Comparison of loss on ignition and thermal analysis stepwise methods for determination of sedimentary organic matter.

1. Loss on ignition (LOI) is a widely used method to estimate organic matter (OM) in the sediment of marine and freshwater ecosystems. Thermogravimetric-differential thermal analysis (TG-DTA) of organic species provides information on thermal reactions, the amount and properties of clay structural water, organic species and carbonates. 2. The accuracy of LOI compared to TG-DTA was evaluated in 33 sediment samples from the Lagoon of Venice (Italy). 3. In all tests conducted with TG-DTA we measured the mass loss of OM and the loss of clay structural water (LCSW) on oxidized samples. The mass loss of OM at 350°C (TG-DTA 350 OM) and the total extraction of organic matter at 567°C (TEOM) calculated from the difference between natural state samples and oxidized samples highlight the presence of both thermally labile and thermally stable substances. 4. The grain size data of sediment samples from the Lagoon shows a variable distribution between slightly muddy sand and mud. Loss of clay structural water at 350°C (LCSW 350) and total extraction of clay structural water at 567°C (TECSW) both estimated by TG-DTA on oxidized samples, were found to correspond approximately to 6% and 10% respectively of the clay fraction (<4 µm). This percentage may be used to correct LOI measurements of OM in sediments with high clay content. 5. LOI 350 (loss on ignition at 350°C) and LOI 550 (loss on ignition at 550°C) proved to be ~80 % and ~200 % respectively of total extraction of mass loss at 350°C (TG-DTA 350 tot) and at 567°C (TEML) estimated by TG-DTA on natural samples, meaning that the LOI 550 value represents a significant overestimate. The difference between the LOI 550 and TEML values indicates that the mass loss excess (MLE) may be accounted for by losses due to breakdown of carbonates.