sb lakes intro pt1

Lakes an introduction

Kettle Lakes in Glacial Outwash

Ephemeral lake on the Great plains

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

Major Lake Types

• Tectonic lakes – basins formed by

movements of the earth’s crust.

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

Major Lake Types• Glacial lakes

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

II. Lakes as Archives of environmental change

Direct Precipitation

Runoff

What comes in? Water

Organic matter Sediments

Pollen

Material from the landscape

Organic matter Sediments Biota

Material produced within the lake

Two primary environmental or climate changes

• Precipitation/Evaporation• Temperature

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

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

For example – recent lake level changes in Great Salt Lake

And these are slight compared

to ancient changes.

Compare Great Salt Lake to Lake Bonneville

Little Mountain Shorelines – Bothwell, Utah

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

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)

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

SedimentologyDown-Core Sediment Properties

SedimentologyDown-Core Sediment Properties

Pollen data from Whitehead and Chrisman, 1979

FOSSIL POLLEN from BERRY POND, MASSACHUSETTS

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.

• 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.

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

salinity and pH

Walden Pond

Ostracodes – small crustaceans

Can also analyze shell chemistry

Chironomids – midges, most common aquatic insect

Chitinous head carapaces preserved in sediemnts

Pollen

Example – The Younger Dryas cold period

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