Variations in the distributions of sedimentary alkyl porphyrins in the Mulhouse basin in response to changing environmental conditions

Alkyl porphyrins from five sediment layers deposited under different palaeoenvironmental conditions in the Mulhouse basin (Alsace, France) have been examined by normal-phase high performance liquid chromatography and liquid chromatography–tandem mass spectrometry. The fossil pigments show marked differences in distribution among the different lithological zones, with around 40 major alkyl porphyrin components detected. Assignment of structures to a number of the porphyrins has been achieved through detailed studies involving analysis of successive losses experienced during multistage tandem mass spectrometry. A suite of bacteriochlorophyll-derived alkyl porphyrins has been detected that exhibits fragmentations in tandem mass spectrometry not consistent with the presence of either n-Pr or i-Bu moieties. Instead, these unusual geoporphyrins may be evidence of previously undetected neo-Pent substituted alkyl porphyrins. The distributions in the different horizons of the Mulhouse basin core reveal that the relative abundances of certain alkyl porphyrins respond to changes in environmental conditions, reflecting variations in the primary producer community during the deposition of the core.