Barrier islands are narrow stretches of sand
deposited parallel to the shoreline
and are super important ecosystems.
They not only provide habitat for many species of
wildlife such as birds and turtles,
but also protect the mainland where lots of people
live and work.
Typically, barrier islands have a lagoon of water between them and
the mainland, then a marsh or wetland environment,
then sandy dunes and a beach face that leads out to the open ocean.
Have you ever been to St. Pete Beach,
Treasure Island, Madeira Beach, or Honeymoon Island?
They're all barrier islands!
Barrier islands may look the same each time you visit,
but they are constantly shifting in shape and size.
They can grow and shrink as ocean currents and storms add and take away sand.
Figuring out how these islands change and what makes them change
is a lot like solving a mystery!
My name is Daniel Ciarletta, and I'm a Mendenhall Postdoc here at the USGS
and I'm a Research Geologist.
What I like most about my job... well, I like getting my hands dirty.
But also, I enjoy actually looking at the past and reading it almost like a book.
That’s kind of the point of geology. It’s like, kind of solving a mystery.
Looking at aerial images, taking cores, GPR profiles,
you know you’re writing the story of the Earth basically
so that’s what I find appealing about it.
Sometimes it takes a long time for these changes to happen.
Here, we see Fort De Soto Beach in Pinellas County
growing gradually from 1995 to the present – a period of 26 years!
Barrier islands can also change a lot over just a single day,
especially when a storm passes by.
In the first picture, we see the barrier island before Tropical Storm Eta,
In the second, we see the island after the storm
where sand from the beach has been pushed over the barrier by storm waves.
This process is called ‘overwash’.
This is what happens to the beach during a storm
and why storms can cause beaches to shrink.
During storms, waves break on the beach
and the water level rises due to tides, surge, and waves.
These processes can take sand off and away from the beach,
a process called “erosion.”
So, we can watch what’s happening in real-time
but we have to do some sleuthing to figure out
what has happened to the island in the past.
To collect some clues, come with us to the beach!
We investigate barrier islands and how they’ve changed over time
using GPR data and sediment cores.
GPR, or Ground Penetrating Radar, is a way to see what’s below the ground.
As barrier islands form over time, sediment piles up in layers.
These layers can pile up in different patterns,
depending on what happened in the past.
The GPR sends electric signals underground which bounce back to the device
providing data about the depth of each sediment layer.
Analyzing GPR data can provide sort of a secret underground map
that we can use to figure out where we want to take a sediment core sample.
We can make interpretations of GPR images, however, we still don’t know what
type of sediment is really under the ground.
This is why we take sediment cores - so we can gather more clues
about what happened here!
A sediment core uses a tube to collect a column of soil, sand, and mud -
or sediment - from beneath the surface.
These layers represent different environments that existed in the past
and show how the present environment at the surface came to be.
The sediments in cores also reveal the impacts of storms, floods,
and other events that cause rapid changes in the environment.
Sediment cores can be collected using a vibracore system,
which vibrates a metal core tube into the ground.
A motor attached to the top of core tube vibrates extremely fast.
The vibration causes the earth beneath the bottom of the tube to move like a fluid
(think quicksand!) allowing the core to sink into the ground.
When the core has finished sinking, it is yanked out of the ground with a winch,
pulling up a column of sediment trapped inside.
Finally, we cap the core and take it back to the lab!
In order to see what’s inside the core and collect clues about the
history of the barrier island, we first have to cut the core tube open.
We slice it from top to bottom using a large, two-sided saw
to cut it into two halves.
One half of the core is photographed, described, and preserved
for future geologists to study.
The other half is used for laboratory analysis.
From the lab half, sediment samples can be removed from individual geologic layers
and analyzed to see how big the grains of sediment are, what color they are,
as well as how old they are.
Here is a little sand card you can put up to the core and can compare and say,
well is this a medium sand, is it fine, is it pretty coarse?
You can just overlay that right here.
For color, we use Munsell soil color charts.
and watch, you can take this and you can put it on top of the core here
and then we can compare the values and figure out what color this actually is.
This information allows scientists to determine the types of environments
that existed there or what kinds of natural events occurred, like storms,
in the past.
Let’s see what our beach core shows.
On the left are photos of how our core site has changed through time.
On the right is a column that represents the contents of the sediment core.
At the very bottom of the core is sediment that is light tan colored.
If we look at the photo to the left, this light tan sediment may have come from
a shallow saltwater pond behind the barrier island.
Photos from 2013 show that a storm pushed sand over the barrier into the pond.
This storm likely left behind a very thick layer of sand,
which shows up in the sediment core as a large layer of darker sand.
After the storm, a mangrove swamp grew on top of everything.
In the core, sediment from the mangrove swamp looks like thin layers of dark mud.
The clues that we collected from inside the sediment core
are helping us put together the mystery
of how the island has changed over time!
Barrier islands are very dynamic.
They’re important because people live on barrier islands
and use them for recreation.
Photos can provide snapshots in time— but they don’t tell the whole story.
Cores and GPR allow us to look deeper into the past
to understand how barriers have changed through time.
With this kind of data, what we’re going to be able to do is develop a baseline
for how these barrier islands were changing in the past.
What that’s going to allow us to do is to project what happens
to them going forward under different scenarios.
So we can do scenarios of say, increasing rates of sea-level rise.
What happens if we’re creating more space for sediment to fill in on the beach?
What happens if you restrict the sediment supply?
What happens if we have groins or jetties updrift
that are now blocking sediment?
You know, how can we expect the island to respond
based on what we know historically?
By looking into the Earth, into the past,
scientists can learn how an environment has changed through time
and what that might mean for the future.
This allows society to adapt to a changing world.