
  uplift, subsidence)
• -- Sediment inputs or removal
• -- Glacial isostacy (ice loading and unloading).

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Sea-Level Varies Significantly Over Long Time
Scales
Millions of Years
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Information about Change in Sea-level over
Geologic Time Scales comes from the use of Oxygen
Isotope Data
Oxygen molecules have several different Isotopes
Remember isotopes have same number of protons but
different number of neutrons.
Two main oxygen isotopes are 16O and 18O
Light Water (H216O)
Heavy Water (H218O)
Light water (H216O) is preferentially removed by
evaporation, so precipitation is enriched in 16O
relative to 18O.
During Glacial Periods
snow falls and is stored in ice caps so polar ice
becomes enriched in 16O.
. and ocean is depleted of 16O
Ratio of 18O 16O can be used to estimate extent
of polar ice and sea-level.
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Which contains Oxygen (with its different
isotopes)
Foraminifera-shells are contain carbonate
CaCO3
So, carbonate shells record the isotopic ratio
of the ocean, in which the organism was living.
So biogeochemist collect very deep sediment cores
and examine the ratio of 16O to 18O to infer
changes in ocean temperature and extent of polar
ice.
18O
Lots of polar ice and cold ocean temperature.
18O
Little polar ice and warm ocean temperature.
16O
16O
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What controls the extent of Glaciers?
Milankovitch Cycles
Wobble in the spin about it axis
Variations in the tilt of the Earths axis
Changes in the Earths Orbit from circle to
ellipse
Few glaciers
Ice age
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Sea-level Change since the Last Glacial Maximum
Last 10,000 years is referred to as the Holocene
Holocene transgression.
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Over more recent times, sea-level change is
inferred from tide gauge data
Current Estimates of sea-level rise are 1-2
mm/year.
Tide gauges may also move vertically with the
region as a result of post-glacial rebound,
tectonic uplift or crustal subsidence.
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Local Sea-Level can be significantly impacted by
glacial rebound.
Weight of glacier depresses crust.
Regions under the ice will rise isostatically
when ice melts.
while adjacent regions may actually sink.
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Sea Level is projected to increase dramatically
over the next 100 years
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Sea-level Change and Coastal Environments
Lectures 4 5
OEAS-306
January 29, 2009

• Outline
• Lecture 4Sea-Level Change
• Cause of Sea-level change
• Long-term Sea-level variations
• Oxygen Isotope Data
• Holocene Sea-Level Rise
• Lecture 5Coastal Environments
• Beaches and Barrier Islands
• Deltas
• Estuaries

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Coasts can be Classified as Erosional or
Depositional.
This classification represents the features of
the coasts, not necessarily whether they are
actively eroding or depositing sediments.

• Erosional Coasts have complex features shape by
  erosion.
• Small pocket beaches
• Sea Cliffs
• Sea Stacks and arches
• Caves
• Wave-cut platforms

• Depositional Coasts have features shape by
  depositional processes.
• Long, sandy beaches
• Barrier Islands
• Bays, Lagoons, and Estuaries
• Sand Spits
• Tombolos
• River Deltas

Many depositional coasts are eroding, however
they were formed through depositional processes.
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Sea Stacks off the coast of Australia
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Wave cut platform and sea cliff along the
California coast.
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Regions with rocks highly resistant to erosion
create irregularities in the coastline.
Focusing of wave energy tries to smooth or
straighten the coastline.
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Characteristics of a Typical Beach
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Sand is maintained on beaches because there is a
balance between on shore and off shore sediment
transport. (Equilibrium)
The velocity of the water switches back and forth
with the passage of a wave.
Gravity acts to move sediment off-shore.
As waves approach shore the velocity becomes
asymmetric, with stronger velocity directed on
shore.
Water percolates through the sand as it rushes up
the beach, so the return flow is weaker.
This pushes sediment toward shore.
Percolation increases with increasing grain size,
so beaches with courser sediments tend to be
steeper.
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This equilibrium shape is maintained as the
coastline erodes.
The Bruun Rule
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Beach profile often changes with wave climate,
leading to seasonal variations in the profile.
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The concept of Equilibrium holds generally, but
real beaches are more complex.
Palm Beach Australia
http//www.planetargus.com/Palm20Beach20Rectifie
d.mov
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Direction of wave approach drives nearshore
currents and net sediment transport.
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Groins at Cape May, NJ
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Ocean City Maryland
Net direction of sediment transport
jetties
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Installation of a Breakwater in Santa Monica, CA
Before
After
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Common Approaches to Beach Erosion
Upham Beach -- St. Pete FL
http//www.planetargus.com/Upham20Beach20Oblique
.mov
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Barrier Islands

• 13 of Worlds coastlines have barrier islands.
• Most barrier islands were thought to be formed
  during the last low-stand of sea-level.
• The theory is that as sea-level began to rise
  18,000 years ago, the coast was flooded
  separating dunes or other sandy deposits from the
  mainland.

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Barrier Islands are Transgressive --They
migrate landward with rising sea-level.
Islands migrate with sea-level rise
This migration takes place because of overwash
mainly during storms.
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Overwash on Ocracoke Island after Hurricane
Isabelle
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Development and Dune Stabilization Prevent
Natural Barrier Island Migration.
Starting in the 1930s, the Works Progress
Administration-Civilian Conservation Corps began
stabilizing the dunes along the Outer Banks of
North Carolina. Dunes were stabilized with sand
fences and sea grasses. This fundamentally
altered the barrier island system, preventing the
natural migration of the islands.
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Deltas
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• Rapidly flowing rivers have high capacity to
  transport sediment.
• When the river enters the ocean, it slows down
  dramatically, depositing its sediment.
• This forms the delta, which progrades seaward
  with time.

Sometimes called a clinoform.
4) As the topset regions extends, it eventually
becomes more efficient for the river to switch
course.
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Mississippi River has change course multiple
times over the past 5000 years, leaving behind
lobes of sediment associated with previous
deltas.
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Estuaries
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• Estuaries are regions where fresh river water and
  salty ocean water meet.
• Salt water is denser than river water.
• The tides flow in and out mixing the water.
• However the mixing is incomplete, so the the
  heavier saltier water is found near the bottom
  with lighter, fresher water near the surface
  (density stratification).
• However, the dense ocean water slowly flows in at
  the bottom (baroclinic pressure gradient).
• This net inflow at the bottom, makes estuaries
  effective at trapping sediment.

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There will be an Exam on the Geological
Oceanography section on Tuesday (February 3rd,
2009).

• Exam will cover all material presented in
  lectures and reading.
• Readings include chapters 3, 4, 5, and 12 from
  Textbook (Garrison).
• Format will be multiple choice, short answer and
  short essay, and a couple of problems.
• Please bring a calculator if you are not good at
  doing math in your head. No computers or phones
  can be used.
• I will be in my office Friday and Monday if you
  have questions, need help, etc ... Please dont
  hesitate to contact me.