Distribution and sources of aliphatic hydrocarbons and fatty acids in surface sediments of a tropical estuary south west coast of India (Cochin estuary)

Surface sediments samples from the Cochin estuaryweremeasuredfor elemental,stableisotopicand molecular biomarkers (aliphatic hydrocarbons and fatty acids) to study the sources and distribution of sedimen- tary organic matter. Concentrations of total organic car- bon(TOC),total nitrogen(TN)andstableisotopicratios of carbon (δ 13 C) ranged from 0.62 to 2.74 %, 0.09 to 0.25 % and −27.5 to 21.7‰, respectively. Sedimentary n-alkanes ranged from 6.03 to 43.23 μ gg −1 with an average of 16.79 μ gg −1 , while total fatty acids varied from 22.55 to 440.69 μ gg −1 . The TOC/TN ratios and δ 13 C suggest a mixture of marine- and terrestrial- derived organic matter in the surface sediments with increasing contributions from marine-derived organic matter towards the seaward side. Long-chain n-alkanes derived from higher plants predominated the inner part of the estuary, while short-chain n-alkanes derived from planktonic sources predominated the bar mouth region. The even carbon preference of the C12-C22 n-alkanes may refer to the direct biogenic contribution from bac- teria, fungi and yeast species and to the potential direct petroleum inputs. The presence of odd mid-chain n- alkanes in the sediments indicates the organic matter inputs from submerged and floatingmacrophytes (water hyacinth). Various molecular indices such as carbon preference index, terrestrial to aquatic ratio, average chain length and the ratios of mid-chain n-alkanes sup- port the aforementioned inferences. The high contribu- tion of odd and branched chain fatty acids along with very low contribution of polyunsaturated fatty acids, suggest the effective utilisation of algae-derived organic matter by bacteria and the effective recycling of labile organic matter in whole settling and deposition process- es. The distributional variability of n-alkanes and fatty acids reveals the preferential utilisation of marine- derived organic matter and the selective preservation ofterrestrial-derivedorganicmatterinsurfacesediments of the Cochin estuary.