varves annually laminated lake sediments

VARVES

Annually laminated lake sediments

Varve Formation

Varve Preservation

Varve Analysis

Sample Preparation

Analysis Techniques

Paleoclimatic Information

Why Bother?

Paleoclimate data provides a baseline record of climatic variability in which we can evaluate current climatic regimes and to understand how climate might change in the future.

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What Causes Varve formation?

• Annual cycles depositing variable sediments= couplets

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Seasonally Variable Components Mechanical Varves• Variation in clastic sedimentation from seasonal

runoff– In Northern environments runoff seasonality is

associated with snow melt– In Tropical environments runoff seasonality

controlled by Monsoonal circulation or other factors controlling the rainy season

– In arid environments clastic layers might represent flood events rather than annual sedimentation because of a lack in rainfall

Biological VarvesVariation in seasonal biological activity

– Diatom varves

Carbonate varves

– Carbonate layers precipitate out of the water column caused by decreased dissolved CO2

– results from increased water temperature

– or increased photosynthesis

Evaporites

– Annual changes in lake level or lake chemistry causing precipitation of minerals

• Varves from Eastern Finland, • light layer = spring flood mineral layer,• dark layer=organic summer-winter layer.

Combinations of annual processes

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Varve Preservation• Absence of Bioturbation

– Reduce the impact of digging animals– Lack of or strong seasonality in

benthic O2

• Stratification of the water column– No mixing to stir sediments

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Basin DynamicsWhere do we find the above conditions?

1) Protected, Deep, Small surface area

2) Large and very deep with stratification of the water column

3) Small lakes that the sedimentation rate exceeds the rate of turbation

Lake Malawi is one of the world's largest and deepest lakes

Shananon dissertation 2006

Varve Lake AnalysisHow do we extract paleoclimatic information from

laminated sediments?

1) Get core

2) Split core

3) Generate age structure

4) Subsample core

5) Analysis

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Split core

marine.rutgers.edu

Develop Age Structure

• Count layers

• Lead 210

• Radiocarbon

• Others?

Preparation

• Slice out a slab from the core half• Freeze dry frozen samples. • Use acetone to remove water from wet

Samples. • Then use resin to remove the acetone and set

sample into a resin puck.

Types of Analyses

Varve Thickness• In clastic varves, varve thickness can be used

to reconstruct past hydroclimatic regimes • Caution: varve thickness can vary over

decadal to century time scales--changing sediment discharge from rivers caused by channel migration or land use change

• In biogenic varves, varve thickness can be used as a proxy for biological productivity--inferences in temp etc.

• Evaporites:

Pollen Analysis

• Potential for high resolution analysis of vegetation change

www.biologie.uni-hamburg.de

remf.dartmouth.edu

Digital image analysis

1) Take a digital image of the core

2) Graph the color density

3) Calibrate this to a climatic variable if you find a match

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Scanning Electron Microscopy (SEM)

Back-scatter electron imagery

Useful for resolving physical and biological components of the annual cycle of sedimentation

Potential for identifying inter-annual microlaminationsSeasonal faunal successions in laminae less than 100µ thick

Scanning XRF Analysis

XRF=X-ray Florescence

Elements from Na-U

XRF allows fro the identification of changes in elemental abundance on 20-100µ resolution

Measurements can be made on clean wet surfaces or resin imbedded pucks

Primary x-ray source the electrons in the elements. The sample then emits (florescence) x-rays with wave lengths characteristic of the elements in the sample.

Shananon dissertation 2006

What can we learn from all this?

• Grain size can provide insight into past runoff intensity

• Organic matter can be used to determine strength and length of the primary production season

• Diatom species can be used to reconstruct past temperature regimes

• Pollen can be used to reconstruct past vegetation regimes

Calibrate varve record to instrumental record

Shananon dissertation 2006

Shananon dissertation 2006

Questions?