Tuesday, January 27, 2015

Maher, University of Wisconsin, LOI procedure

First a review of the basic idea behind loss-on-ignition; Dean (1974) is a convenient reference. Most sediment is composed of a mixture of clastic silicates and oxides (sand, silt and clay), organic material, various carbonates (usually CaCO3), and water. If a sample of sediment is heated to around 100°C in an oven, the water will be driven off, and the difference between the raw sample's weight and its dry weight will be the amount of water that was in the sample. This can be expressed as the weight-percent solids in the raw sediment, or its complement, the percent of the raw sediment's weight that is water. This simple measurement can be useful in characterizing changes in lake sediment cores. The sediment's weight-percent solids tend to increase gradually with depth owing to compaction, but differences in inorganic and organic content will often modulate the trend. Although this should be considered loss-on-drying rather than LOI--because no ignition occurs--the weight of the dry sediment serves a useful base to which true ignition losses can be compared.

When the dry sediment is placed in a muffle furnace and heated to 550°C organic material is consumed; the amount is the difference in weight between the dry sediment and the 550°C ash. The organic matter can be multiplied by a constant to estimate the amount of organic carbon in the sample. (The constant generally lies between 0.4 and 0.6; for example the molecular weight of Cn/(CH20)n = 12/30 = 0.4. For a default value I use 0.58, the average of the range of the weight-fraction of carbon in dry peat and lignite [Waksman, 1936, p. 284, as cited in Erdtman, 1943, Table 2, p. 17].) When estimating organic matter it is important to restrict the temperature to 550°C; clay minerals can release water from their structure at higher temperatures.

Some workers return the ash to the furnace and heat it to about 1000°C in an effort to estimate the sample's carbonate content. The difference in weight between the 550°C ash and the 1000°C ash often is assumed to result from the loss of CO2 as carbonates break down. Loss-on-ignition cannot indicate which carbonate minerals are present; the different carbonate minerals give off carbon dioxide at slightly different temperatures--generally in the range from 700-850°C. Calcium carbonate is a common carbonate in lake sediments. The ratio of molecular weight between CaCO3 and CO2 = 100/44; an estimate of a sample's calcium carbonate content can be obtained by multiplying the ash's weight loss by 2.27 to convert CO2 loss to the original CaCO3. (N.B. Many common lake sediments may contain a significant clay fraction yet have a low carbonate content. Clays contain up to five percent lattice OH units that are liberated as water when the sample is heated from 550-1,000 C. Therefore, the 550-1000°C firing would contain a significant (3-4%) amount of lattice water loss in samples with high clay and low carbonate content. The "carbonate content" results must be interpreted with care. chrono.qub.ac.uk

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