Geology Unit: Sea level history NSF Grant DRL-1316782

Geologic Time Things to keep in mind: 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 than in ancient history The time scale for the climate change issues is much smaller than the time scale that geologists study

Geologic Time Find a zoom in of qua

“Recent” Geologic Time: the Quaternary Period Find a zoom in of qua

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. Uniformitarianism is a key principle in geology.

Historical Sea Levels A major climate change issue for coastal areas, like Tampa Bay, is potential sea level rise. Geologist look at how sea level has changed in recent geologic history (150,000 years ago). To help us to understand possible sea level change in the future. Uniformitarianism is a key principle in geology.

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 Generally, sea level has been much higher than the current level ~250 mya (million years ago) supercontinent Pangaea resulted in lower sea levels May need to explain to students that “~” means “approximately”. http://www.wiley.com/college/strahler/0471480533/animations/ch13_animations/animation3.html 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 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

~20,000 years ago sea level was ~130 meters lower Now let’s look at something even closer to today and directly related to Florida Sea level record over the last 20,000 years ~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.

Note: This can also tell us what Florida will look like in the future. 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 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 Shoreline 20,000 bp Last Glacial Maximum

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.

Sea Level 6-8 meters higher 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.