Sulphur, low temperature ash and minor elements in humid–temperate peat of the Fraser River Delta, British Columbia

Abstract

Recent peat deposits of the Fraser Delta have accumulated in a variety of depositional settings and from a wide range of vegetation. Studies of these deposits have enabled evaluation of some factors that control the distribution of sulphur, ash and minor elements in coal. Brackish peat of the lower delta plain is characterized by high total sulphur (up to 5.6%), low C/N (12-5), C/S (< 11) and total polysaccharide/C ratios and an ash mineralogy dominated by quartz, feldspar and clays. Freshwater peat has lower total sulphur (0.5%), higher C/N and C/S and higher polysaccharide/C ratios and a clay mineralogy enriched in kaolinite. The highest sulphur values occur in brackish sedge peat whereas the lowest values were obtained from Sphagnum peat. Organic S is the main S form in all peat ( > 70% of s): carbon-bonded S comprises 28-88% and ester sulphate 6-40% of total S. Pyritic S, elemental S and soluble sulphate S are minor, whereas hydrogen sulphide occurs in trace amounts. Silicon is the most abundant element (2.8-28%) followed by Al (334 ppm-6.4%) or Fe (357 ppm-5.0%), Na (43 ppm-2.3%), Ca (404 ppm-1.1%), Mg (217 ppm-1.2%), and Ti (40-580 ppm). All the minor elements are correlated with low temperature ash. On an ash basis, brackish and freshwater clay facies have higher Al and K and lower Ca contents than organic facies and brackish facies have twice the Fe and Na content of freshwater facies. The abundance of total S and the ash content of the peat reflects the depositional environment and the peat-facies. The higher pH of brackish peat has facilitated bacterial reduction of seawater sulphate leading to higher S peat. The higher kaolinite content of freshwater peat reflects the acid conditions (pH 3-4.5) of the peat bogs and the in situ formation of kaolinite. There is no evidence for formation of pyrite which reflects the low pH that has inhibited microbial mobilization of organic sulphur. The minor element abundance is a product of the distribution of detrital and authigenic ash. The lower total polysaccharides/C and C/N ratios of brackish facies reflects the greater degree of decomposition in this facies.