DATING OF LATE-QUATERNARY DEPOSITS Sediment Varves, Glacier Layers, Corals, Tree Rings, Speleothems
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
- MECHANICAL: discontinuous, interupted by slumping and storms, record built from many correlated
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.
- 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)
calcite "varves" light layers CaCO3
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
CO2 used in photosynthesis during summer months, pH changes at
autumn overturn result in calcite supersaturation
- SEDIMENTARY: seasonal pulses in alluvial or aeolian discharge.
Elk Lake Varves (Bradbury et al., in prep.)
- 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),
Example: Castile Fm., New Mexico - Texas. (260,000 layers).
gypsum redeposited from earlier playas?
anhydrite (CaSO4) & halite summer,
carbonate CaCO3 (organic, dark) in winter.
hard, massive layers in summer (melted) vs.
Chemical: seasonal variation in
lighter, less dense in winter
Aeolian (Dirt) Layers: (Thompson, 1979, 1986)
Quelccaya Glacier (Thompson 1986) 21-level average
Huascarán, Peru, (Thompson et al. 1995)
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)
Annual? see Betancourt et al. (2002)