Sources of n-alkanes in an urbanized estuary: Insights from molecular distributions and compound-specific stable and radiocarbon isotopes

Diagnostic molecular ratios and compound-specific 13C and 14C analyses were used to identify n-alkane sources in surface sediments collected along a transect from an urbanized estuary draining a peat-rich catchment (Tyne, UK). The most abundant homologues were generally C29 or C31, and the carbon preference index (CPI; 1.8 to 6.4) and average chain length (ACL; 28.5 to 29.5) of C25–C33n-alkanes became progressively lower in samples closer to the mouth of estuary. δ13C signatures of C19–C31n-alkanes ranged from − 37.1 to − 29.3‰ and in general became more depleted with increasing carbon number. Δ14C values for C21 (− 945 to − 738‰) were significantly more depleted compared to C29 (− 591 to − 65‰) and C31 (− 382 to − 96‰), pointing to a much higher component of fossil (i.e., 14C-free) carbon in the shorter homologue. The radiocarbon contents for these three n-alkanes decreased toward the North Sea, which in conjunction with an up to 4‰ seaward 13C-enrichment in C29 and C31 and seaward decreases in CPI and ACL pointed to petrogenic hydrocarbon contamination in lower estuarine sediments. Independent 13C and 14C mass balances used to calculate the relative proportions of modern (i.e., plant wax) and fossil (i.e., petrogenic) n-alkanes yielded similar results and demonstrated that mixing with marine-derived organic matter (OM) or microbial degradation during estuarine transport led to a seaward decline in modern, longer-chain homologues, resulting in an increasingly larger fossil carbon contribution toward the mouth of estuary. The ability to clearly distinguish anthropogenic from natural inputs suggests that compound-specific radiocarbon analysis can successfully delineate the age of terrigenous OM delivered to the coastal zone even near historically polluted systems such as the Tyne.

Research highlights
► We examine variability in sources and ages of n-alkanes in estuarine sediments.
► Molecular distributions combined with compound-specific 13C and 14C analyses.
► Converging lines of evidence point to greater petrogenic component in lower estuary.
► Independent isotope mass balances delineate relative proportion of fossil n-alkanes.
► Ages of terrigenous n-alkanes exported to sea can be assessed in polluted systems.