Pleistocene -the Ice AgesSleeping Bear Dunes National LakeshorePleistocene - the Ice Ages• Stage is now set tounderstand the nature offlora and vegetation ofNorth America and GreatLakes• Pliocene (end of Tertiary)- most genera had alreadyoriginated (in palynofloras)• Flora was in place• Vegetation units (biomes)already derivedVegetationFloraThe vegetation and flora as we see it now (Holocene) was dramaticallyaffected by Pleistocene eventsPleistocene - the Ice Ages• In the Pleistocene, earthexperienced intensification towardsclimatic cooling• Culminated with a series of glacial-interglacial cyclesPleistocene - the Ice Ages• North American flora andvegetation profoundly influenced bythese “ice-age” events• Wisconsin glaciation (last epoch)most important• Assembly of flora and vegetation ofmost Great Lakes was during the latePleistocene and Holocene - (18,000years ago to present)Pleistocene - the Ice Ages• In the Pleistocene, earthexperienced intensification towardsclimatic cooling• Culminated with a series of glacial-interglacial cycles• North American flora andvegetation profoundly influenced bythese “ice-age” eventsLouis Agassiz [1807-1873]• Swiss geologist who studied mountainglaciation in Europe• First scientist to articulate continentalglaciation in the northern hemispherebased on landscape features he “read”in eastern North America• His subsequent studies of NorthAmerican glacial evidence beganthe study of the field ofcontinental glaciation• Evidence for continentalglaciation is overwhelmingPleistocene - the Ice AgesWhat evidence did Agassiz see?• ice scouring (rock markings, flatplains,U-shaped river valleys, erratics)• glacial till (outwash, direct deposit)• drainage channels• temporary glacial lakes• kettles (permanent lakes formed by icechunks)• moraines• perched dunes• plant and animal “disjunct” distributionsPleistocene - the Ice Ages• Terminal moraines are some of the mostvisible “footprints” of glaciers• formed as ice front “stagnated” for a timeand deposited large amounts of unsortedrock, pebbles, sand, and clayPleistocene - the Ice Ages• Terminal moraines (oftenfrom different lobes) areevident from past glacialstages - before the lastWisconsin stageLast 3 glacial and interglacialstages in IllinoisPleistocene - the Ice Ages• Glacial lakes - now gone orreduced - formed as glaciers firstbegan melting• Wisconsin Dells and Devil’s Lakegeology is largely due to suchglacial eventsPleistocene - the Ice Ages• Perched dunes — as in SleepingBear Dunes National Landscape —provide evidence of higher LakeMichigan water levels during iceretreatPleistocene - the Ice Ages• Erratic boulders - such as thisPetoskey Stone at the top of aSleeping Bear Dune moraine -also are evidence of glacialactivityPleistocene - the Ice Ages• Perched dunes — as in SleepingBear Dunes National Landscape —provide evidence of higher LakeMichigan water levels during iceretreatLake sediment pollenprofile through time• Pollen profiles in lake or bog sediments/peat show sweep of vegetationand flora through time in once ice-covered regionsPleistocene - the Ice Ages• Disjunct distributions of arctic species at edges of glacial advances inWisconsin River Dells and Kickapoo River valley a lot further souththan expected for the species suggest “left behind” scenariosArctic “bird’s-eye-primrose” distribution in WisconsinPleistocene - the Ice AgesEvidence for Pleistocene glaciation cycles?• A number of “paleothermometers” indicate thatthese ice ages occurred on a cycle about 100,000 +/-yearsPleistocene - the Ice AgesPaleothermometers - telling temperature back in timeForaminifera ("forams")Foraminifera - zooplankton of marine water- produce outer hard coverings of CaCO318O/ 16O ratio is important in CaCO3 samplewhen δ18O is big, the ratio is large and the sample is heavywhen δ18O is small, the ratio is small and the sample is light16O - typical, lighter form18O - rare (0.2%), heavier formThe oxygen in calcium carbonate comes intwo basic isotopes from H2O:Pleistocene - the Ice AgesForaminifera ("forams")Lighter isotopes become concentrated in theatmosphere through evaporation - it is easierto vaporize a light molecule. Rain furtherconcentrates lighter isotopes because heavierisotopes preferentially precipitate.Paleothermometers- telling temperature back in timePleistocene - the Ice AgesCombined with the net poleward movementof water vapor in the atmosphere, and thefinal precipitation of water vapor into the icecap, the net result is that the ice caps areisotopically lighter than the oceans.Foraminifera ("forams")Paleothermometers- telling temperature back in timePleistocene - the Ice AgesThe average isotopic composition of theoceans is controlled by how much ice isstored in the caps. The more ice stored inthe caps . . . the isotopically heavier theoceans and the Foraminifera become.Foraminifera ("forams")Paleothermometers- telling temperature back in timePleistocene - the Ice AgesForaminifera ("forams")Paleothermometers- telling temperature back in timePleistocene - the Ice AgesEvidence for Pleistocene glaciation cycles?• including ocean temperatures using 18O/16O• rise/fall of fossil corals in Bahamas• CO2 concentration in atmosphere• stomate density on Pine needlesPleistocene - the Ice AgesWhy did Pleistocene glaciation occur?Warm Pacific water Warm Pacific water1. Continentalmovements bringingcold water into Atlanticor stopping flow ofwarm water into AtlanticPleistocene - the Ice Ages1. Continentalmovements bringingcold water into Atlanticor stopping flow ofwarm water into Atlanticand disruption of thewarm shallow “salt”current (not surfacecurrent) that brings extraheat to North Atlantic2. Albedo effect - once continental glaciers form, thereflectance of sunlight off white glacial ice furtherdecreases heat budget of north AtlanicPleistocene - the Ice AgesWhy did Pleistocene glaciation occur?3. Milankovitch cycles:a. Ellipticity cycle - 105,000 yb. Tilt cycle - 41,000 yc. Axis wobble - 26,000 yImportance is the synchrony orasynchrony of the cycles(especially first two)Pleistocene - the Ice AgesWhy did Pleistocene glaciation occur?What happened in the Pleistocene?• Holocene (Recent) - the presentinterglacial started 10,000 ya• Wisconsin - the last glacial(Wurm in Europe) occurredbetween 115,000 ya - 10,000 ya• Height of Wisconsin glacialactivity (most intense) was18,000 ya -