cc >> Tom Zinnen: Welcome,
everyone, to Wednesday Nite at the Lab.
I'm Tom Zinnen. I work here at the UW Madison
Biotechnology Center. I also work for UW Extension
Cooperative Extension, and on behalf of those folks and our
other sponsors, Wisconsin Public Television, Wisconsin Alumni
Association, and the Science Alliance, thanks for coming to
Wednesday Nite at the Lab. We do this every Wednesday
night, 50 times a year. Tonight, I have the pleasure to
introduce to you Eric Carson. He's a fluvial geomorphologist
with the Wisconsin Geological and Natural History Survey, and
he's an assistant professor in the UW Extension Department of
Environmental Sciences. His current research interests
include Quaternary landscape development and fluvial systems
near forming glaciers, uh-oh, near former glacier margins in
the upper Midwest and intermontane west.
I take that to mean he studies the geology where the glaciers
drew back. >> Exactly.
The impacts of post-glacial climate change and historic land
use on sediment mobility, flood behavior, and land from genesis.
And applications of Quaternary geochronology to the glacial
record of North American mid-continent, which is right
here. He received his BS from West
Virginia University in 1996, his MS and PhD from UW Madison here
in 1998 and 2003, respectively, and was a lecturing professor at
San Jacinto College in Houston, Texas, from 2003 to 2008.
Please join me in welcoming Eric to Wednesday Nite at the Lab.
[APPLAUSE] >> Eric Carson: Thank you, Tom.
Thank you all for showing up. I appreciate the turn out and
the opportunity to talk about some of the research that I'm
doing here at the Wisconsin Geological Survey.
I was hired at the survey four years ago now, started in the
summer of 2008, and the Wisconsin Geological Survey has
had a long record of Earth surface processes research
relating to the glaciers, relating to the glacial record
of Wisconsin, which certainly isn't surprising given the
extensive record here, but haven't had much of a record
with having people with an interest in streams working in
the state. And so when I was hired the idea
was that I would be turned loose in the southwestern corner of
the state, the driftless area, which was never covered by
glaciers and allowed to do my own thing and study the geologic
processes there. So as a good geologist, I tend
to think of Wisconsin looking something like this rather than
the way we see it today. [LAUGHTER]
I'll wait and see if we can get a pointer instead of just an
hour glass symbol there. Okay, well, regardless, the idea
is if we go back in time, maybe 18,000-20,000 years ago, this is
the image of what Wisconsin would have looked like.
It was covered by the southern margin of the Laurentide Ice
Sheet. So the ice sheet that covered
all of modern Canada and extended down into northern
United States came down, and as it came down into the upper
Midwest and into Wisconsin, it divided into several sub-lobes
and partitions. And across Wisconsin, you can
see they're labeled here, the Superior, Chippewa, Wisconsin
Valley lobes, and then down in our neck of the state, we have
the Green Bay lobe and the Lake Michigan lobe.
And the Green Bay lobe you can see extended right down here
into the area of the Baraboo Hills and down to just south and
west of Madison. You can also see that it dammed
up Glacial Lake Wisconsin. But then you can also see this
large area here in the southwestern part of the state,
and this is the driftless area. Now, if we pull away the
glaciers and look at things as it is today, the driftless area
itself still stands out pretty clearly, pretty distinctly in
the landforms that we have generated.
If we look at it, you can still see the effects of the last
glacial cycle on the landscape. So here's the margin and behind
it the landscape is entirely derived from the glacial
processes that have gone on. Whereas, down in the driftless
area, it's primarily stream incision that's happened.
Now, in the previous slide we saw what the last glacial cycle
looked like with ice coming down primarily from the north and
from the northeast, but during the Quaternary period, during
the last 2.5 million years or so, there have been numerous
cycles of glaciations. And so during this time this
driftless area stands out in the Midwest as an area that has
escaped glaciation entirely.
We will restart later.. . [INAUDIBLE]
Okay, so we see a landscape that's very distinct
in the upper Midwest. We have upland surfaces that are
covered by a thick mantel of wind-blown silt that's blown in
during the glacial cycles. We have the Lower Wisconsin
River Valley that's incised several hundred feet.
This is taken from Ferry Bluff just near Sauk City, and it's
about 250 feet, I believe, from where this photo was taken down
to the river. And then in much of the Lower
Wisconsin River Valley, the valley itself was actually dug
out another 200-300 feet down and has filled up with sediment.
You can see the nude beach in the distance there.
If you want to look that way, you can see that.
[LAUGHTER] And in between these upland
surfaces and the deeply incised Lower Wisconsin River
Valley, we have a landscape that has a level of topography that's
not seen anywhere else in the Midwest.
And so if we sort of take a step back and look at the Midwest as
a whole, we can see how this area came to be and why it came
to be when we look at the record of Quaternary glaciations.
So on here we have the three major episodes of glaciations
that have occurred during the Quaternary period.
So in the gray area here in Iowa down through Missouri we have
what we refer to as the pre-Illinoian glaciation.
This was a cycle of glaciations that occurred starting about
2.5 million years ago and extending to about 500,000 years
ago. Primarily, that ice was centered
in western Canada and flowed down from the north and the
northwest into the upper Midwest.
Then from a period of about 300,000 years ago to 130,000
years ago we had what was referred to as the Illinoian
glaciation. So these deposits are shown in
gold, and there's our pointer. We can see that they come down
and clearly derive their name from being well expressed in the
state of Illinois and bound to the driftless area just down
here on the south and the southeast.
