Stable isotope compositions of sedimentary organic matter off Zanzibar town

Authors

• ANN Muzuka, Institute of Marine Sciences, University of Dar es Salaam, P.O. Box 668, Zanzibar, TanzaniaFollow

Section

Physical Sciences

Abstract

Stable isotope compositions of sedimentary organic carbon for sediments collected off the Zanzibar Town averaged -17.9%. Low isotope values of organic carbon, which were associated with high C/N ratios, were observed within Zanzibar harbour suggesting a significant contribution of terrestrial organic matter to the harbour. Sediments collected outside the Zanzibar harbour were richer in ¹³C than those collected within the harbour area. This 13 enrichment in ¹³C can be attributed to a decrease in the influence of terrestrial organic matter (0M) and high contribution of' sea grasses. A decrease in the influence of terrestrial OM is further supported by an offshore increase in the stable isotope compositions, which averaged 4.7%. The stable isotope compositions of sedimentary organic carbon and nitrogen indicate that allochthonous materials were further offshore. The calcium carbonate (CaCO₃) content off the Zanzibar Town increased offshore from less than 20% to more than 80%. An increase in CaCO₃ suggests that dilution by siliciclastic material was low as one goes offshore, thus supporting the stable isotope results. Highest concentration of CaCO₃ was observable north of the Chapwani-Bawi Islands. Lateral variations in the contents of organic carbon and nitrogen in the study area shows preferential preservation of OM in the harbour relative to other areas. High content of organic carbon and nitrogen in the harbor may be attributed to high proportion of terrestrial OM that is resistant to biodegradation. Low contents of organic carbon and nitrogen observable outside the harbor suggest poor preservation resulting from a high rate of oxidation.

Recommended Citation

Muzuka, ANN (1999) "Stable isotope compositions of sedimentary organic matter off Zanzibar town," Tanzania Journal of Science: Vol. 25: Iss. 1, Article 5.
Available at:https://doi.org/10.65085/2507-7961.2387

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