Organic carbon, phosphorus and nitrogen in surface sediments of the marine-coastal region north and south of the Paria Peninsula, Venezuela

The C/N ratio averaged 23.67 with a minimum of 10.65 and a maximum of 67.78 (SD 3.74%). Overall, depending on the authors and their approach, these values are within acceptable ranges as reported in the literature (Talbot and Laerdal, 2000; Tyson, 1995, Meyers, 1994, Hecky et al.1993, Hedges 1992; Romankevich 1984, Healey and Hendzel, 1980), in the Table II summarizes the ratios of C / N given by various authors according to various criteria and sedimentary environments. Figure 8 shows the significant correlation between C and N in the sediments in the study area, R ² = 0.80. However, it can be observed that the GP samples group separately from those of the PP. As mentioned previously, there are a number of factors that make the GP sediments different from those of the PP in terms of composition, texture, and distribution. The GP is characterized by a having a mix of waters with different properties and compositions, originating from continental, transitional and marine sources, this makes the lack of a relationship marked by C/N in the GP. In the PP, however, in spite of the influence of some currents, the behavior of the C/N ratio is that expected for a marine environment, with its variations of dissolved oxygen, temperature and other physicochemical parameters depending on depth and the influence of coastal upwelling. 

The spatial distribution of the C/N ratio (Fig. 8) in the surface sediments in the PP is fairly uniform with values not exceeding 30 units, whereas in the GP the distribution is highly variable, with some areas having values exceeding 50 and others where the values are as low as those of the PP. It can be argued that even with values near 30, the OM in the PP sediments is a result of marine primary productivity, especially in the westernmost sector where values of less than 20 coincide with the most productive zone in the area due to coastal upwelling processes. In the GP, the higher variability may indicate different OM sources. For instance, values exceeding 55 units are due to the presence of OM from higher plants discharged by the Orinoco River and the waters from the Amazon brought by the Guyana Current. The mean, 23.67 (SD 3.74%) is slightly more than twice 10, which according to Bonilla et al. (1995) indicates that the geochemical balance between the sedimentation and decomposition of organic matter in marine sediments has been reached.

High Corg, P and N values associated with shallower depths in the western area of the PP, indicate a zone of substantial primary productivity that can be attributed to coastal upwelling processes and the influence of the waters of the Orinoco and Amazon rivers brought by ocean currents.

The C/N ratio clearly distinguishes the OM sources between the two sectors of the study area: of marine origin for the PP and a mixture of marine and continental origin for the GP, indicating the influence of the Orinoco and Amazon rivers in these areas. It also indicates the presence of two types of sedimentation environment: one slightly anoxic, thereby conserving the OM in the sediment, and the other oxic, not permitting its conservation. In addition, the C/N ratio reveals a deficit of nitrogenous elements, limiting the productivity of the area. Productivity in the PP, however, is not limited by TN deficiency due to a steady supply of this nutrient to the area from the contribution of the rivers, as mentioned earlier.

The silt and clay fractions differ in their content of C and N, whereby apparently the less degraded or more difficult to degrade N-rich organic compounds accumulate preferentially in the clay fraction, generating a relative enrichment of nitrogen compounds in this fraction and a relative enrichment of Corg in the silt fraction.

We found considerable differences in the distribution and content of Corg, TN and TP in the sediments of the PP and GP, with higher concentrations in the PP, possibly due to different geomorphological conditions as well as hydrochemical and river inputs, between the two environments. Ocean currents and the sediment source may be the most important factors that control the composition, texture and spatial distribution of the sediments and elements considered in this study.

According to Gomez (1996), the the fertility of the waters off northeastern Venezuela are caused by: 1) Upwelling; a hydrographic phenomenon that occurs during the first months of each year, 2) The contributions of dissolved and particulate organic matter from the great South American rivers, particularly the Amazon in the first few months and the Orinoco during the second half of the year, 3) Coastal lagoons and other coastal water bodies that enrich the adjacent sea, especially from May to November and 4) The local supply of nutrients from the erosion of the numerous islands and islets, headlands and cliffs on the continental shelf by internal waves. According to Odriozola (2004), a surface layer of low salinity observed during each cruise of the SIMBIOS-Orinoco indicates that both the Gulf of Paria (GOP) and the southeastern Caribbean (SEC) are under the influence of freshwater inputs during the wet and dry seasons. According to this author, in the GOP, this surface layer is a direct result of the discharge from the Orinoco River plume, whilst in the SEC it could be due to the discharges from both the Orinoco and the Amazon rivers, transported to the SEC by the Guyana Current. However, the similarity of the silicate-salt mixture found by Odriozola (2004) to that reported by Froelich et al. (1989) for the Caribbean, suggests that water from the Orinoco River plume, rather than the Amazon, dominates this region.This finding supports the observations made by Muller-Karger et al. (1989) using satellite imagery.

To PDVSA CAMUDOCA for their financial support of the second phase of the project "Baseline Environmental Study, Mariscal Sucre" and to the Research Council of the Universidad de Oriente for financing the project "Geochemistry of superficial sediments of the coastal-marine region of the north and south continental shelf of the Paria Peninsula, Sucre State, Venezuela "(CI-02-030700-1404/08). To the Institute of Earth Sciences and the Oceanographic Institute of Venezuela for their institutional support of this study.

 

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