And then most recently we have the Wisconsinan or the Wisconsin
glaciation. So this is the entire area
that's shaded in green. And this was an ice cap that was
centered more in central Canada over towards the Hudson Bay
more. So the ice was actually flowing
down from the north and the northeast into the region.
And so when we see this, we see that the driftless is this one
spot that stands out that was never surrounded by ice all at
one time but at the different time periods, it has been
surrounded and it is the one area that has never been covered
by ice. Okay, if we look back at the
topographic map, what I'm going to do here is highlight the two
areas where I've been primarily working since I've started at
the Wisconsin Geological Survey, and these are the two areas that
I'm going to be talking about. First, I'll be talking about
some work here in Grant County. So the southwestern most county
in the state. This is the area where the
pre-Illinoian, or oldest, glaciations have budded against
the driftless area. So I'll talk about some of the
work that has been teasing out how we can learn about the
distribution of those deposits. And then back here closer to
home in Madison, right along this last glacial maximum
margin, work that I've been doing pinning down the exact
timing of the glacial record. Okay, so let's start down in
Grant County. So up in the left corner here we
have a DM of the county. Wisconsin River coming down
here, the Mississippi River flowing along here, and the area
that's covering the majority of the slide is highlighted here in
gold. I guess it's probably about 23
years ago now my PhD adviser, Jim Knox, was doing some work
down in this area, put in a core hole right here above the town
of Glen Haven. That core location is up on the
high bluffs, so right at the very top of the bluffs above the
Mississippi River. Much of the landscape here is
buried by wind-blown silt. So there's 15, 20, 25, as much
as 30 feet of wind-blown silt covering the bedrock.
When Jim put in this core, he found, I think my recollection
is, right around 28 feet of this wind-blown silt on top the
bedrock. And then right down at the base
right before he hit bedrock, he hit about two feet worth very
dense, dark gray clay that had flecks of minerals that aren't
found in the bedrock around here.
And what he interpreted this is as one of the pre-Illinoian
glacial advances. That this was a remnant of till
from one of these pre-Illinoian advances that was coming in from
the west from Iowa and Minnesota and actually topping up onto the
modern bluffs above the Mississippi River.
Not surprisingly, he wasn't able to find it in any other
locations. Partially this has to do with
the fact that back in the '80s the coring equipment that he had
didn't give him the ability to core many locations very
effectively and partly because if we're thinking of a glacial
deposit that may be a half million or a million years old,
there's probably not a very extensive record of it left.
Probably most of it has been eroded away.
So it might have been just good fortune on his part that he
found that deposit in that one location.
Now, when I showed up and began working, I had access to a
quicker, more portable coring mechanism, and so in the past
two years I've put in a whole series or cores here trying to
find more of this same deposit. And to give you an idea of how
patchy this material can be, here is Jim's original core.
He calls this his Adrian site because the Adrian family owns
the land. The first core I put in was
within a hundred feet of where his core was.
And when I put the core in, it went down to bedrock and there
wasn't any tilling in my coring location a hundred feet away.
So sort of an idea of how patchy this old material could be.
So I put in maybe two dozen, two and a half dozen, cores.
What you can see here is a very limited area right along the
bluff where I found several additional cores that had this
old till right at the base. So it's a really good indication
that we do have the presence of ice up on this surface.
Like I said, the fact that there are the blue squares which are
locations that there was no till, the fact that we're
finding those intermixed, like I said, isn't at all surprising
because the age of this till. One of the other interesting
things that I found in the course of this coring are these
two locations that are shown in gold, and rather than finding
till, what I found in those two locations was thick
accumulations of sand and gravel.
And that's exactly the kind of thing that you typically see out
right in front of a glacier. Glaciers that are in this
landscape position, in this part of the world, in this time in
the geologic history tend to have a lot flowing around at
their margin flushing a lot of water out from underneath them
and carrying a lot of sediment, and a lot of that is sand and
gravel, that this outwashes, what we refer to it.
And it was in some ways unsurprising but in some ways
very gratifying to be able to find it in these two locations,
further evidence of this glacial advance.
So it gives us an idea that we had ice here on the landscape
coming up. Now, the Mississippi River at
this time probably was not incised as deeply as it is now,
probably in somewhat a similar landscape position, but we still
have the good evidence of ice coming across what is today the
Mississippi and occupying just barely into Wisconsin with
outwash coming out. Now, when we compare this to
additional work in the driftless area, we see that it's not the
only pre-Illinoian deposit that we can find in Grant County and
in western Wisconsin. So this blue dashed line is
going to be the margin of the glacial deposits that I
identified. There are also additional
deposits right here in the mouth of the Mississippi River.
These are referred to as the Bridgeport Deposits.
These were identified by Jim Knox in the geography department
and John Attig in my office. It is also a pre-Illinoian ice
advance. In the case of the coring that
I've done, if I can get my pointer, the coring that I've
done in this area, the fact that the deposits are pre-Illinoian
are pretty much demonstrated by the fact that they're covered by
this thick accumulation of wind-blown silt, 25 or 30 feet
of silt that dates well back into the geologic record.
So that's what demonstrates the age of these deposits.
In the case of the Bridgeport deposits here in the mouth of
the Mississippi, there is additional information that
gives us a little bit better chronologic control.
