Stable carbon and nitrogen isotopes and elemental composition and origin of organic matter from the Neogene Euphrates, Injana and Dibdibba formations in Iraq : discrimination between marine and terrestrial environments

Three Neogene sedimentary formations reflecting different depositional environments have been investigated for the purpose of diagnosing the isotopic source of the sedimentary organic matter and their palaeoenvironmental significance in discriminating between marine and terrestrial environments. One of these formations is composed of carbonate and marl of a shallow-marine environment (Euphrates Formation), and the other formations are composed of a clastic terrestrial (fluviatile) deposits with some differences in the depositional conditions (Injana and Dibdibba formations). The isotopic δ¹³C%, δ¹⁵N% values, organic carbon (OC) and total nitrogen contents, and atomic C/N ratios are used as evidence of depositional environment. Average δ¹³C values (in ‰) for the Dibdiba, Injana and Euphrates formations are –25.4, –25.7 and –24.5, respectively; average δ¹⁵N values (in ‰) are 4.44, 5.24 and 7.24 and average OC (in wt.%) is 0.13, 0.3 and 1.1, respectively. A significant variation in the stable carbon and nitrogen isotope compositions between fluvial and marine deposits was recorded. The δC, δ¹⁵N and C/N values indicate that the organic matter in the Euphrates Formation is of marine origin, whereas it is of terrestrial origin (fluviatile) in the Injana and Dibdibba formations. The C/Natomic values are mostly high in the Injana (37) and Dibddiba formations (51), suggesting greater input and/or preservation of terrestrial organic matter derived from land plants; the shallow-marine environment (Euphrates Formation) was characterized by a low C/N ratio (9) due to the algal-derived organic matter with limited input of terrestrial organic matter.