Lakes

Distribution Modern –1% of continental surface –<0.02% water Ancient –Much more significant! –Useful for studying basin processes on smaller scale

Limnology Lakes are sensitive to climate change –One of few paleoclimate indicators in terrestrial environment –Varves / Rhythmites Annual cycles of sedimentation Rapid! changes over small distance Very diverse –Hydrologically open (have an outlet) –Hydrologically closed (no outlet) –Can change phase –May lead to evaporitic lakes Ultimately doomed –Will fill up!

Types Open –has outflow of water –Ppt + inflow = Evap + outflow clastic sedimentation common Closed –no major outflow –inflow > Evap –chemical sedimentation may dominate due to concentration of ions Perennial –inflow from at least 1 perennial stream –rarely dry up Ephemeral –salt-pan basins –dry up seasonally; fed by springs, runoff, groundwater –Salts important ephemeral deposits salts are bedded

Origins Formation –Meteors- Hudson Bay –glacial processes (ice damming, etc)-Glacial Lake Missoula –wind drives accumulation of sands –oxbow lakes –tectonics (Large lakes) Rift valleys (Lake Tanganika) Subsiding intracontinental regions (Chad)

Size Ponds to Caspian Sea –Caspian Sea saline lake- 1.2% 371,000 km² maximum depth of about 980m –Great Lakes ~ 1/4 to 1/8 of size of Caspian Shallow to deep –Lake Baikal 1700m

Caspian Sea The northern part of the Caspian freezes during the winter, in harsh winters, the whole northern area of the sea is covered with ice. Ice can occur in the southern regions of the sea in December and January. In mild winters, ice forms in shoals in the shallow areas near the coast.

Lake Baikal

Controls on Lake Sedimentation Processes and properties of water –Currents, Waves, Tides, Turnover/ Varves Chemistry of water –Ca, Mg, Na, K + bicarbonate, sulfate, chloride –High concentrations (Evap>Ppt) lead to saline lakes Shoreline fluctuations Abundance of sediment –Fluvial origin –Autochthonous

Physical Processes Wind- waves –shore erosion River inflow –sediment input Atmospheric heating –size and shape affects heating of layers of water Barometric pressure –lake size can lead to seiches (standing waves) Gravity- tides –Present in large lakes (Great Lakes)

Sub-environments Beach and nearshore Delta Offshore –In some ways, like mini- oceans Can have continental “shelf” Energy levels vary with respect to position of rivers and waves Sedimentation reflects wide variety of sub- environments and types

Identification Terrigenous indicators Biota Thin and laterally continuous Varves (light/dark layers) Other terrigenous environments adjacent –e.g., fluvial, periglacial, evaporites CAN be confused with shallow marine!

Varves Very thin light and dark layers –In lakes, form from turnover –Summer surface heats, lowers density, stratifies water column rivers active –fine sediments can’t fall through density layer; become isolated –Lighter colored coarse grained sediments accumulate on beds –Winter surface cools, density increases (4°C = max density) –Surface waters sink, deep waters rise –when frozen, isolated from rivers »lower sediment input »waters are calm; fines accumulate »Organic matter accumulates (dark, fine clays)

Varves Often glacial –Summer = light –Winter = dark NON- Glacial forms exist –in summer, can precipitate carbonate especially in humid regions Look at all factors! –Don’t assume it’s glacial Rhythmite –Sequence of regularly alternating layers Couplet or triplet

Paleoclimate Cycles of Lockatong Fm of NJ (Mesozoic) –Climatic variations derived from varves –P.E. Olsen chemical (light) –evaporative –higher concentration of dissolved ions –led to ppt of carbonate detrital (dark) –humid periods; open conditions (Van Houten Cycles)

Paleoclimate Upper Triassic Lockatong lacustrine deposits in central New Jersey and adjacent Pennsylvania are arranged in short asymmetrical "detrital" and "chemical" cycles that resulted from expansion and waning of the lake, presumably due to cyclic variation in climate.

An orderly asymmetrical repetition of the common rock types, reflected in weathered profiles, is accompanied by a regular vertical variation in color, composition, and sedimentary structures. Approximately 80 short cycles are present in a well-exposed 1,400-foot section of the middle part of the formation

Lower platy black mudstone and gray marlstone of gray chemical short cycle, showing various types of disruption by shrinkage. (1) Very dark-gray analcime-rich mudstone at top of preceding cycle. (2) Very light-gray crystalline calcite with layers of dark-gray dolomitic mudstone. (3) Light-gray, extensively disrupted dolomitic marlstone with scattered small patches of calcite. (4) Medium- gray brecciated dolomitic marlstone with shrinkage-crack casts of gray calcitic and dolomitic (5) Very dark gray to black dolomitic mudstone with medium-gray, dolomitic crumpled shrinkage-crack casts. (6) Black dolomitic mudstone with densely scattered crystals of calcite and medium-gray, dolomitic shrinkage-crack casts.

Paleoclimate-Cyclicity Varve-counts of black mudstone suggest that short cycles resulted from 21,000-year precession cycles. Bundles of detrital and of chemical short cycles occur in intermediate cycles 70 to 90 feet thick; these in turn occur in long cycles 325 to 350 feet thick. The patterns apparently resulted from alternating wetter and drier phases of intermediate and long climatic cycles, producing through-flowing drainage and a bundle of detrital cycles or a closed lake and a bundle of chemical cycles.

Paleoclimate Varves –Read past annually! Derive from sediments and geochemistry: –Vegetation plays role in sediment supply also –Temperature freezing cuts off sediments –Storm frequency storms aid erosion