Analysis of n-Alkanes and n-Alkenes in Chukchi Sea Sediments
Sasu, Salomey Asantewaa
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This study determined the concentrations, distribution and sources of n-alkanes in core sediments from four locations: Stations H24, H30, H32 and BarC5 in the Chukchi Sea, which is in the Arctic Ocean. n-Alkanes are valuable tracers of aquatic, terrigenous and petroleum inputs and may give evidence to the extent of climate change. The study of climate change in the Arctic is important since the Arctic is warming faster than any other part of Earth. The increase in temperature is causing enhanced evaporation leading to a higher precipitation rate and hence enhanced erosion. The n-alkanes C12-C35 were analysed and characterized using GC-MS in all four core sediments. The core sediment concentrations varied from 6.3 µg/g dry weight at depth 12-14 cm in Station H24 to 136 µg/g at depth 10-12 cm in Station BarC5. In most cases, vertical profiles of n-alkane concentrations did not exhibit systematic trends. A predominance of odd and high molecular weight n-alkanes is observed which is indicative of terrestrial input in the sediments. Thus, the most abundant n-alkanes determined were C27, C29 and C31 which originate from higher plant waxes. n-Alkane source apportionment was quantified using carbon preference index (CPI), odd-even predominance (OEP), terrigenous aquatic ratio (TAR) and low molecular weight (LMW)/high molecular weight (HMW) ratio. Most of the CPI, OEP, TAR and LMW/HMW values suggest terrestrial over aquatic input of n-alkanes. Contribution of aquatic sources was minimal given the low concentrations of low molecular n-alkanes at all four stations. Predominance of even numbered short chain n-alkanes is unusual and is being reported for the first time in Chukchi Sea sediments. This predominance is attributed to various sources including bacteria, fungi and yeast. Unresolved complex mixtures (UCM), which is a sign of petroleum input, was observed in some core sediments at Station H24; however, odd-even predominance was observed which is indicative of biogenic input. Even numbered n-alkenes in the range C12-C20 were detected in some core sediments at Stations H30, H32 and BarC5. The even numbered n-alkenes are being reported for the first time in Chukchi Sea sediments and are thought to be derived from algae, cyanobacteria, zooplankton and phytoplankton. In general, results from this study clearly show terrestrial input is the main source of n-alkanes in Chukchi Sea sediments with a minor input from aquatic organisms. There was no apparent correlation between nalkane concentrations with sediment depth, total organic carbon or arsenic concentrations. The highest concentrations of n-alkanes determined at certain depths for Stations H24, H30 and BarC5 correlated to increase in temperatures in the Arctic Ocean. However, this relationship was only consistently observed for Station H30. Therefore, climate change was most apparent in Station H30 core since it was the only core that had the highest CPI and OEP values for the surface (0-1 cm) sample which decrease down the core.