Transcript - Catalyst - University of Washington

Eustatic sea level has fluctuated between -120 and 200 meters relative
to modern sea level over the Phanerozoic Eon (542 Ma – Present).
Causes of Eustatic (Global) or Relative Sea Level Change
1. Tectonism- Largest and most gradual changes. Caused by changes in
the volume or holding capacity of the ocean basins. Rates of sea
floor spreading have the greatest impact on impact on ocean basin
holding capacity. The volume of mid-ocean ridges increase during
periods of high sea floor rates; thus resulting in higher eustatic sea
level values, such as the transgression (rise in eustatic sea level)
during the Cretaceous Period. The Tertiary Period marine regression
(fall in eustatic sea level) was likely related to the enlargement of the
Atlantic Ocean basin and the denser ocean crust distal of the
spreading ridge.
1.
Ice Sheet Growth and Decay- The growth and melting of land ice is responsible for the second major cause of eustatic
sea level change, and the most dominant during the Quaternary Period (prior 2 million years). As ice sheets build up
sea level lowers ~1 m/1000 years. Maximum lowering was ~120-150 m during the most extensive glacial cycles. As the
ice sheets rapidly decay sea levels rises at an average rate of 5-10 m/1000 years. Ice sheet growth and decay also
influences the water mass over the ocean basins.
1.
Local isostatic and tectonic effects which primarily affect a local setting. These relative sea level effects are primarily
due to glacio-isostasy (ice loading and unloading effects) and hydro-isostasy (water loading and unloading effects).
Causes of Eustatic (Global) or Relative Sea Level Change
1.
Tectonism- Largest and most gradual changes. Caused by changes in the volume or holding capacity of the ocean
basins. Rates of sea floor spreading have the greatest impact on impact on ocean basin holding capacity. The volume
of mid-ocean ridges increase during periods of high sea floor rates; thus resulting in higher eustatic sea level values,
such as the transgression (rise in eustatic sea level) during the Cretaceous Period. The Tertiary Period marine
regression (fall in eustatic sea level) was likely related to the enlargement of the Atlantic Ocean basin and the denser
ocean crust distal of the spreading ridge.
1. Ice Sheet Growth and Decay- The growth and melting of land ice is
responsible for the second major cause of eustatic sea level change,
and the most dominant during the Quaternary Period (prior 2 million
years). As ice sheets build up sea level falls ~1-3 m/1000 years.
Maximum lowering was ~120-150 m during the most extensive glacial
cycles. As the ice sheets rapidly decay sea levels rises at an average
rate of 5-10 m/1000 years. Ice sheet growth and decay also
influences the water mass over the ocean basins.
1.
Local isostatic and tectonic effects which primarily affect a local setting. These relative sea level effects are primarily
due to glacio-isostasy (ice loading and unloading effects) and hydro-isostasy (water loading and unloading effects).
Eustatic sea level has gradually
lowered since the Cretaceous
transgression.
The development of continental ice
sheets has had major impact on
shorter term fluctuations as the ice
sheets have grown and decayed.
The largest fluctuations occurred
following the build-up of ice sheets
on the northern hemisphere
continents.
Causes of Eustatic (Global) or Relative Sea Level Change
1.
Tectonism- Largest and most gradual changes. Caused by changes in the volume or holding capacity of the ocean
basins. Rates of sea floor spreading have the greatest impact on impact on ocean basin holding capacity. The volume
of mid-ocean ridges increase during periods of high sea floor rates; thus resulting in higher eustatic sea level values,
such as the transgression (rise in eustatic sea level) during the Cretaceous Period. The Tertiary Period marine
regression (fall in eustatic sea level) was likely related to the enlargement of the Atlantic Ocean basin and the denser
ocean crust distal of the spreading ridge.
1.
Ice Sheet Growth and Decay- The growth and melting of land ice is responsible for the second major cause of eustatic
sea level change, and the most dominant during the Quaternary Period (prior 2 million years). As ice sheets build up
sea level rises ~1 m/1000 years. Maximum lowering was ~120-150 m during the most extensive glacial cycles. As the
ice sheets rapidly decay sea levels rises at an average rate of 5-10 m/1000 years. Ice sheet growth and decay also
influences the water mass over the ocean basins.
1. Local isostatic and tectonic effects which primarily affect a local
setting. These relative sea level effects are primarily due to glacioisostasy (ice loading and unloading effects) and hydro-isostasy (water
loading and unloading effects).
Ice loading can produce major isostatic adjustments to the lithosphere as the
ductile asthenosphere is displaced outward from the ice load. As the major ice
sheets retreat the asthenosphere will slowly flow back causing the lithosphere to
rise. Returning ocean water will also have corresponding isostatic effects.
