Paleolimnology


I. Rationale
    A. Academic – scientific curiosity
        1. lake typology
        2. evolution of lake ecosystems
        3. biogeography and evolution of organisms
    B. Anthropogenic changes to lakes
        1. baseline for restoration
        2. confirming time course and extent of changes
    C. Climate – understanding past climates so that you can predict future trends/responses

II. Problems
    A. Selective destruction
    B. Spatial averaging – mixing spatially separate communities

Sediment focusing

        1. sediment focusing
        2. apparent changes in productivity over time

Potential problems caused by spatial averaging of sediments -- changes in apparent productivity over time
        The bottom levels integrate the whole lake’s productivity into a smaller area – can result in an apparent change in
            productivity, even if constant over time
    C. Mixing and bioturbation

III. Tools
    A. Fossils – these indicators can be proxies not only for what lived in the lake or around the lake, but also for other
        conditions (habitats for organisms) when we can’t measure those conditions directly
        1. diatoms, chrysomonads
            a. the siliceous frustules are preserved
            b. dissolution if Si is undersaturated in the sediments
            c. different diatom species can have very different tolerances for conditions (acidity, salinity, eutrophication…)
        2. cysts, resting stages
            a. crysophytes, dinoflagellates
            b. zooplankton
        3. pollen, spores
            a. some from macrophytes
            b. lots of pollen falls on the lake from the surrounding area
            c. local versus regional effects
        4. crustaceans
            a. ostracods (benthic); have different requirements
            b. water column crustaceans – mostly cladoceran carapaces; copepods not as well preserved
        5. chironomids
            a. again, different species have different tolerances
            b. especially good for oxygen tolerances
        6. mollusca
            a. again, looking at preferences
            b. can look at isotopes in shells to find out temperature when formed
            c. must be a calcareous environment or the shells will dissolve
        7. fish
            a. bones or scales
            b. fairly rare
        8. sponges
            a. siliceous structures – spicules
            b. hard to interpret
        9. macrofossils
            a.  fruits, seeds, pieces of wood
            b. rare, but very distinctive and informative
            c. from catchment; not blown from long distances
        10. grass cuticles
            a. have siliceous structures that can be distinguished to family
            b. from catchment area
    B. Other indicators
        1. Pigments
            a. Distinctive to different algae
            b. Degradation products of chlorophylls
            c. Accessory pigments unique to specific algal groups
            d. Need to correct for degradation
        2. Organic matter – chemistry of C and N
            a. Bulk form, bulk amount, chemical characteristics…
            b. Originate in algae and/or land plants (are biochemical differences – structural compounds; C:N is higher in land plants)
            c. Alkanes and alkenes
                (1) Higher plants versus algae
                (2) Stable isotopes for CO2
        3. Minerals, elements – N, P, S
            a. Cations, nutrients, inorganic precipitates can be used to infer water chemistry
            b. E.g., Gypsum deposits – must have been lots of sulfate (SO42-)
        4. Grain size – large versus small; how turbulent the environment was
        5. Paleothermometers

            Stable isotopes (O, H)
            In diffusion the lighter isotope is enriched at the endpoint
            As temperature increases this difference between fractionation of light and heavy isotopes decreases
                (fractionation greater at low temperatures)
    C. Dating – needed to resolve depth with respect to time
        1. varves – annual laminations in the sediments
        2. 0-150 years
            a. 210Pb
            b. 137Cs
            c. Pb rise from the use of automobiles
            d. Ragweed pollen (Ambrosia) rise with human settlement; this pollen can mark human arrival in an area
        3. 150-75,000 years (often only used until 40,000 years ago)
            a. 14C – 5,568 years is the half life
            b. for every 14C atom there are 1012, 12C atoms
            c. bomb produced 14C can also be used from 0-40 years
                i. 1950 – 100% modern (defined)
                ii. 1975 – 140%
                iii. 1995 – 117% modern (the extra 17% is all due to bomb testings)
        4. >75,000 years
            a. other isotopes with longer half-lifes
                i. K/Ar – half life of 1.31 x 109 years
                ii. Uranium series – 238U half life of 4.51 x 109 years
                iii. Half lives in billions of years – not precise for tens of thousands of years
            b. Thermoluminescence
            c. Paleomagnetism
                i. Remnant magnetism (e.g., hematite, Fe2O3)
                ii. Pole reversal
                iii. The last reversal was 730,000 years ago
                iv. Iron in the sediments is aligned with the poles
            d. Tephrachronology -- Volcanic ash

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