So this is a cross-section looking along the Wisconsin
River. So looking from as far west as
about Sauk City, Wisconsin, right near the Baraboo Hills all
the way down to the mouth of the Wisconsin River at Prairie du
Chien. So we see a number of different
surfaces here. This the bedrock surface of the
Wisconsin Valley that's buried by several hundred feet of sand
and gravel. And we'll once again restart
later. [LAUGHTER]
We have the modern river surface shown here in the solid line
and then a series of terraces, a series of outwash terraces.
So surfaces built up of this sand and gravel that gets washed
out of glaciers. So this would have been from the
ice that was at the Baraboo Hills carrying sand and gravel
down the Wisconsin River Valley and producing these terraces
that are mirror images to the modern river topography, just
higher surfaces. But then here in the blue we
also have these Bridgeport deposits.
In the mouth of the river, there is an actual moraine.
There's a topographic landform which is exceedingly uncommon
for glacial deposits of this age.
There is a terrace associated with it, and there is outwash
deposits associated with it, and they actually, you notice that
these ones that are associated with the modern river grade
downward to the west. So they're grading downward in
the direction that the water is flowing.
This Bridgeport deposit grades in the opposite direction.
It grades downward to the east. So right here is the Bridgeport
terrace, grades down to the east, and what this tells us is
that when ice was at this Bridgeport moraine location it
was coming from the west, coming out of Iowa and Minnesota, and
was completely blocking the mouth of the Wisconsin River.
And, in fact, it was completely reversing the flow of the river.
And during this time, the river was flowing back to the east
back here towards the Madison/Sauk City area and in
some unknown direction flowing off by another root into the
Mississippi or perhaps flowing into the Great Lakes basin.
It's demonstrated by both the slope of the terrace and also
the structure of the sediments within the terrace tell us that
they were deposited by water flowing to the east.
As far as age control on this, Jim Knox and John Attig took
paleomagnetic measurements from the sediment.
So they measured the magnetism of the Earth's magnetic field
that was preserved in the sediments, and they have what we
refer to as a reverse polarity. So these were deposited, these
sediments were deposited prior to the last time that the
Earth's magnetic field reversed. So that occurred 790,000 years
ago. So these deposits of the
Bridgeport moraine and Bridgeport terrace are something
greater than 790,000 years ago. In order to have a soft sediment
landform like this Bridgeport moraine still expressed on the
Earth's surface is actually pretty intriguing to us.
What this tells us is we had multiple advances of
pre-Illinoian ice up against the western margin of modern day
Wisconsin. We have some pretty good
evidence that those two deposits, the deposits from my
coring up on the bluffs and the deposits from the Bridgeport
moraine, almost certainly were not the same age.
They were probably two completely different
pre-Illinoian ice advances. But what we're seeing is at
multiple times we had ice coming up and delineating this western
margin of the driftless area. Now, if we move to the opposite
end of things, if we move to the east edge of the driftless area
right here in the Madison/Sauk City/Baraboo Hills area, we're
up against the most recent ice margin.
We're up against the late Wisconsin ice margin.
And this is an area that's intriguing to us, first because
the landforms are much more present on the landscape so it
should be a much easier proposition to tackle.
It's also intriguing to us because despite the fact that
the Wisconsin deposits are the most recent, we have very poor
age control on the exact timing of events.
I told you at the beginning that the Wisconsin deposits are, in
general, something less than 30,000 years old, and we as
geologists have a ballpark understanding of the timing of
the late Wisconsin glaciation, but we don't have a very refined
understanding of it. And that's kind of interesting
considering that the history of studying glacial deposits here
in Wisconsin goes well back into the 19th century with TC
Chamberlin and Charles Van Hise here at the university.
And there's some very good reasons for it.
This quote up here is from a recent survey publication of the
glacial deposits of Wisconsin. This is by Kent Syverson and Pat
Colgan, and what they know is that the late Wisconsin maximum,
the timing of exactly when the ice was fluctuating isn't
constrained in Wisconsin because there isn't organic material
during that time period. And organic material, of course,
is important because radiocarbon dating for a half century now
has been our bread and butter tool for producing chronologic
control, for developing chronologic control.
And so in Wisconsin there simply hasn't been found any
significant amount of organic material during this huge window
that spans the peak of the Wisconsin glaciation.
Now, we can look at this in the larger picture if we look at the
radiocarbon data that are available.
So this is from a publication that Lee Clayton from our office
put together back in 2001. What he's gone through here,
he's gone through the published literature and he's pulled out
all the radiocarbon ages from large wood.
That's important because having large organic material gives us
some context to where it was deposited.
And he's plotted it up with age on the vertical axis.
So going from 12,000 to 15,000 years ago back to over 35,000
years ago. And then on the horizontal axis
he's plotted it in its position from north to south.
So here's the Illinois/Wisconsin state line, Illinois extending
off to the right, Wisconsin extending up to the left.
And as we move forward in time from, say, 35,000 years ago,
what we see is that the northern limit of organic material keeps
receding. It recedes farther and farther
south until a peak somewhere in the mid '20s.
And then this front of organic material begins advancing north
again. Now, the obvious interpretation
of this is that this is a reflection of the climate
changing and the ice advancing out onto the landscape, and as
the landscape begins to get covered with ice, clearly you're
not going to have organic material, certainly in the areas
that are covered by ice but also there's going to be an
additional fringe of land that isn't going to be supporting
much vegetation growth. We have quite a bit of geologic
evidence that during the late Wisconsin maximum here in the
upper Midwest there were permafrost conditions.
So the ground was permanently frozen year-round.
