Levels, spatial distributions, and sources of n-alkanes and acyclic isoprenoids (pristane and phytane) in surface sediments from Hodeidah city, Yemen

Samples of surface sediments from two coastal areas, namely outer the entrance (Zone O) and at the inlet to the seaport of Hodediah (Zone I), were collected and examined to assess total organic carbon (TOC) and aliphatic hydrocarbons. The analysis aimed to investigate the distribution, concentrations, and origins of aliphatic hydrocarbon n-alkanes as well as acyclic isoprenoid pristane and phytane. Sediments were collected using a Van Veen grab sampler, dried, extracted with a dichloromethane/methanol mixture and then analyzed by gas chromatography-mass spectrometry (GC–MS). Total organic carbon (TOC) in weight percent (wt. %) was determined by a simple oxidation method. The total extractable organic matter was separated by thin-layer chromatography (TLC) into aliphatic fraction (F₁, mainly aliphatic hydrocarbons). The average total n-alkane concentrations relative to TOC were 3016 ± 4317 µg/g_(TOC) in Zone O and 690 ± 405 µg/g_(TOC) in Zone I. The average concentrations of the isoprenoid pristane (Pr) and phytane (Ph) were 175 ± 307 µg/g_(TOC) in Zone O and 164 ± 472 µg/g_(TOC) in Zone I, and 269 ± 346 µg/g_(TOC) and 258 ± 6624 µg/g_(TOC) in Zone O and Zone I, respectively. The major sources of n-alkanes were petroleum residues, based on the organic geochemical parameters and indices (e.g., CPI(o/e), LMW/HMW, Pr/Ph, Pr/nC₁₇ and Ph/nC₁₈). They were 93 ± 2% of the total n-alkanes in Zone O and 90 ± 4% in Zone I. The biogenic origins of n-alkanes were relatively low levels in the two zones. Microbial sources ranged from 3.2 ± 2.8% in Zone O and 6.2 ± 4.2% in Zone I; higher plant wax inputs were 2.7 ± 1.8% in Zone O, and 2.3 ± 1.7% in Zone I, whereas algal sources were 1.5 ± 2.2% in Zone O and 1.4 ± 2.0% in Zone O. Additional researches are required to characterize the rate, accumulation, and transformation of diverse organic matter sources in the coastal region of Yemen.