L'indice de réflexion ou réflectance donne une mesure de la teinte du sédiment entre le blanc (100 %) et le noir (0 %). Sa valeur est directement liée à la teneur en matières organiques.
Les relations entre l'indice de réflexion et les teneurs en carbone organique, en azote Kjeldahl et en carbonates sont données pour les sédiments de dix lacs différents.
Une étude particulière de prélèvements superficiels et de prélèvements profonds montre qu'une simple mesure de la réflectance donne une valeur approchée de la composition chimique.
- Sédiments lacustres,
- composition chimique,
Use of a «reflectance index» for a quick study of lake sediments
Thorough studies on many lakes in Franche-Comté, especially chemical and physical investigations concerning sediments, have been carried out for several years. Seventy-one sediment samples have been extracted at different points and depths of ten lakes in Franche-Comté. The upper stratum between 0 and - 2,5 cm were isolated and studied.
A comparison of visual observations and results of chemical analysis have shown that the darkest sediments are those which contain the greatest amount of organic matter. This finds its origin in a particularity of Franche-Comté lake sediments, which are often composed of dark black organic matter in different evolutionary states, and clear mineral compounds such as calcium carbonate and detrital particles.
Differences in colouration were particularly difficult to determine, so colours were defined by the reflexion factor of the sediment surfaces, after the sediments had been well dried and homogeneized.
The reflectance index was measured with a reftectometer, which is generally used for determinations of dark smokes in air pollution. The reflectance index was defined between the values 0 and 100. The value 0 was obtained with a standard black glass and the value 100 with a white filter paper.
Comparing reflectance indices and ignition loss has shown that there is an exponential relationship between these two parameters. These relationships were found to exist between the reflectance index and organic carbon, Kjeldahl nitrogen and carbonates.
Ignition loss, organic carbon and Kjeldahl nitrogen proportions increase, but, on the contrary, carbonate proportions decrease when the reflexion index increases.
The values calculated and observed were then compared and generally the difference does not appear to be too great. Once, relationships have been established, it becomes possible to use them, after measuring reflexion indices, to obtain quickly and easily a good approximation of the composition.
Special relationships may also be calculated for an individual lake, as shown for St-Point Lake. If one mineral constituent prevails in the composition, it is possible to obtain a good estimation of this component.
For this lake twenty-eight sediment samples were extracted and for each sample three strata were separated. The reflectance index was again defined between the values 0 and 100, but the value 100 was now obtained by using a flat calcium carbonate surface, as the principal mineral part of St-Point Lake consists of calcium carbonate.
Recent results obtained from two lake sediments of the Massif Central indicate that such relationships also exist for these lakes and that this method of estimating the chemical composition can be generalised.
The calculated composition may often be sufficient to estimate values required for others studies on the same lakes, especially biological studies, thus avoiding laborious and extensive chemical determinations.
- Lake sediments,
- chemical composition,