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

Ahad, J. M.E., Ganeshram, R. S., Bryant, C. L., Cisneros-Dozal, L. M. , Ascough, P. L. , Fallick, A. E. and Slater, G. F. (2011) Sources of n-alkanes in an urbanized estuary: insights from molecular distributions and compound-specific stable and radiocarbon isotopes. Marine Chemistry, 126(1-4), pp. 239-249. (doi: 10.1016/j.marchem.2011.06.002)

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Abstract

Diagnostic molecular ratios and compound-specific <sup>13</sup>C and <sup>14</sup>C 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 C<sub>29</sub> or C<sub>31</sub>, and the carbon preference index (CPI; 1.8 to 6.4) and average chain length (ACL; 28.5 to 29.5) of C<sub>25</sub>–C<sub>33</sub>n-alkanes became progressively lower in samples closer to the mouth of estuary. δ<sup>13</sup>C signatures of C<sub>19</sub>–C<sub>31</sub>n-alkanes ranged from − 37.1 to − 29.3‰ and in general became more depleted with increasing carbon number. Δ<sup>14</sup>C values for C<sub>21</sub> (− 945 to − 738‰) were significantly more depleted compared to C<sub>29</sub> (− 591 to − 65‰) and C<sub>31</sub> (− 382 to − 96‰), pointing to a much higher component of fossil (i.e., <sup>14</sup>C-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 <sup>13</sup>C-enrichment in C<sub>29</sub> and C<sub>31</sub> and seaward decreases in CPI and ACL pointed to petrogenic hydrocarbon contamination in lower estuarine sediments. Independent <sup>13</sup>C and <sup>14</sup>C mass balances used to calculate the relative proportions of modern (i.e., plant wax) and fossil (i.e., petrogenic) <i>n</i>-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.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Fallick, Professor Anthony and Ascough, Dr Philippa and Bryant, Dr Charlotte and Cisneros, Dr Malu
Authors: Ahad, J. M.E., Ganeshram, R. S., Bryant, C. L., Cisneros-Dozal, L. M., Ascough, P. L., Fallick, A. E., and Slater, G. F.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Marine Chemistry
Publisher:Elsevier
ISSN:0304-4203
ISSN (Online):0304-4203
Published Online:12 June 2011

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
375511A Dedicated Radiocarbon AMS for Carbon Cycle ScienceAnthony FallickNatural Environment Research Council (NERC)UNSPECIFIEDSUERC