Transcript Marine Geology PPT

Geology Unit: Sea level history
NSF Grant DRL-1316782
Geologic Time
Things to keep in mind:
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Geologists study the Earth’s history
with a time scale of millions of years
Our understanding of the Earth is much
more detailed in recent history
• for example: past 200,000 years
more detailed than 65 mya
The time scale for the climate change
issues is much smaller than the time
scale that geologists study
Geologic Time
“Recent” Geologic Time:
the Quaternary Period
How do geologists think?
Uniformitarianism
• The present is the key to
the past…
• …so the way things
form now are assumed
to be how they were
formed in the past.
• RESULT: we can predict
what will happen in the
future from present and
past events.
Historical Sea Levels
• A major climate change issue
for coastal areas, like Tampa
Bay, is potential sea level rise.
• Geologists look at how sea
level has changed in recent
geologic history (150,000
years ago).
• To help us understand
possible sea level change in
the future.
How Sea Level has
Changed Over Geologic
Time
Note: The message here is that what happened
before can also happen in the future
Let’s look at sea level over a very long time – the last
590,000,000 years
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Generally, sea level has been much higher than the current
level
~250 mya (million years ago) supercontinent Pangaea
resulted in lower sea levels
By the way, this particular sea level curve, developed mostly by geologists working
for Exxon, has provided a guideline for geologists to look for oil. In other words, this
graph is related to the gas you put in your car.
Now let’s look at something closer to today and directly related to
Florida
Sea level record over the last 140,000 years
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Generally, sea level has been lower than the current level
The last time the sea level was higher than today’s sea level was
125,000 years ago.
• Sea level was 6-8 meters higher
• Resulting in the formation of the Florida Keys and the
Everglades
Now let’s look at something even closer to today and directly
related to Florida
Sea level record over the last 20,000 years
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~20,000 years ago sea level
was ~130 meters lower
Sea level rose very fast at 1
cm/year from 20,000 to 8,000
years ago.
• sea level rise slowed down
to about 3 mm/year from
8,000 to 3,000 years ago.
In the last 3,000 years, it has
been rising very slowly at
about 1 mm per year.
By the way, the predicted sea level rise rate for the near future is about 3-6 mm per year.
(We will discuss this in more detail later.) This is much slower than the rate between
20,000 to 8,000 years before present. So, our prediction is not out of line at all.
So, why was the sea level so low at 20,000 years bp …
because the water was frozen in glaciers.
Last Glacial Maximum
~20,000 years before present
Laurentide Ice Sheet
95,000-20,000 years ago
Geologists can tell where the glaciers were from studying the sediment on the
Earth’s surface. That’s how they know the extent of the ice sheet.
What did Florida look like 20,000
years before the present (bp) and
125,000 bp?
Note: This can also tell us what Florida will look like
in the future.
Florida 20,000 bp
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The Florida peninsula
was more than twice as
wide as it is today.
Tampa was about 200
km from the shoreline
and 130 m above sea
level
Last Glacial
Maximum
Shoreline
20,000 bp
How do we know that sea level was about
130 m lower 20,000 years before present?
One example:
A barrier island was
found at a depth of 70 m
below the sea through
sea floor imaging.
A barrier
island found
at a depth of
70 m.
Caladesi
Island today.
Barrier
islands only
form near sea
level.
Florida 125,000 bp
• The Florida peninsula was
about 2/3 of the length
and much narrower than
it is today.
• Tampa and the entire
South Florida was under
water.
Sea Level 6-8 meters higher
Shoreline
125,000 bp
How do we know that sea level was about
6-8 m higher 125,000 years bp?
Evidence: Fossil Coral Reef in FL Keys
• 125,000 years old.
• Fossil coral found at Windley
Key indicates a barrier reef
environment
• means shallow salt
water.
• The top picture is a fossil
coral in growing position.
• The top of the Key Largo
Limestone is about 2.9 m
above present sea level.
Windley Key Fossil Reef Geological State Park in the FL Keys
How do we know that sea level was about
6-8 m higher 125,000 years before present?
Evidence: Anastasia Formation in
South Florida
• 125,000 years old.
• 4-6 m above
current sea level.
• Top layer is
described as a
fossil worm reef
• Means a
shallow
marine
environment
Beach rock found on South Hutchinson Island in Florida
How do we know that sea level was about
6-8 m higher 125,000 years before present?
Evidence: Anastasia Formation in
North Florida
• 125,000 years old
• 8-10 m above
current sea level
• Rock used to build
St. Augustine Fort
Beach rock found in St. Augustine, Florida
Take home message
Geologists have found that the sea level has been
going up and down throughout Earth’s history.
The rate of change can be much faster than what is
predicted for the future.
So, we really need to understand how climate and
sea level will change in the future.