Variation of δ15N in Indian coal, lignite and peat

Abstract

Nitrogen (N) stable isotope ratio (δ¹⁵N) in coal organic matter (OM) provides information on the N source and dominant mechanisms affecting isotopic fractionation during coalification. However, published data on δ¹⁵N distribution in coal is rare. The present study is one of the first reports on the δ¹⁵N composition of peat, lignite, sub-bituminous and anthracite coals in India and one of the first attempts to understand the processes influencing δ¹⁵N composition at different stages of coalification from peat to anthracite. Peats were collected from the western coast of North Andaman Islands and Lake Loktak in Manipur. Plant samples were collected from the peat sampling locations. Cenozoic lignites were collected from Panandhro, Matanomadh, Umarsar and Tadkeshwar mines in Gujarat and Neyveli in Tamil Nadu. Cenozoic sub-bituminous and Permian anthracite coals were collected from Assam and Sikkim, respectively. Variation of δ¹⁵N in plants is attributed to the differences in rainfall, plant type and N sources. Lower δ¹⁵N values in peats (mean 1.19) compared to the plant samples (mean 2.77) indicate a nonlinear response of δ¹⁵N to the relative enrichment or loss of N during peat formation in Lake Loktak and decomposition of OM under anaerobic conditions leading to selective preservation of ¹⁴N in the Andaman Islands. The δ¹⁵N composition of the studied peat (−1.4–1.6), lignite (−1.4–1.8) and coals (−2.8–5.0) retains their OM source signature. Overall higher δ¹⁵N values of Cenozoic lignites compared to the Cenozoic sub-bituminous coal reflects regional differences in climate. Higher δ¹⁵N (1.3–5.0) values in Gondwana anthracites reflects the tectonic influence of Himalayan orogeny.