Isotopic signatures of organic matter in sediments of the continental shelf facing the Orinoco Delta: Possible contribution of organic carbon from savannas

The Sr, C, and N isotopic signatures of organic matter in sediments from the continental shelf facing the Orinoco Delta were measured to determine the contribution of sediments transported from the Amazon River by the coastal Guayana current and the sediments transported by the Orinoco River. Box core samples between 60 and 300 m water depth collected along 4 transects located eastwards to the Orinoco Delta were analyzed. Carbon and nitrogen concentrations decreased with depth under water on the shelf, and were strongly correlated indicating homogeneity of organic matter composition. Phosphorus content was also associated to organic matter in most samples, but some of them revealed deposition of P-enriched sediments. The Sr/Sr ratios showed a strong continental signature averaging 0.7117, therefore, limiting the possible carbon and nitrogen sources associated with these sediments to C3 trees, C4 grasses, or freshwater phytoplankton. The dC values were relatively high averaging 21&, above values reported for sediments on the Amapá shelf and the Amazon River in Brazil. Average dC values did not differ significantly among transects. High dC values point to the influence of organic matter transported from the C4-plants dominated savannas in the northern fringe of the Orinoco River. dN values were positive and averaged 5&, being within the range of values measured in the Marajo island (Amazon River) and the estuary of the Pará River. The dN values differed significantly among transects (4.9– 5.2&), lowest values corresponding to the northernmost transect near the coast of Trinidad, and the highest values corresponding to the transect located at the southernmost position. 2005 Elsevier Ltd. All rights reserved.