So this is a kind of condition where you wouldn't expect
organic material to be growing. So this makes good intuitive
sense to us but it's problematic because it crimps our ability to
understand the exact timing of when the glacier fluctuations
were occurring. Beyond that, there are a couple
issues with using radiocarbon dating in glacial environments.
Not only in the upper Midwest with the southern Laurentide Ice
Sheet margin but in many of the former ice sheet margins, the
permafrost conditions limit the amount of material you're going
to have. So there very often isn't a
whole lot of radiocarbon material, datable material, that
extends back to the maximum ice conditions.
In the cases that we do find organic material close to ice
margins in glacial settings, there are further complications
with interpreting the ages. The first of them is that a
glacial environment, a glacial landscape, that's having ice on
the landscape, having sediments deposited by the ice and by
water in front of the landscape, this is an inherently messy kind
of setting. There's a lot of water, a lot of
very saturated sediments, a lot of mobility of sediments and
mobility of the landscapes in whole.
There are a couple cases where geologists have been able to
document how long glacial landscapes tend to be mobile
after the glaciers recede, and in the published literature
there is cases where it's been documented that the landscape
has been mobile for hundreds to as much as 2,000-3,000 years
after the ice retreated away. Now, this is important to us
because what we as geologists are interested in is the time of
landform deposition because that's when the glacial action
is occurring. What the organic material and
what the radiocarbon dates are able to tell us is the timing of
landform stabilization. And so if we have a lag of
hundreds to thousands of years between the two, it's going to
crimp our ability to use radiocarbon.
The final one is because we have this lag in stabilization of
landscapes, because we have a spatial lag, often, between
where we find radiocarbon material and where the ice
margin was, it's very rare that we have cases that we find
organic material that we can correlate to exact ice margin
positions. So we can tell from the
geomorphology, from the landscape and the landforms that
are posited, we can tell where the ice was we just don't have
the ability to pin down an exact time for when the ice was there.
What this leads to is sort of some circuitous routes, I guess
is what you'd say for coming up with how and when the ice
margins were fluctuating. One of the very typical ways of
dealing with these issues of radiocarbon dates in glacial
environments is try to accumulate them and use them in
mass. So if radiocarbon dates can't
tell us exactly where an ice margin was, at least they can
tell us when a specific location was covered or uncovered by ice.
And a great example of that is the Two Creeks forest bed that's
up in east central Wisconsin, up just north of Sheboygan.
So this is within the geology community, within the
geomorphology and certainly radiocarbon community, this is a
very significant site. This is actually one of the very
first sites that was dated using radiocarbon dating.
Two Creek forest bed is a location up in Sheboygan.
It was well behind the ice margin when ice was at its
maximum, so the location was buried by ice, and following the
glacial maximum, ice was melting back and eventually melted back
far enough to expose the Sheboygan area and uncover it
from ice. And after it was exposed, a
spruce forest grew in the location.
And once this spruce forest had grown to maturation and was a
fully grown forest, the ice re-advanced and physically ran
over the forest. Up in Two Creeks you can find
places where there are mature tree stumps still in place with
the trunk snapped off. You can find entire tree trunks
mixed into the till. And so the radiocarbon dates
there give us very good control on when that location was
uncovered by ice as it melted back and when it was buried
again as the ice advanced forward.
What it doesn't tell us, though, is how far north the ice
retreated to expose it or how far south the ice advanced
again. Now, if you take numerous
locations like that, numerous locations where you have
radiocarbon dating that tells you a specific location was
exposed and uncovered by ice, you can start to marshal them
and get an idea in space and time what areas were covered and
what areas were uncovered by ice.
So we express this in the geomorphology world in a diagram
like this which is called a time distance diagram.
So this has age on the vertical axis, going from old up to
young, and then distance along the ice margin.
So in this case, we're looking from, say, Green Bay down to
Madison. And then the curve is the
generalized location of where the ice margin was.
Advancing we know that it had to back off to expose Waterloo and
Cactus Rock here. We know it had to back off
farther to expose Valders and the Sherwood sites.
Then there was an advance, there was a retreat to expose
the Two Creeks forest. So we can get the general
pattern of ice advances and retreats, it still doesn't give
us exact ice margin locations and times and that's actually
really crucial. It's crucial for us if we're
going to try to model the dynamics of the Laurentide Ice
Sheet, if we're going to understand how fast and what
mechanisms lead to its melting. It's crucial if we're going to
model climate, if we want to have firm starting points and
firm midpoints for climate change as indicated by exact ice
margin positions in space and time.
It's important for, say, biogeography.
If you're interested in the migration of plant species in
the landscape, it certainly helps if you can know when
advances and retreats were occurring and in a very detailed
sense. So refining beyond this has long
been an interest within the geomorphology community.
If radiocarbon isn't a possibility, if radiocarbon
dating is simply compromised in the upper Midwest during the
last glacial maximum, what we need to be interested in is
other methods in dating. And about the time that I showed
up at the Wisconsin Geological Survey, a few of my colleagues
and now collaborators had began exploring the utility of what's
referred to as optically stimulated luminescence dating,
or OSL dating. And OSL dating is certainly very
attractive in this kind of environment because OSL dating,
rather than dating organic material, the life and death of
organic material like radiocarbon does, OSL dating
dates the amount of time that a sand grain has been buried away
from the sun. And so certainly and environment
like this an any glacial environment, certainly in
Wisconsin we have plenty of sand around.
And so if you can measure the time that these sand grains have
been buried, then that's going to get around the problem of not
having organic material. Now, the fundamentals of OSL
dating are up here. I'll run through it really
quickly. The idea is that mineral grains,
particularly quartz grains, tend to accumulate electrons in their
crystal lattice that are released by the decay of
surrounding radioactive elements.
