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JPT Nº9

 

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DISCUSSION

Contrary to static sieving methods discussed by Ward (1981), this technique uses considerably less water, thus, it is inexpensive and environmentally friendly. Our method uses only 10 - 15l per sieve. The technique described here allows samples to be collected during a field campaign and processed at a later date. Making it suitable for seasonal regions where short, intense fieldwork is only possible during warmer/dryer parts of the year. It is also an excellent technique for a reconnaissance sampling by investigating the potential of a new site without excessive time or money being invested.

Nevertheless, this technique could be easily adapted to remote areas. By using jerrycans for water storing water, and by using the plastic bowls and kitchen sieves (widely available in major cities), the technique here described could be employed with little modification. Only the lack of running water would make it difficult to process sediment <500µm, but the rest of the sediment could be greatly reduced in weight for transport. In this experiment, approximately 200 kg was processed by four volunteers sieving for two weeks working 4 to 5 hours a day. In our case, three series of sieves were used simultaneously, which allowed processing two, and in some cases three 5-kg samples a day, depending on the nature of the sediment.

Although no number of sieves and people is specified in McKenna et al. (1994), the technique here described processes approximately 45 kg per day, compared to 1800kg per day of other techniques. The severity of this limitation is not clear, because no specific information is available in the literature for comparisons. Nevertheless, the diversity of fossils recovered in fossiliferous layers was remarkable: each 5-kg sample produced fish remains (e.g. scales, teeth); amphibian and lizard jaws; archosaurian teeth and ostracods. As a negative control, oxidized paleosols, known to be poorly fossiliferous, barely produced any fossils when using the same technique. Although not quantified under controlled experiments, the amount of time that should be spent on sieving is a tradeoff between concentrating and separating grains of appropriate size and preservation state of the fossils (since the abrasion during sieving could potentially damage the fossils).

It is difficult to assess the amount of information from different techniques based on the yield, compared to the time required and amount of money invested. To make a fair assessment of information gained from each methodology, when compared to its time and financial investment, would require a comparison of techniques based on the same sedimentary horizon. In turn this would also require different lithologies and horizons to be processed to assess the most efficient technique for different geological settings. However, there is still the risk of confounding the intrinsic productivity of the method instead of the method efficiency, yield amount being only one factor of method efficiency.  A qualitative and quantitative analysis of the fossil damage or number of rare taxa recovered, under different sieving methods, could serve as proxies for the method efficiency.  These comparative studies will be attempted in the future seeking to determine which sieving technique should be used under what situation, but are beyond the scope of this paper.

For each 5kg sample, the 750µm and 500µm sieve fractions recovered well-preserved fossils, with no fracturing or damage. Whereas techniques described by Hibbard (1949), McKenna (1962, 1965) and McKenna et al. (1994) imply that the loss of specimens smaller than the mesh size is unavoidable (since sediment below the smallest sieve is dumped), the technique described in this paper recovers specimens of almost all sizes, even enabling the collection of small ostracods and carophytes (100 - 200µm), which are useful for paleoecological reconstructions. Finally, this methodology does not require an extra step of concentration of the residue by use of acids or heavy liquid flotation (Gibson and Walker 1967), which is known to cause damage potentially to both the desired fossils and the environment (Cifelli et al. 1996).

In essence the “Henkel technique” and the method here described share the “static sieve” idea, but the technique described by Henkel (1966), although hard to compare, seems to be more aggressive to the fossils (see figs. 2 and 3 in Rauhut, 2001, showing many tooth apex broken) and it does not retain below mesh sized matrix.

The method described in this paper is designed to be an inexpensive technique, to recover the maximum amount of microfossils with a minimal amount of damage and cost. This method is also designed to use minimal amounts of chemicals. This helps to prevent chemical damage to specimens, which can obscure or destroy textures on fossils (Cifelli et al. 1996).

A major step to improve this technique could be to adapt it to process larger amounts of material. This could be accomplished by using larger sieves, which would not influence greatly the final budget. Following McKenna’s technique, although inexpensive, it does not adapt to laboratory conditions and requires the construction of sieves.

 

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