Carbon and nitrogen stable isotopes in suspended matter and sediments from the Schelde Estuary

• Published in: Marine chemistry 1998-04, Vol.60 (3), p.217-225
• Main Authors: Middelburg, Jack J, Nieuwenhuize, Joop
• Format: Article
• Language: English
• Subjects: carbon nitrogen; Earth sciences; Earth, ocean, space; estuaries; Exact sciences and technology; Geochemistry; Isotope geochemistry; Isotope geochemistry. Geochronology; Marine; Mineralogy; Scheldt River; Silicates; stable isotopes; suspended materials; Water geochemistry
• ISSN: 0304-4203; 1872-7581
• DOI: 10.1016/S0304-4203(97)00104-7

The C/N and stable C and N isotope ratios ( δ 13 C , δ 15 N ) of sedimentary and suspended particulate matter were determined in the Schelde Estuary. Suspended matter was divided into 2 to 5 size fractions by centrifugation. Four major pools of organic matter were recognized: riverine, estuarine, marine and terrestrial materials. Terrestrial organic matter ( δ 13 C ≈−26‰, δ 15 N ≈3.5‰, C/N≈21) is important for the sedimentary pool, but suspended matter is dominated by the marine ( δ 13 C ≈−18‰, δ 15 N ≈9‰, C/N≈8), riverine ( δ 13 C ≈−30‰, δ 15 N ≈9‰, C/N≈7.5) and estuarine ( δ 13 C ≈−29‰, δ 15 N ≈15‰, C/N≈8) end-members. In the upper estuary, the suspended matter size fractions vary systematically in their carbon and nitrogen biogeochemistry, with the small particles having low C/N ratios, depleted δ 13 C and enriched δ 15 N values relative to large particles. Moreover, sedimentary and suspended matter differ significantly in terms of C/N ratios (17 vs. 8.9), δ 13 C (−26.3 vs. −28.9‰) and δ 15 N (+6.9 vs. 12.0‰). In the lower estuary, suspended matter fractions are similar and sedimentary and suspended organic matter differ only in terms of δ 13 C (−23.5 vs. −20.1‰). Our data indicate that autochthonous organic matter contributes significantly to the total suspended matter and that the suspended organic matter composition cannot be explained in terms of conservative mixing of riverine and terrestrial sources on the one hand and marine sources on the other hand.