Now, in any sedimentary package, you have a sand bar on a river,
there's going to be quartz grains, there's going to be
grains of other mineralogies that are going to have
radioactive elements. Thorium, potassium, and uranium
are the most common, and so as a sand grain that's buried in a
sand bar, buried in a dune or wherever, it's going to be
surrounded by these radioactive elements and the longer it's
buried, the more it's going to accumulate these electrons.
And so if we measure the amount of electrons that have been
accumulated in a sand grain during the time since it was
last buried, it's going to give us an indication of how long it
has been buried. The measurement of that
accumulation of energy is actually the optically
stimulated part of it. Optically stimulating is what
they do to the sand grain in the laboratory setting.
We need to know a couple things. We need to know the dose rate,
so that's basically how much uranium, potassium, and thorium
are in the surrounding sediment. So that can be relatively easily
measured with a salinometer. We also have to have to have
some confidence, and this is an important one for our research,
we have to have some confidence that this sand grain has been
what we called zeroed out, and what that means is that all the
electrons that accumulated in it were flushed out of it prior to
its most recent burial. What we know is that short
exposure to direct sunlight, something on the order of 15 to
20 seconds of exposure of a sand grain to direct sunlight, will
flush its crystal lattice clean. So this is interesting if you
think of the life cycle of a sand grain that it can be buried
and exhumed and transported and buried and exhumed and
transported multiple times over. And as long as the last time it
was transported it was exposed to enough sunlight to flush out
all the previously accumulated electrons, then it's going to
give us a good date for how long it was buried that last time.
Now, within the OSL dating community, OSL is a relatively
new methodology. It was originally conceived and
developed 35 or 40 years ago. It's been about the last two
decades that it's been in common use, and the way to deal with
this has been traditionally to work in geologic environments
where you can intuitively demonstrate that the sand grains
were transported far enough to be flushed clean.
The way that's been done has primarily been working in areas
where the sand was transported in the air column, transported
wind-blown. So, intuitively, you can see if
a sand grain was carried up in the air column it was going to
be exposed to enough sunlight. What we're doing is starting to
press the boundaries of this, starting to work in different
geologic environments and address whether or not OSL
dating can be used and address whether we can verify whether
the sediments that we're going to be interested in have been
flushed clean and are going to be viable dating methods.
As a general rule, OSL dating is applicable and time ranges from
about 300 years as a minimum, that's how long it needs to be
buried to begin accumulating a measurable amount of electrons,
up to about 100,000 years. So that's when the sand grain
becomes saturated with electrons and can't accumulate any more.
So that's the dating range. That fits perfectly into the
late Wisconsin time frame. So it's the ideal tool in the
respect for using. The methodology, the location
and the setting that we're applying this, is in sand that
was deposited in lakes right on the ice margin.
So this is a different setting from the wind-blown environments
that have typically been used, but it's got some great benefits
for us with dealing with the Wisconsin geologic record.
The first application of it, this is work that was started
right when I showed up, we had this published last year in the
journal Geomorphology, was using OSL dating in two small lake
basins right here in the Baraboo Hills.
And so these are both up high on the Baraboo Hills.
They're not down in Devil's Lake.
They're actually up high on top the quartzite.
Let's zoom in on that. So here we have the Baraboo
Hills. The red line on here is the ice
margin. So everything to the east of
that was covered with ice, and then we zoom in and the ice is
sort of shaded in white in the larger image.
And you can see that very convoluted ice margin in the
Devil's Lake area. So ice coming in, blocking the
Devil's Lake gorge to the north, blocking it to the south and
looping around, and there was this area in here up on top the
Baraboo Hills and this area over here on top the Baraboo Hills
that were ice free. And in these two locations,
Feltz Basin right here with this large lake shown and South Bluff
Basin here with this smaller lake shown, there were two lakes
that were formed precisely because the ice was there.
These were small valleys up on the quartzite that normally
would drain down into the lowlands down to the Wisconsin
River, but during last glacial maximum time, those drainages
were blocked by the presence of ice.
And so lakes formed and those lakes only existed while ice was
at its exact maximum. As soon as the ice started
thinning and retreating back to the east, those lakes drained
and no more sediment accumulated in it.
So the idea for us is that this is a perfect environment where
if we can derive reliable OSL dates from these, these dates
can be unambiguously and entirely solidly correlated to
the glacial event that happened to the ice margin position.
And so with numerous dates from the Feltz Basin area, numerous
dates from the South Bluff area, all of them converged to an
average age of 18,500 years ago. Certainly this was the first
case published of OSL dating in a glacial environment.
Certainly it was one of the very first numeric controls on the
timing of ice fluctuations in Wisconsin.
So it was certainly significant work in both those regards.
What does KA stand for? >> KA is thousands of years ago.
So 18.6 KA is 18,600 years ago. Sorry.
When we work with dating techniques in the geologic
realm, we're very often concerned with two issues:
accuracy and precision. So accuracy is how well does the
date we have correlate to the geologic event?
And the precision is how tightly does it constrain it?
How much error is involved in it?
And one of the failings that we have with OSL dating right now
is that the, certainly with this work we weren't able to test the
accuracy portion of it. It was unsupported by other
dating techniques, which was a worry to us but something we
weren't able to address. The second is that the precision
is relatively poor for OSL dating.
