Inventory of aliphatic hydrocarbons in a tropical mangrove estuary: a biomarker approach

Abstract

Aliphatic hydrocarbons in the sediments of the estuarine mangrove forests of Cochin region, Southwest coast of India were characterized to assess the sources of organic matter (OM) and to validate the effectiveness of the various geochemical indices to identify multiple OM sources and the complex geochemistry of these tropical coastal environments. The total concentration of the n-alkanes varied from 606.39 to 826.25 µg/g [dry weight (dw)] in mangrove sediments, to values between 230.10 and 287.67 µg/g in estuarine sediments. The n-alkane distribution in the study region indicated a strong odd over even predominance in both mangrove and estuarine sediments. Assessment using terrestrial OM indices such as Carbon Preference Index (CPI), Terrigenous Aquatic Ratio (TAR), and Terrestrial Marine Discriminant (TMD) suggested that a large fraction of OM in the mangrove sediments was derived from mangrove plant litter and a better preservation of mangrove OM in the sedimentary environment has occurred. These findings are supported by the higher content of total lipid, tannin, and lignin, as well as by the depleted δ¹³C values. Low ratios of CPI, TAR, and TMD at estuarine sites E2 and E3 indicated a mixed origin of OM. Natural or biogenic indices such as natural n-alkanes ratio (NAR), Σn-alkanes/n-C16, n-C29/n-C17, and LMW/HMW revealed that all the mangrove stations and the estuarine station E1 received OM from natural or biogenic sources, whereas a significant anthropogenic input was noticed for estuarine stations E2 and E3. Indices like Pr/Ph, Pr/n-C17, Ph/n-C18 further confirmed the mixed nature of OM accumulated under the anoxic environment.

Keywords:

• Organic matter
• mangroves
• hydrocarbon
• biomarker
• geochemistry

Acknowledgements

The authors gratefully acknowledge the facilities and the support provided by the Dean and Director, School of Marine Sciences, Cochin University of Science and Technology (CUSAT). We also express our heartfelt gratitude to the Head, Department of Chemical Oceanography, School of Marine Sciences, CUSAT for providing necessary laboratory and instrumental facilities. We also acknowledge the help rendered by Dr. T. V. Sankar, Principal Scientist and Dr. R. Anandan, Senior Scientist, Central Institute of Fisheries Technology, Kochi, for GC-MS analyses. We are grateful to Dr. Aninda Mazumdar, Senior Scientist, Geological Oceanography Division, National Institute Oceanography, Goa for stable isotope analysis. We are sincerely thankful to the Editor and anonymous reviewers for their comments that have improved the quality of the manuscript.