Key Idea 3: Fossil evidence for climate changes

• Pollen Analysis

  • good proxy: chemically stable/physically strong (survives for long time to give idea of climate) + affected heavily by oxygen; only survives in anaerobic conditions + abundant
  • able to study changes in vegetation community in response to climatic conditions - European pollen record consistent with Greenland Ice Cores; during YD, interval artic herbs dominate pollen records due to glacial conditions in Europe (YD interval ended and warmer conditions returned)
• Pollen Analysis

  • valuable climate indicators: preservation due to structural chemistry (exine is chemically resistant; prevents degradation in anoxic environment) + abundance in sediments + transported and dispersed over wide area + identification of pollen easy - show index of vegetation at that time/space
  • criticisms: open to interpretation + amount of pollen produced by species not consistent (season/annual variations); some species may produce small amounts of pollen but be abundant + relatively small scale (local/regional)
• Pollen Diagrams

  • show pollen counts by relative abundance (%) of arboreal/herbs genera - pollen easily accessible (close to surface) with extraction equipment (readily available) + unconsolidated
  • zoned on basis of the abundance of different types of pollen present; abrupt change in species marks boundary between different climate
• Beetles as perfect proxy:

  • sensitive to environmental change + great species diversity + highly mobile (distribution shifts rapidly from environmental change) + many species within narrow climatic tolerances
  • remained extant throughout whole study period + proxy group of organisms well known - well prepared, abundant fossil record; beetle exoskeleton reinforced with chitin (resists decay)
  • many species stenothermic (adapted to narrow range of temperatures) + modern ecological requirements well understood (well studied insect)
• Location of Fossil Beetles:

  • wide variety of sedimentary environments (anoxic water-laid sediments concentrate remains in layers of organic detritus); lake/pond sediments and deltaic deposits
  • fluvial sediments have fossil beetles; accumulation of organic detritus in channel beds/backflows - bogs/fens have good accumulations of fossil beetles
• • chitinous beetle remains preserve well in quaternary sediments (match with modern sediments)
  • useful characteristics; shape/size of exoskeleton + colour patterns useful identifier + depth/shape/distribution of surface features
  • uniformitarianism applies; ancient beetle populations same environmental tolerances as modern counterparts
  • species consistent throughout quaternary; exoskeleton morphology constant + species found together in fossil assembleges ramin ecologically compatible today
• • climatic factors limit beetle populations towards edges of their environmental ranges (large scale, climatic changes in pleistocene shifted insect distributions)
  • thermal tolerances of beetles; modern distribution patterns of beetle species in temperate/high latitudes coincide with climatic zones (distribution of individual species reflect temperature regimes)
• Mutual Climatic Range (MCR) Analysis of Fossil Assembleges:

  • method of climate reconstruction based on concept proxy organisms live within bounds of set climatic parameters
  • climate envelope developed for species of predators/scavengers found in fossil assembleges; envelopes for species found together in fossil assembleges put together to find area of overlap (MCR)
  • method allows for paleoentomologists to produce quantified paleotemperature estimates for summer/winter seasons for fossil assembleges
• Woolly Mammoth - Mammuthus Primigenius

  • extinct species of elephant found in pleistocene/holocene fossil deposits (2.6Ma-present) in Europe, northern Asia, and North America; thrived in pleistocene ice ages
  • lived in steppe tundra (ecosystem of low shrubs/grasses) widespread across Eurasia/North America (some populations inhabited forests) - herbivorous; consumed stems/leaves of tundra plants
• Distribution of Woolly Mammoths:

  • evolved from steppe mammoths 800,000-600,000 years ago in Asia + spread to Europe 200,000 years ago and from Asia across Bering Land Bridge to North America 125,000 years ago
  • distributed throughout northern america, europe and asia - glaciers, mountain chains, deserts, and changes in sea level/vegetation restricted their distribution

  
• Extinction of Wolly Mammoths:

  • largely extinct 10,000 years ago due to pressures from warming climate out of ice age (reduced habitat for cold-adapted animals; moved north as glaciers retreated) and by overhunting
  • small populations survived in North America 10,500-7,600 years ago and persisted until 5,600 years ago on St. Paul Island, Alaska + last mammoths 4,300 years ago on Wrangel Island, northern Russia - extinction from inbreeding/loss of genetic diversity
• Woolly Mammoths as Proxy Environmental Indicators:

  • useful as lived in specific environments - when fossils found and dated it indicates what the environment was like at that time
  • mammoths moved with changing climate to live in favourable conditions (tundra); moved further south during glacials, then further north as temperatures increases/glaciers retreated
• Radiocarbon dating (C14) of organic material:

  • dates material <50,000yrs + half life of 5570yrs
  • dates shells, peat, wood, bone, and ash (organic materials)

  Small proportion of atmospheric carbon is radioactive isoptope C14 - living things absorb atmospheric carbon (same C14:C12 ratio as air); organism dies, C14 begins to decay - ratio of C14:C12 decreases with age so used to date organic material.
• Limitations of Radiocarbon Dating:

  • C14:C12 ratio in sample compared with ratio in atmosphere today; ratio has changed in past 200yrs from burning of fossil fuels that produce C12 (ratio not consistent through time)
  • assumption rate of cosmic ray bombardment (converts N14 to C14) remained constant for thousands of years
  • leakage of daughter product of radioactive decay + short half life limits dating to 50,000yrs
• Varves (incremental dating)

  Alternating light/dark layers of sediment in glacial lakes - the pattern of thick/thin bands same for all lakes in same climatic region:

  • summer: glacial ice melts fast and increased flow carries down silt into lake to make thick, pale layer
  • winter: no meltwater (frozen lake) results in low energy deposition of clay particles/organic matter that grows under ice to produce thin, dark layer
• Varves

  • one year is recorded as a pair of layers on the lake bed; can count layers to find how many years are represented in the sequence - in recent sediments, can count years back from present to give chronostratigraphic ages
  • dating using varves limited as only apply since last glacial advance (limited time scale)
• Tree Rings/Dendrochronology (incremental dating)

  • annual growth of tree rings reflect climate of an area; as trees get older, each year's growth spread over a wider circumference
  • variations in ring width each year indicate presence/absence of growth-limiting factors: semi-arid areas, availibility of moisture affects amount of annual growth/ring width + boreal forest, length of growing season determined by temperature (frost controls annual cycles of growth)
  • dating limited to 10,000yrs with accuracy
• Isochronous Marker Beds, eg. Volcanic Ash Layers

  • pyroclastic deposits blasted tens of kilometres into atmosphere and laid down over large areas at the same time
  • closer to the eruption, the deposits may be tens of metres thick, but further away form thin layer that alters over time to clay (bentonite)
  • marker horizons recognised across planet (form deposits on land/in water) + unique geochemical signitures + can be dated from K-Ar containing minerals
  • can be destroyed by erosion or interrupted
• Problems with lithostratigraphic correlation (varves/ash layers):

  • lateral variations means variation in sediments as they change horizontally; closer to sediment source, beds are thicker
  • diachronous beds (same lithology but vary in age); as conditions change with time, a sequence of beds may have parts of bed with different ages (confuse lithostratigraphic correlation)