If we had, with this date here of 18,600 from the South Bluff
Basin, if we had radiocarbon date of 18,600 year, the lab
error on that would probably be, today with AMS dating, would
probably be 18,600 plus or minus 60 years.
That's how precise radiocarbon dating is.
With OSL dating, currently the errors associated with it are
plus or minus 10%. So 18,600 plus or minus 1800 or
1900 years. >> What is OSL dating?
OSL is dating how long the sand grain has been buried, how
long ago it was deposited. >> What is OSL stand for?
Optically Stimulated Luminescence.
Thank you. >> You're welcome.
When we're looking at this in the big picture, we wanted to
address both of these. We wanted to try to address the
accuracy issue to determine whether OSL dating can be
verified to have reasonable accuracy and hopefully through
laboratory techniques deal with the precision as well.
And since we found several lake basins right here in the lower
Wisconsin River Valley, this area is actually quite fertile
in having lake basins associated with the ice margin position.
So from previous mapping that had been done by my colleagues
at the Geological Survey, we're aware of a whole host of small
lake basins that exist on the landscape.
We have several of them up here that are part of Glacial Lake
Wisconsin. We have a whole series of them
that are out in front of the maximum ice margin position down
along the Lower Wisconsin River. There's a few extra lake basins
that are behind the maximum position.
So we have lakes in a variety of settings with a variety of
different mechanisms forming them that we can continue to
test the OSL dating mechanism. That's what we've been doing the
last year or two. And for the rest of this talk
I'm going to focus in on these two locations.
So this is Black Earth Creek flowing from the Middleton area
down into the Lower Wisconsin River, and this northwestern
site I'm going to refer to as Marsh Valley.
It is in Marsh Valley. And then this southeastern site
I'll refer to as the Swamp Lovers site or Swamp Lovers
Valley. It's in an unnamed tributary
that much of the land is an LLC by the landowners.
And they love swamps so they named it the Swamp Lovers LLC,
that owns their land. So Swamp Lovers site or Swamp
Lovers Valley is what I'll be referring to.
And what I'm going to do is you're going to be seeing that
map area there quite a few times.
You'll be seeing it with geologic maps, you'll be seeing
it zoomed in with topographic and digital elevation models.
So kind of keep that in mind of where we are in the bigger
picture with the Wisconsin River here in the northwest corner and
the late Wisconsin maximum here on the eastern boundaries of it
with Black Earth Creek cutting across the sort of central
portion of that area. So let's take a look at this
from the geologic perspective. One of the primary tasks that
our office works on is creation of geologic maps.
So this is the surficial geology map that was created for Dane
County. So this shows all the surficial
deposits in the county, mapped by two of my colleagues.
And this is the area that we are looking at.
And just to give you sort of a big picture idea, basically
everything in the greens are the glacial deposits and everything
in the washed out sort of whitish buff color with the
oranges and pinks, those are stream deposits and the deposits
in the eastern most driftless area.
So we're seeing right up against this margin of the glaciated
train against the driftless area.
So let's zoom in on here and talk about the Marsh Valley site
and the Swamp Lovers site. Here's Marsh Valley right here.
During their mapping they actually identified this as a
location that would have had a lake in it.
There's not a lake currently, but these are lake deposits that
are shown in, oops, excuse me, that are shown in blue.
Here in the Swamp Lovers Valley, they identified that there were
similar deposits just probably not extensive enough for them to
map. So they knew that there were
lakes there, and the idea of why there were lakes in these two
streams is because both of them drain into streams that carried
outwash, carried that sand and gravel being flushed out from
the glaciers. So Marsh Valley drains directly
into the Wisconsin River, Swamp Lovers Valley drains down into
Black Earth Creek, and you need to imagine that during glacier
times this outwash was coming down Black Earth Creek, it was
coming down the Wisconsin River, and it was actually filling up
and choking those valleys. They were what we call
aggrading. They were filling up with sand,
and as they were filling up, it was damming up Marsh Valley, it
was damming up Swamp Lovers Valley, it was damming up, this
was Dunlap Creek up here. It did the same thing to it.
We focused in on these two for a very specific reason, though,
because not only were they dammed up like that by the
outwash on the main stream, but they were also in a position
where the ice at its very maximum position, at its very
maximum extent during the late Wisconsin, got just barely into
their headwaters. So we can see the green off on
the eastern part of the map area.
And here we have the headwaters of the Swamp Lover Valley.
Here we have the headwaters of the Marsh Valley, and the ice
extended no more than a mile or a mile and a half into those
valleys. So it was only when the ice was
right at its maximum extent did it get into the headwaters of
those valleys. Why is that important?
It's important if we think of the processes that were going on
in those two valleys. And so this next set of maps
that I'm going to show are a conceptualization of how
sediment was accumulating in these two valleys.
And these actually date back to the time when John Attig and Lee
Clayton mapped this area. This is how they conceived that
sediment would have been accumulated.
So here we have that same map area I've been showing.
Wisconsin River up here, Black Earth Creek flowing off to the
west, and this is a time slice taken while ice was advancing to
its maximum. So we can see the ice up here in
the northeast corner of the map, and we had outwash coming down
both Black Earth Creek and down the Wisconsin River damming up
lakes in our two sites. So a lake here and a lake here.
And then if we step forward in time, we get to when ice was at
its maximum and it was in the headwaters of Marsh Valley and
Swamp Lovers Valley. And because ice was in the
headwaters of those valleys, water was coming down and
outwash was coming down those valleys.
