sb lakes intro pt1

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Lakes an introduction

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Page 1: Sb lakes intro pt1

Lakes an introduction

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Kettle Lakes in Glacial Outwash

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Ephemeral lake on the Great plains

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Major Lake Types• Volcanic lakes

– basaltic so relatively unproductive because nutrient concentrations are low.

– Two types: • Maars - small depressions in

craters of explosive origin resulting from lava coming into contact with groundwater or degassing of magma;

• Caldera - formed by collapse of the roof of partially empty magma chamber.

Big Soda lake, NV

Crater Lake, OR

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Major Lake Types

• Tectonic lakes – basins formed by

movements of the earth’s crust.

• Ex. Lake Tanganyika• Ex. Eagle lake, CA

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Major Lake Types• Glacial lakes

– most common lake origin due to erosion and deposition associated with glacial ice movements.

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II. Lakes as Archives of environmental change

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Direct Precipitation

Runoff

What comes in? Water

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Organic matter Sediments

Pollen

Material from the landscape

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Organic matter Sediments Biota

Material produced within the lake

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Two primary environmental or climate changes

• Precipitation/Evaporation• Temperature

That may initiate some response in the environment that may be recorded by lakes

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Possible lake responses to climate• Physical

– Change in water balance and lake level– Change in sediments delivered to lake

• Biological– Diatom assemblages– Ostracode assemblages– Insects, e.g. chironomids

• Chemical– Isotopic ratios of lake water, e.g. D/H– Change in salinity

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For example – recent lake level changes in Great Salt Lake

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And these are slight compared

to ancient changes.

Compare Great Salt Lake to Lake Bonneville

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Little Mountain Shorelines – Bothwell, Utah

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Major shorelines of Lake Bonneville and the early Great Salt Lake

Shoreline Name

Elevation*** at Antelope

Island

Approximate Age in Calendar

Years before Present

Gilbert* 4,275 12,800 - 11,600

Provo** 4,840 16,800 - 16,200

Bonneville** 5,220 18,000 - 16,800

Stansbury** 4,445 24,400 - 23,200

*    Great Salt Lake shoreline **  Lake Bonneville shoreline ***Feet above mean sea level

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Change in Sediments• Lithostratigraphy

– Color, grain size, bedding, structures

– Etc.• Water content• Petrographic examination

and x-ray• Image Analysis• Phys. Properties

– Magnetic suspectibility– GRAPE (gamma ray

attenuation (density)

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Paleolim 591L

Core Lithology-PG1351

Silty-Clastic Layer

Massive Grey Silty Clay Brownish-Grey Silty ClayOlive-Grey Laminae

Deposited under anoxic conditionsi.e. Glacial-type climate

LithologyD

epth

(cm

)

UnitOlive-Grey Silty Clay

Massive Interglacial-type sediments

UnitUnit

Sandy Layer

Depth: 385 cm

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SedimentologyDown-Core Sediment Properties

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SedimentologyDown-Core Sediment Properties

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Pollen data from Whitehead and Chrisman, 1979

FOSSIL POLLEN from BERRY POND, MASSACHUSETTS

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Paleoclimatogical reconstructions based on biota

• Based on the concept of ecological niche (a given population is adapted to survive and reproduce under a set of biotic and abiotic conditions).

• A niche can be devised as a multi-dimensional space in which the axes represent variables.

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• In paleoclimatology, useful species are those with limited niches; whose abundance, distribution, and presence or absence are controlled by climatic factors.

• However, one needs to keep in mind that niches are dynamic and transient and that species can evolve and adapt.

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Diatoms – single celled algae with a ‘shell’ made of silica, sensitive to

salinity and pH

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Walden Pond

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Ostracodes – small crustaceans

Can also analyze shell chemistry

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Chironomids – midges, most common aquatic insect

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Chitinous head carapaces preserved in sediemnts

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Pollen

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Example – The Younger Dryas cold period

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named after an indicator genus, the alpine-tundra wildflower Dryas octopetala

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