DATING OF LATE-QUATERNARY DEPOSITS
ANNUAL DATES:

Sediment Varves, Glacier Layers, Corals, Tree Rings, Speleothems

dendrochronology varves speleothems corals ice cores pages

1. VARVES: annually-deposited sediment

    Developed by O. Heer (1865)
    1912 Gerard DeGeer developed the Swedish Varve Chronology, the first accurate dating of the late-glacial and Holocene.

  • Couplet: pair of adjacent different layers
  • Lamination: layers of sediment differing in composition.
  • Rhythemites: geological deposit containing regularly alternating (not always annual) layers of different composition

Baron Gerard DeGeer in Sweden


Swedish Varve Sequence

PRESERVATION critical - waves and biological-mixing destroys varves

TYPES OF VARVES
  1. MECHANICAL: discontinuous, interupted by slumping and storms, record built from many correlated surface exposures
      Coarse Layer, summer, runoff, silt, organic (light)
      Fine Layer, winter, slow settling, clay, (dark)
        Example: Swedish varve chronology, Baltic Sea. Tied to European Climatic Sequence through pollen analysis of the sediments.
        Example: Western U.S., Ice-dammed valleys in N. Rockies (Wait, 1984) varved sediments used to date floods in glacially-dammed lakes.
  2. BIOLOGICAL: discontinuous, deposited only when activity of bottom-dwelling organisms is suppressed (anoxia), interrupted by layers of homogenized sediment, layers often indistinct, layers thin, hard to count.
      diatom "varves" light layers may be pure diatoms
        Example: Gulf of California, 150 m core 12-15 laminae/cm (Donegon, 1982)
        light layers 90% diatoms, deposited in winter when NW winds favor upwelling, increased nutrients, greater productivity
        dark layers terigenous, deposited in summer, when monsoon precipitation leads to increased river discharge
      calcite "varves" light layers CaCO3
        CO2 used in photosynthesis during summer months, pH changes at autumn overturn result in calcite supersaturation
  3. SEDIMENTARY: seasonal pulses in alluvial or aeolian discharge.
    Elk Lake Varves (Bradbury et al., in prep.)

  4. EVAPORITES: discontinuous, deposition interrupted or layers changed by changes in lake chemistry, changes in lake level.
      Example: Searles Lake, CA,
        (light layer) salts precipitated during summer months.
      Example: Lake Estancia, NM, layers of silt/clay and
        (light layer) gypsum (gypsarenite),
        gypsum redeposited from earlier playas?
      Example: Castile Fm., New Mexico - Texas. (260,000 layers).
        anhydrite (CaSO4) & halite summer,
        carbonate CaCO3 (organic, dark) in winter.
  5. TIDAL

READINGS


2. GLACIER Layers

Mechanical:
    hard, massive layers in summer (melted) vs.
    lighter, less dense in winter
Chemical: seasonal variation in
    acidity
    volcanic ash
    nitrate concentration
    18O/16O ratio
    CO2
Aeolian (Dirt) Layers: (Thompson, 1979, 1986)

Quelccaya Glacier (Thompson 1986) 21-level average


Huascarán, Peru, (Thompson et al. 1995)

www.ngdc.noaa.gov/paleo/icecore/icecore-varlist.html

READINGS


3. CORAL LAMINATIONS

Potential source of annual-resolution data from tropical seas.

Growth Banding: dense dark band during dormant season

Fluorescence: organic compounds from terrestrial plants - record of river discharge in Great Barrier Reef (Isdale, 1984) Geochemical studies: (18O, 13C, Isotope ratios: Cd Mn Ba /Ca) temperature reconstructions based on oxygen isotopes in Gallapagos Islands (Shen et al., 1992)

El Niño records from corals


Seychelles coral δ18O (Charles et al., 1997)


4. SPELEOTHEM LAYERS


Annual? see Betancourt et al. (2002)


READINGS

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