And the outwash was actually being deposited on top of those
lake sediments. And then as soon as ice began to
retreat, as soon as it backed off no more than a mile or so,
it got out of Marsh Valley, it got out of Swamp Lovers Valley,
and the supply of outwash down these valleys was cut off.
But there was still outwash coming down Black Earth Creek,
there was still outwash coming down the Wisconsin River, so
they continued to aggrade. They continued to fill up with
outwash. And so once ice began to
retreat, lakes were reestablished in those two
valleys. Now, that's the conception that
they had from doing their mapping, from understanding the
geologic processes. They never had means or reason
to go out and test it by coring. Step forward 20 years in time to
now when I'm working with the Survey and we're interested in
this OSL dating, all the sudden there is a reason to do it.
And so out we went to go and core.
This is us with what's called a geoprobe coring equipment.
The track vehicle on the left. It's a direct push system from
with hydraulic pressure it pushes a core barrel down into
the Earth and retrieves about an inch and a half diameter core
barrel. Really nice, really quick coring
unit that we have access to. You can see here we're certainly
in the dead of winter, and this was previous winter when we
actually had a winter and had snow and that kind of thing.
[LAUGHTER] This is the Swamp Lovers site.
We had to do it in the winter. We have a short window at the
tail end of the summer to do this kind of coring in the sites
because the ground is either frozen or that tiny window when
it's dry. Otherwise, this is a wet swamp
that we couldn't get this coring rig out on to.
So we took advantage of the cold weather back in February of
2010, put in some cores to verify what the stratigraphy was
like, to see if it was like what had been conceived there and if
it was going to be a good setting for radiocarbon dating.
Collected cores, I want to tell you a little bit about what we
found in them before I actually show you the stratigraphy, show
you the layers of sediment in them.
We were pleasantly surprised at the volume of material, what we
refer to as paleoenvironmental proxies, so indicators of what
the conditions were like. We found abundant and diverse
assemblages of pollen, of phytolith, which are plant
remains, and also of diatoms, which are algae that lived in
the water column. That's what's shown on the far
side of the image. Those are freshwater diatoms.
All of those are very nice for us because they give us
indications about what the climate, the water conditions,
the water temperature, the salinity.
So they started filling in and flushing out the story of what
the conditions were like while this lake was in existence.
What we also found that really shocked us and put big smiles on
our face for weeks after finding it was an incredibly rich record
of organic material in this lake.
What is truly and honest to god, I know we as scientists love
to inflate the value of our work...
[LAUGHTER] Truly and honest to god,
an unparalleled record of organic material for radiocarbon
dating in the upper Midwest. From top to bottom in this lake
in these cores we found plant macrofossils, identifiable
pieces of aquatic vegetation. We found both terrestrial and
marine gastropods, snail shells that were living either in the
freshwater column or living on land and then getting washed in.
And we found them from the top of our core all the way down to
within a few centimeters of the base of the core.
So stunning find in and of itself to have all that.
So let's look at the stratigraphy.
So this is a column showing what the layering of the sediment
looked like. Like I said, it is a marsh today
in the Swamp Lovers site. So we found several meters of
marsh sediment, organic, rich mats of vegetation and fine
grained sediment mixed together. Beneath that we found right
around four meters of laminated, layered, silty clay.
Exactly what we'd expect from a lake.
So lake sediment. And on top of that we found
outwash, sand and gravel, beneath that we found yet more
lake sediment. So we found exactly what we'd
expect to find based on the understanding of the setting
from the mapping. This throws us for a loop when
we actually verify the things we think.
[LAUGHTER] But we're going to go with it.
In the very small red dots up here in this upper lake
sediment, these are four of the organic samples that I've had
radiocarbon dated. So you can see that they go
entirely through that lake section from right at the top
where it transitioned into a marsh all the way down to the
bottom. So this is the point right here
where we transition from sand and pebbles back into a lake
when ice was retreating. So all the sudden, literally for
the first time in the upper Midwest, we have a location
where we can easily identify what was going on as far as
glacial processes and all the sudden we have radiocarbon
material to give us a control. And this is over and above
everything we were originally looking for which was whether
we'd have good sediment for OSL dating, which, sure enough, we
have. So we have a sample here for OSL
Dating, two samples from here at the bottom in the outwash for
OSL dating. This is ideal for us if we're
interested in accuracy and precision.
All the sudden we get the precision of the radiocarbon
date, we also have the cross-verification of the
radiocarbon and the OSL dating to tell us whether or not the
accuracy is there. So this is perfectly thrilling
for us. If we look in a little bit
bigger setting here, this is a cross-section going up the
length of the valley from northeast to southwest.
This is what we found from multiple coring locations.
So here's a location up at the head of the valley that went
through a similar stratigraphy. Here's our core shown on the
right that went down through the outwash and into lake sediment.
And we actually weren't able to retrieve samples, but we
penetrated all the way down to the bedrock at the bottom of the
valley. And then down at the southwest
end, there's outwash that was piling up in the Black Earth
Creek. And that visually has a
different characteristic to it. Much larger material, different
mineralogy in it. So visually recognizable as
being different outwash than this that was coming down the
valley. And then although it's subtle on
this diagram, we've got the flat surface of this marsh, this
former lake basin, and right down at the mouth where it
empties into Black Earth Creek there's a little rise here where
the outwash at Black Earth Creek had piled up, and that's
actually there. You can drive out on highway 14
and just on the east side of Cross Plains, you can actually
see that. There's a house on it, and you
can actually see that it drops down a couple feet into this
Swamp Lovers Valley. So exactly identical to what we
had expected. If we look at our dates, we have
a series of dates. So I'll go through these from
top to bottom. With radiocarbon dates we have
an age of about 7700 calendar years BP when this lake
converted over to a swamp. We have at the other end of the
lake section we have dates that are right around 17,100 calendar
years for when the lake was established.