Sea Level Reconstructions
Coral reefs
Raised and Drowned Deltas
Drowned river valleys
Paleoshoreline Indicators (marine terraces)
Marine Oxygen Isotope Records (Global Ice Volume Records)
Coral reefs will drown if sea level rise is too rapid for the growth rate to keep up
with the rising water. Coral reefs grow at rates between 0.8 and 80 mm/ year.
Drown reef complexes found off
the coast of Hawaii were likely a
casualty of meltwater pulse 1A
between 14,700 and 14,200 years
ago.
Tectonically uplifted terraces dated
along the Huon Peninusula, Papua New
Guinea document marine high-stands
formed during interglaciations.
Ice loading can produce major isostatic adjustments to the lithosphere as the ductile
asthenosphere is displaced outward from the ice load. As the major ice sheets retreat
the asthenosphere will slowly flow back causing the lithosphere to rise. Returning
ocean water (from melting ice) will also have corresponding isostatic effects. The
magnitude and rate of isostatic depression and recovery will be related to the
thickness of the ice sheet, thickness of the lithosphere and viscosity of the mantle.
Relative sea level change in a previously glaciated
regions will be dependent upon eustatic sea level
change and isostatic effects.
Post-glacial marine terraces on
San Juan Island, Washington:
A product of multiple
Meltwater Pulses 1A
Hope Sisley, Terry Swanson, John Stone
University of Washington
Terraces form when relative sea level is stable. How
would you reconcile terrace formation on San Juan
during a period of rapid isostatic uplift (i.e., 10-20
cm/yr)?
Be-10 dated till boulders on Cattle Point moraine and terraces.
1A
1B
Late wisconsinan glaciomarine
deposition & isostatic rebound,
N.puget lowland, WA
D.P.Dethier et al., 1995
Fig.1.Puget Lowland
Scope
• Events between 13.6k - 11.3k years ago
in the Puget Lowland:
– Rapid retreat of cont.ice
– Everson marine incursion
– High rates of isostatic rebound
• http://www.washington.edu/burkemuseum/w
aterlines/glaciation.html
Study Emphasis
• Glaciomarine Deposits
– Distribution
– Stratigraphy
– Paleoecology
– Geochemistry
• Ice recession, marine incursion, isostatic
rebound
– Geomorphic evidence
– 14C evidence
• Fig.5.Glaciomarine deposits, inferred depositional
mechanisms and environments. Modern analog
in coastal Alaska.
Facies
•
•
•
•
•
Proximal
Transitional
Distal
Marine and Estuarine
Emergence
Relative sea level can be determined
using geomorphic features such wave-cut
terraces, topset beds of marined deltas,
and relict channels on Whidbey Island.
Proximal Facies
• Sorted deposits generally become finer from N to S in the
moraines
• Preserved thicknesses as much as 100m
• Kettled landscape
Transitional Facies
-Finer grained
-Richer in dropstones
-More diverse fauna
-1-10km away from ice margins
-Pebbly silt, 3-10m thick
-Locally rich in fossils of invertebrates
-Accumulated at about the same time
as proximal facies
Distal Facies
• >10km away from ice margins
• Absence of dropstones suggests that about 13.5k years ago,
the floating ice was uncommon near Marysville, and did not
reach Mt.Vernon
Marine and Estuarine Facies
-Thin, fine grained
-Shallow, subtidal origin for
sandy sediment overlying
pebbly silt
emergent facies
marine facies
ice-rafted pebbly silt
ice-proximal (flow till)
Emergence Facies
• Emergence of land, shallowing of water
• Contain boulder lags and finer sandy deposits
• Wave-cut surfaces
emergent facies
ice-rafted pebbly silt
• Fig.6.Stratigraphy and lithology of
Everson-age deposits
Fraser Glaciation (25k-10k y ago)
• Cont.ice attained thickness of ~1700m at the
Int’l boundary (Canadian border), and about
1200m near Everett
• The retreat of ice took only a few hundred
years, caused
– rapid glacio-isostatic rebound
– marine incursions (over 10 000km2 of WA)
– deeply embayed margins on ice lobes
• Fig.2. Marine limit at ~13.5ka
• Fig.3.Altitude of marine limit in Puget Lowland, in meters
• Fig.4. Stratigraphy of
Everson-age
deposits.
• Site 1-Mt.Vernon N
• Site 2-Mt.Vernon S
•
•
•
•
•
Fig.4.(continued)
Site 3-Big Lake
Site 4-Davis Bay
Site 5-Cattle Point
Site 6-Marysville
Isostatic Rebound
• Present day land area S from Everett lay above
sea level
• Present day area N of Everett was covered by
40 to >150m of marine water
• After the ice progressively retreated, most of
the uplift was occurring within ~3.5k years
• The relative sea level fall persisted 3-6k years,
since rebound rates exceeded eustatic sea
level rise before 7k years ago
• Fig.7.Maximum altitude of marine limit
from Everett to Int’l Boundary