Paleoecological Reconstructions

In this approach, paleoecologists use the available evidence for reconstruction purposes at a range ofecological scales.

1. Past biota - what taxa were present in the past?

2. Past populations - what were the population sizes in the past?

3. Past communities - what communities or 'life assemblages' were present in the past?

4. Past landscapes - what was the past landscape and how did it vary in space and time?

5. Past environments - what was the environment (e.g., climate and lake-water pH) at particular times in the past?

6. Past ecosystems - what was the ecosystem at particular times in the past?

Reconstructions can be based on a few 'indicator species' or on the fossil assemblage as a whole. The reconstructions may be qualitative or quantitative. An important approach for quantitative environmental reconstructions in Quaternary paleoecology involves modern organismenvironment transfer or calibration functions to transform fossil assemblages into estimates of the past environment (Figure 4).

Transfer functions are mathematical regression-type models that express the relationship between modern assemblages of organisms (e.g., pollen) preserved in surface sediments (Ym) and the contemporary environment (e.g., mean July temperature - Xm):

Xm Um Ym where Um is the modern transfer function estimated by inverse regression or calibration.

The transfer function is assumed to be invariant in time and space and is applied to fossil assemblages, Yf, to derive estimates of the past environment, Xf :

This general approach has been used to estimate sea-surface temperatures from fossil foraminifers, radiolarians,

Contemporary climate data

Calibration dataset (191 lakes)

Contemporary climate data

Calibration dataset (191 lakes)

Reconstruction

6380.100 6700.140 7680.90 8150.115 9250.100

20 40 20 40 60 80 % of total pollen and spores

1250 1300

f 1400 ¿5 1450 1500 1550

6380.100 6700.140 7680.90 8150.115 9250.100

20 40 20 40 60 80 % of total pollen and spores

Pollen diagram

1250 1300

f 1400 ¿5 1450 1500 1550

Reconstruction

12 13 14 -16-12 -10 400 500600

mm yr

12 13 14 -16-12 -10 400 500600

Figure 4 A schematic representation of the stages involved in deriving a quantitative reconstruction of past environment from pollen-stratigraphical data using a modern calibration training set and transfer or calibration functions. Based on an unpublished diagram by Steve Juggins.

mm yr coccolithophorids, diatoms, and dinoflagellate-cyst assemblages, terrestrial climate from fossil pollen, chironomid, cladoceran, diatom, mollusk, and insect assemblages and from tree-rings, lake conditions (e.g., pH, salinity, total P, and anoxia) from fossil diatom, cladoceran, chironomid, chrysophyte, and ostracod assemblages, bog moisture from fossil moss and testate amebae assemblages, and atmospheric CO2 from stomatal density of fossil leaves.

As all the biotic evidence is generally used for reconstruction purposes, it is not possible to use the reconstruction as a basis for interpreting the observed biotic changes, except in the rare cases where the environmental reconstruction is based on independent sources of evidence such as stable isotopes, sediment geochemistry, or one biological proxy that is used solely for reconstruction purposes.

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