So this lake was established then.
That gives us good radiocarbon control on when the ice was
retreating. It also tells us that this lake
was in place on the landscape for something on the order of
10,000 years before it rolled over to being a marsh.
The OSL dates we have, and these dates you can see are converted
to calendars. You can see how tight the error
is. This has an error of, what is
that? 170 plus 450, so about 600 years
is the total error on this laboratorian conversion to
calendar years. The OSL date, this is 21,400
years plus or minus 3300 years. So a much larger spread
consistent with typically what we find with OSL dating and
particularly what we're finding with OSL dating in lake
settings. Down here we have an age
centered on 23,000, 23,500 years BP and 26,500 years BP.
Because of the errors, these three dates are actually
statistically indistinguishable from one another.
We can't separate them, but what we can say are a couple things.
We can say with confidence that the ice was at its absolute
maximum position probably for a relatively short amount of time
centered around 26,000 to 21,000 years BP.
And we can also say that all of the dates, both radiocarbon and
OSL, show complete internal consistency.
As you get lower, you keep getting older and older ages
which is exactly what we want to see.
This is wonderful for us for our confidence for using OSL dating.
It's wonderful for our empirical knowledge of glacial processes
to now have these radiocarbon ages.
If we look at this setting, we now have an age for 17.1 KA,
17,100 years ago, when a lake had been established here.
We can go to the Marsh Valley site.
So this is a view of it. This is just an absolutely
gorgeous little lake setting just east of Mazo.
So nice, flat lake plain setting that is now being cultivated.
If you're out there in this little lake basin, you can
actually look around it and all the way around it you can see a
nice little break-in slope that was the former beach surface.
So you can still see the morphology of this lake.
This lake was actually in existence for quite a bit
longer. It was established by about
20,100 calendar years ago based on, again, abundant radiocarbon
dating from within it. And it existed as a lake until
about 1500 years ago. So this lake was in place on the
landscape for something on the order of 18,500 years.
Okay, if we take the combination of all these, if we take the
Baraboo OSL ages, if we take the OSL ages from the Swamp Lovers
site, and the radiocarbon ages from the Swamp Lovers site and
the Marsh Valley site, we can plot them up on here and see
what kind of progress we're making of getting into that
window of no age control during the late Wisconsin maximum.
And this is what we get. So here's our radiocarbon age
from Swamp Lovers. The, oops, excuse me,
radiocarbon dates from Marsh Valley are the two lower red
dots. This is the average OSL date
from Swamp Lovers, and this is the OSL age from Baraboo.
Put them all together, they actually have, as a group of
three different locations that are under similar conditions,
they have very nice consistency with the ages.
They're going into this wilderness that we've had
existing for a century now of no age control.
So this is something that is simply thrilling for us as
geologists working in this state.
Now, if we pull back up a step to wrap up here, and I showed
all these different locations, this has us really excited,
excited on a number of fronts. In retrospect, looking back,
it's not all that surprising that we didn't find any
radiocarbon datable material here in these two sites.
These were up extremely high on the landscape, would have been
incredible hostile environments during the Wisconsin maximum.
They were also up on the Baraboo quartzite, which in general is a
tough place for stuff to grow because that bedrock is so
inert. So it's not surprising to not
have found organic material here.
To have found such an abundance of organic material from here in
Swamp Lovers and here in Marsh Valley really gives us
confidence as we start coring into all these other lake basins
that are shown here that they're going to have a high probability
of also having radiocarbon material.
So we're going to build what is truly, it's not world class
because we're only talking North America here, but truly a
continental record or glacial events that has no parallel as
far as its chronologic control. The nice thing is when we look
at these, these are all tied into different glacial events as
well. So not only can we date that
last glacial maximum that I've been talking about, but there's
a series of events that we can date as well.
I mentioned these three lake basins up here are part of
Glacial Lake Wisconsin. So mapping and previous research
into Glacial Lake Wisconsin indicates to us that it probably
existed multiple times going back prior to the last glacial
maximum. So OSL dating in those basins
could give us information about ice advances prior to the last
glacial maximum. We have a number of different
lake basins here in the Sauk City area, right near Badger
Ammunition Plant. There are related to Johnstown
position, they're also related to recessional Elderon position
because these were dammed up by outwash deposited by the
Johnstown ice, and then they drained when ice was receded to
the Elderon position. So they formed because of the
Johnstown position, they drained because of the Elderon position.
We have this lake here in the East Baraboo Basin that was only
dammed up when ice was at this recessional position that we
call the Elderon phase. We have lake over here.
This is Glacial Lake Middleton. It only existed when ice was at
this recessional phase that we call the Middleton phase.
So we have a whole range of different timing questions and
chronology questions that we can address with this.
Really high level of confidence, really high level of excitement
on it. By the time we're done with
this, particularly if we get the amount of radiocarbon material
that we're expecting, we'll have fully confirmed the viability of
OSL dating in these environments.
And so moving forward from that, we can actually expand back out
and start looking across the entire margin of the southern
Laurentide Ice Sheet and begin dating large-scale events.
And with that, I'd like to thank you, and I'd be happy to take
any questions. [APPLAUSE]