Download Holocene Climate Change Interpreted from Lake

Holocene Climate Change Interpreted from Lake-level
Reconstructions, Bighorn Mountains, WY
Marc Serravezza
December 3, 2010
Fundamentals of Research
Outline for today’s talk
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Motivation for research: Why study Holocene climate change?
Brief background: What do we need to estimate paleolake-levels?
Study site: the Bighorn Mountains!
Methods and Results: What we will produce and how we will do it.
Motivation/Relevance of Research
Why study Holocene climate change?
Retrieved from www.usgs.go v
• 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.
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
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Proposed Research/Expected Results
Here are the results that we expect to generate during completion of this
project:
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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).
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.
A time-line of lake-level changes throughout the Holocene for each lake.
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
Shuman (2009)
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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.
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.
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/cm3)
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…
Courtesy of J. Marsicek
…we can correlate the facies along our transects, extending from the current
shoreline out to the lake’s depocenter.
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…
3) Time-line of Holocene lake-level changes
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).
Thus, the end result is a Holocene time-line of lake-level changes, produced
for each lake.
4) Contemporaneous changes in lake-level with previously studied sites
Analyzing for past climate change during the Holocene
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)
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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.
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.