Holocene Climate Change Interpreted from Lake-level Reconstructions, Bighorn Mountains, WY

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22 févr. 2014 (il y a 3 années et 1 mois)

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Holocene Climate Change Interpreted from Lake
-
level
Reconstructions, Bighorn Mountains, WY

Marc
Serravezza

December 3, 2010

Fundamentals of Research


Outline for today’s talk

I.
Motivation for research: Why study Holocene climate change?

II.
Brief background: What do we need to estimate
paleolake
-
levels?

III.
Study site: the Bighorn Mountains!

IV.
Methods and Results: What we will produce and how we will do it.


Motivation/Relevance of Research

Why study Holocene climate change?


Water resource planners are very interested in quantifying the possible effects
of future climate changes on their state’s or municipality’s water supply.


By studying
contemporaneous
changes to lake levels caused by climate
changing events during the past, we
can
attain
a better understanding of the
effect that a similar event would have on our water supply in the future.



Retrieved from www.usgs.go

v

Background

What
do we need
for estimating
paleolake
-
levels?


We need a lake with the following characteristics:


Small lake (<50 hectares) located at the outlet of a small catchment.


Shallow and wide (<10 m deep).


Minimal surface water inputs/outputs; a ‘quiet’ hydrology dominated by
groundwater flow.


Solid, homogenous bedrock under basin sediments.


These characteristics help us isolate climatic inputs to lake level change from
local inputs, as well as…

…allow for the preservation of lacustrine
facies
:









Specifically, we need to observe
nearshore

lacustrine
facies
:


Forms in the shallow waters near the lake’s current shoreline, due to wave
action and currents.


Contains relatively coarse sediment, minimal organic content, and a higher
bulk density.


Contains macrofossils unique to shallow freshwater environments; e.g.
nymphaea

(lily pads) and
alisma

(water weeds).


Relatively low sedimentation rate.


Facies

should extend nearly continuously around the lake.

Proposed Study Sites





Primary Sites

Alternate Sites

1)
Lower Paint Rock Lake

3) Trigger Lake


2)Lower
Medecine

Lodge Lake

4) Bear Lake


2

1

3

4

Proposed Research/Expected Results


Here are the results that we expect to generate during completion of this
project:


1.
3
-
D lake basin profiles
, using ground penetrating radar (GPR), for both Lower
Paint Rock Lake and Trigger Lake (Lower Medicine Lodge Lake and/or Bear Lake if
alternate sites are selected).

2.
Nearshore

facies

correlations
along a transect for each lake identifying past
landward or
lakeward

shifts of the shoreline.
Facies

identification will be based
on laboratory analysis for grain size distribution, organic content, bulk density,
macrofossil content, and sedimentation rate.

3.
A time
-
line of lake
-
level changes
throughout the Holocene for
each lake.

4.
Analysis of contemporaneous changes in lake
-
levels with previously studied
sites

within the Northwest Rocky Mountain region, indicating past climate
changing events during the Holocene.



1) GPR profiles

We can generate 3
-
D GPR profiles fairly quick (within 1 to 2 days), and thus
analyze them immediately in the field for:


Lake basin characteristics


Potential
nearshore

facies

and
onlap
/
offlap

sequences


If the GPR images look good, we can continue with collecting sediment
cores along a transect on that lake.


If the GPR images look poor, we can move on to one of the alternate lake
sites.


Light and dark contrasts indicate boundaries between
sediments with different dielectric constants.


Thickness of bands gives an idea of sedimentation
rate


Light colored bands
may

correspond to
nearshore

facies
.

Shuman (2009)


2) Analyses of Sediment Cores /
Nearshore

Facies

Correlation

Transects of sediment cores will be collected with a 70mm


piston corer, and taken back to the
Shuman Lab

for analyses…

Nearshore lacustrine
facies

will be identified based on:

1.
Coarse sediment content (>63µm)


measure wt. % of subsample (+/
-

0.1%)
after wet sieving and drying.

2.


Organic content


measure % loss on ignition (+/
-

0.1%).

3.

Bulk density


measured by gamma ray attenuation (+/
-

0.1g/cm
3
)

4.


Sedimentation rate


measured by radiocarbon dating of sedimentary
charcoal.

5.
Macrofossils



identification of plant macrofossils unique to shallow freshwater
environments.



Now, with the
nearshore

facies

clearly identified…

The final correlation produced for each lake will indicate both magnitude
and direction of past shifts in the lake shoreline.


It should agree fairly well with observations from the 3
-
D GPR profiles…

…we can correlate the
facies

along our transects, extending from the current
shoreline out to the lake’s
depocenter
.

Courtesy of J.
Marsicek


3) Time
-
line of Holocene lake
-
level changes

Thus, the end result is a Holocene time
-
line of lake
-
level changes, produced
for each lake.

Each shift of the shoreline corresponds to a change in lake
-
level. We can then
bracket the time that these changes occurred using
radiocarbon dating of
sedimentary charcoal
(
ubiquitious

in western lacustrine sediments).


4) Contemporaneous changes in lake
-
level with previously studied sites
Analyzing for past climate change during the Holocene

We will isolate climatic inputs to lake
-
level change from local inputs by correlating
with multiple lakes in different watersheds, but within the same climate region.


This should provide us with an improved understanding of climate change within
Wyoming during the Holocene.

1

2

3

4

5


Previously studied sites:

1
)

Foy Lake, MT (Shuman et al.,
2009);
2)

Park Pond, WY (Lynch,
1988);
3)

Bear Lake, ID
(Rosenbaum, 2010);
4)

Long
Lake, WY (Shuman et al., in
progress);
5)
Hidden Lake, CO
(Shuman et al., 2009)

References

Lynch, E.A. (1998). Origin of a park
-
forest vegetation mosaic in the Wind River Range,

Wyoming
,
Ecology,
79,1320
-
1338
.

Rosenbaum, J.G. (2010).
Paleo
-
lake levels of Bear Lake (Utah/Idaho): a record of Holocene

aridity
.
2010 GSA Denver Annual Meeting
(31 October


3 November 2010
).

Shuman, B., Henderson, A.K., Colman, S.M., Stone, J.R., Fritz, S.C., Stevens, L.R., Power, M.J.,

and
Whitlock, C. (2009). Holocene lake
-
level trends in the Rocky Mountains, U.S.A.

Quaternary
Science Reviews,
28,1861
-
1879.