We're here in Wellington New Zealand's capital city and it's renowned for its shakiness.
We've got the Wellington Harbour on one side and the CBD on the other
filled with all of these buildings. Now Wellington is surrounded by fault lines
and these faults influence the sediment around the Wellington basin. We're going to take you
on a journey of how we use the drill rig to core down into the sediment. We need to look at
these sediments to understand how we build these buildings, to engineer them, but they also tell
a lot of other different stories as well and I'm going to show you what those sediments look like
and about the history of Wellington uncovered from these sediments.
So we're in the office of ENGEO of Wellington, and we've got a few examples of core boxes we'd like
to show you from under the ground surface. So not starting at the surface but in this example
this one's at 13 meters - further and further we go down into the ground. At about 13
meters - quite sandy, very very green in colour, there's often a few rocks and stuff found in there
and we come down these cores - keep coming down into these layers, and you see it's starting to
get a bit browner. And that could be just a change in the amount of oxygenation in the sediment at
that time. So this one here is really quite red. That's caused from a lot of air in the environment
oxygenating the iron which is in this sediment. Whereas in another example is this greeny colour
so this could have been deposited at the bottom of a river or a lake where there's less ability
for the sediment to become oxygenated. So based on those examples of colour changes and sediment
changes, we can come down and interpret what this environment might have been like.
So for example this might have been more like a river or a stream, or this might have been like
an overbank adjacent to the stream. So the stream is diverted off course and gone somewhere else.
And then all of a sudden we come to this finer white layer. And when you analyze this under
the microscope you can actually see lots of little glass particles and they are airborne transported
particles that have made their way all the way from Taupo Volcano 25,500 years ago.So this
is the super volcanic eruption of Lake Taupo and it's the world's youngest super volcanic eruption.
So the total volume of rock erupted out of the ground equates to something like 1,100 cubic
kilometers which is just flabbergasting! I've recently been to the Chatham Islands and seen it
in deposits in the soil adjacent to the road there. So we're here in Wellington 350 kilometers
away from Taupo and we have a deposit that is over 20 centimetres thick. So this 20 centimetre thick
deposit of ash would have slowly built up and had an impact on the organisms living around the time,
whether that be in the soil underneath or on the plants as the ash came through the forest. This
impacted the environment and and vegetation and climate around New Zealand at that time.
Moving on - around about 30 something metres we still have these gravelly layers,
could be river deposition into this greyer material, and then all of a sudden this abrupt
transition into these organics. We've taken a little hole out of here because
we've aimed to radiocarbon date how old this sediment is. This is really fine material
analogous to what could be deposited in the bottom of a lake. This sediment can contain broken
leaves, twigs, pollen. We've got more lake material and you come into this abrupt transition
to where it could have been a more oxygenated environment again, into a riverine system.
Had you ever thought that in the bottom of Wellington was once this big lake? So we've
jumped across to a new box. This one's down to about 65 meters. This is really sandy and green,
really coarse so the environment maybe somewhat like a river - high energy,
more coarse material and then all of a sudden we have this quite pinkish layer. And we've
also analyzed this under a microscope and this is more volcanic glass. So this is
about 40 meters lower than the other eruption the Oruanui eruption and this is actually the
Whakamaru eruption. This produced a volcanic glass tephra and is around about 345,000 years ago.
The age of this eruption is a unique time marker and can tell us about
what the overall earth's climate was like at that time. We know that in terms of the global scale
there was actually a colder climate but after the eruption we know that the overall global
climate started to warm. So we can see evidence of this in this lake material coming in here,
going up the core again we've got evidence of peat formation so it could be traces of pollen
and particles in here, and even come across and you can see a fragment of wood in this sediment
here as well. The difference between this colder climate to this warmer climate here equates to
around about a six degrees change in global temperatures. Five or six degrees is enough
to change sea levels by around about 120 meters and it's enough to change the global climate from
a glacial period - an ice age -to a warm period an interglacial. So we've just jumped further back in
time. A nd then we start to transition into more brownie gravels and then finally here: this marks
the change between the sediment and the underlying basement rock of Wellington. And this is weathered
this basement here, but if you go further and further deeper back in time we eventually
hit some rock. So these sediments represent around about half a million years of Wellington's history
and we have this massive time break because these ones underneath were deposited around about 150 to
200 million years ago. So this is a time break between the underlying rocks and the overlying
rocks, and it's a gap of around about 150 to 200 million years difference. From those sediments
that we've seen today looking at Wellington's history we've had evidence of lakes, rivers,
movement of the land going up and down, movement of the ocean up and down, and we have evidence for
super volcanic eruptions coming from Taupo landing on Wellington surfaces over time.