Hello marine biology students.
In this video we're going to talk about marine
sediments and continental margins.
[Intro Music]
So marine sediments are materials that accumulate
on the bottom of the seafloor.
Sediments 
are composed of small fine material that settles
on the seafloor.
The composition of the sediments and the accumulation
of the sediments
will vary for various regions of the seafloor
of an ocean basin.
The rates of accumulation and really the length
of time that ocean basin has been around will
all factor into the quantity and again the
type of sediments that are there.
The amount of sediments can be an indication
of the age of an ocean basin.
Sediments can also provide information about
the water column above that specific section
of the seafloor and also provide insights
about local geographical features.
There are two primary sources for marine sediments.
There are those that come from the weathering
down of rocks, usually from the continents
we call these 
lithogenous sediments, but there can also
be sediments that are formed by organisms
we call these biogenous sediments.
So, lithogenous, biogenous, these make up
the sediment that accumulates on the sea floor.
Now biogenous sediments usually are going
to be composed of microfossils, whether we’re
talking about the shells of foraminiferns
or maybe the little plates from cocolithophores.
In fact chalk is a deposit of these little
calcareous discs from microorganisms known
as cocolithophores.
The age of the sediments themselves can be
determined using carbon dating and other types
of isotope testing.
In fact, for some of these microorganisms,
like the foraminiferans, the way their shell
forms is actually an indication of the ocean
temperatures that those organisms lived and
so ocean temperature 
at the time that those microfossils were deposited
can also be determined.
In graduate school, I had a friend who was
a paleoclimatologist, where he would look
at ocean sediments and be able to determine
information about weather and climate in the
distant geological past.
So here we see what some of these shells look
like.
These are the tests of foraminiferans and
again through isotope testing, whether it's
carbon dating or even oxygen isotope testing
to determine temperature, these types of microfossils
can provide a good deal of information about
what the oceans used to be like in the past.
The rest of this video I'd like to spend some
time talking about continental margins.
So this is really where the continents are
meeting the oceans.
So the margins of continents are boundaries
between continental crust and oceanic crust.
You can really think of these as being the
sides or edges of the continent.
These edges of the continent, these continental
margins, generally consist of a continental
shelf, the continental slope, and the continental
rise.
So, looking at a diagram of these continental
margins, we can see the continental shelf
and then a shelf break that transitions to
the continental slope and then the continental
rise down at the bottom of the continental
slope, as an interface between the ocean floor
and abyssal plane, and this continental margin.
So let's talk about these different sections
of the continental margin.
So the continental shelf this is actually
composed of continental crust, not oceanic
crust, and this continental crust 
is actually below sea level.
So in this part of the ocean, the seafloor
is chemically very similar in composition
to the rocks on the land that are on the shore
above.
Continental shelf ends up making up about
8% of the ocean surface, yet it is one of
the richest areas of the ocean in terms of
the number of species.
The biodiversity 
of this region is significantly higher than
a lot of other places in the ocean, such as
the open ocean.
The width of the continental shelf varies
depending on which shore line you're looking
at.
Anywhere from as small as 1 kilometer up to
750 kilometers.
Now, the end of the continental shelf is a
region known as the shelf break
and this shelf break typically happens at
a depth of around 120 to 400 meters.
After that shelf break, the edge of the continent,
this continental margin is known as the continental
slope and again this continental slope really
can be thought of as the edge or side of the
continent.
It begins at the shelf break and extends down
to the deep sea floor.
Its descent is at a much steeper angle than
the continental shelf and the continental
rise.
The continental rise is formed by sediments
that had been pushed down from the continental
shelf and slope.
So these are sediments that are either entering
the ocean from rivers or streams or erosion
of the continental crust above.
They enter the sediment and as they travel
down the continental slope they accumulate
at the bottom and this is known as the continental
rise.
It can almost be thought of as in underwater
river delta
or an alluvial fan, but in this case, the
river is formed by the sediments.
Now, if you have an active margin, the continental
rise may be absent and in its place you might
have an oceanic trench.
And so, with that, let's talk a bit about
the different types of continental margins.
In a passive margin, the edge of the continental
plate is not directly against an oceanic plate,
but you have both oceanic crust and continental
crust as a part of the same lithospheric plate.
Passive margins are relatively inactive geologically
speaking.
There's going to be a wide flat coastal plain,
wide continental shelf, and the continental
slope will be at a gentler angle than on an
active margin.
And examples of this would be the East coast
of the United States and also on the coast
of Brazil.
Here you have a wide continental shelf, a
gentle continental slope, and you would have
a significant continental rise at the bottom
of those continental slopes.
Conversely, in an active margin, these are
sites of more intense geologic activity including
earthquakes and volcanoes and trenches.
Active margins typically are categorized by
steep rocky shorelines, narrow continental
shelves, and very steep continental slopes.
Sometimes the continental rise may be totally
absent and replaced instead with an ocean
trench.
The west coast of the United States and the
coast of Chile are examples of this.
So, when we look at the difference here between
the East Coast and the West coast of the American
continents, on the west coast we have a separate
oceanic plate that is coming up against the
continental plate and this is a subduction
zone.
The oceanic plate, being more dense, is going
beneath the continental plate, but this is
going to result in mountains and volcanoes
and earthquakes and geologic activity.
We also see a very short continental shelf,
a very steep angle on the continental slope
and down at the bottom of the continental
slope little or no continental rise at all,
and that's because the sediments as they're
coming down, they are also being subducted
into that trench.
Whereas, if we look on the Atlantic coast
of the American continents, here the Atlantic
Ocean is on the same lithospheric plate as
the continent itself.
There's a wide continental shelf, a gentle
sloping to the continental slope and a significant
continental rise.
All of these are indications of a passive
margin.
That brings us to the end of this video.
Now, one thing I would like you to consider
before our next video is “How would life
be different if you were surrounded by water
instead of being surrounded by air like we
are on land?”
Alright, see you in the next video.
