Occurrence of C25 highly branched isoprenoids (HBIs) in Florida Bay: Paleoenvironmental indicators of diatom-derived organic matter inputs

C25 highly branched isoprenoids (HBIs) are potentially valuable indicators of diatom-derived organic matter (OM) inputs to sediments and the C25 alkane has been used previously as a biomarker in paleoenvironmental studies. In this work, a suite of C25 HBI monoenes and dienes was detected in a sediment core from Russell Key, central Florida Bay, USA. Hydrogenation proved all these compounds to be acyclic alkenes with the parent structure of the C25 HBI alkane 2,6,10,14-tetramethyl-7-(3′-methylpentyl)pentadecane. The tentative double bond positions and geometry of three monoenes and one diene were also established on the basis of comparison of retention indices (RI) and mass spectra with those published for synthetic or isolated compounds. The abundance of individual alkenes showed significantly different depth profiles. The concentrations of dienes decreased rapidly with increasing depth, but this trend was not observed for the monoenes. The highest concentration of total C25 HBIs was observed at mid depth in the core, suggesting strong historical inputs of diatom-derived sedimentary OM during that period. In fact, the depth profile of C25 HBIs reflected quite well historical variations in diatom abundance and variations in diatom species composition in central Florida Bay, based on the results of fossil diatom species analysis using microscopy. This study provides further evidence that, with care, some C25 HBIs can be applied as biomarkers for diatom inputs in paleoenvironmental studies although our data did not allow the identification of one specific diatom group as the main HBI contributor.