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

Marc SerravezzaDecember 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 ResearchWhy 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

BackgroundWhat 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 Sites1) Lower Paint Rock Lake3) Trigger Lake

2)Lower Medecine Lodge Lake4) Bear Lake

21

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/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…

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

23

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)

ReferencesLynch, 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.