Relationship of Metal Contaminants with Acid-Volatile Sulphides (AVS) in Tropical Estuarine Sediments: Potential Influen
The river transport of anthropogenic trace metals to coastal areas and the extent to which these elements are trapped within coastal systems are issues of great interest (Salomons and Förstner 1984). Many studies have investigated the input of metals tran
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Springer-Verlag Berlin Heidelberg GmbH
Luiz Drude Lacerda . Ricardo Erthal Santelli Egbert Duursma . Jorge Joao Abrao (Eds.)
Environmental Geochemistry in Tropical and Subtropical Environments
With 128 Figures and 80 Tables
,
Springer
Editors
Prof. Luiz Drude de Lacerda Instituto de Ciencias do Mar, Universidade Federal do Ceara, Av. Aboli-
6 8 10 12
17080 ±890 - .
20420 ±1600-
20 22 24
"U
Fig. 2.6. Core BM6 (Lake Barombi, Cameroon). Simplified lithological log compared with profiles of
TOC, C, arboreal pollen contents and C/N ratios. The wavy lines on the lithological log indicate slump intervals (after Giresse et aI. 1994)
The geochemical parameters indicate the existence of two main units. The lower unit (from the base up to 11 m) is characterised by TOC, C/N and £513 C values that show large amplitudes of variation around 8%, 18 and in the range -24 to -30%0, respectively. In the upper unit, by contrast, these parameters show little variation, with average values around 5%, 15 and -32%0, for TOC, C/N and £513 C, respectively.
14
A. Sifeddine . D. Wirrmann
2.4
Interpretation and Discussion
For the different studied sites, organic matter with 613C values between -8 and -36%0 is broadly characteristic not only of aquatic plants and/or phytoplankton but also land plants from forest or savanna environments. Thus, the 613C value alone cannot be used to interpret the results in terms of changing environmental conditions. On the other hand, the C/N ratios for all sites lie in the range 5-40, which enables discrimination between lacustrine and terrestrial environments. However, in the latter case, the C/N ratio does not allow a distinction to be made between forest and savanna environments. In order to discriminate a dominant lacustrine component with respect to terrestrial inputs, and also to differentiate forest from savanna environments, it is proposed here to cross correlate the C/N and 6 13 C values (Fig.2.7a). The interpretations are then confronted with palynological results expressed as percentage abundance of arboreal pollen (%AP). For Lake Ossa, the combination of C/N and 6 13 C data on a cross-plot diagram (Fig.2.7b) shows a general distribution of points lying close to the planktonic (algal) organic matter end-member. However, the scattering of certain points towards the terrestrial end member indicates a slight contribution from C3 land plants. Thus, the lack of marked variations - not only in lithology but also in geochemical and petrographic parameters - may be attributed to a clear predominance of algal organic matter. Despite the persistence of forest cover around Lake Ossa as revealed by palynological analysis, the T/Pratios are always less than unity (see Fig. 2.2), which confirms the algal trend suggested above. In the case of Lake Carajas, the distribution of C/N-613C pairs (Fig. 2.7C) provides evidence for the presence of two types of organic matter. One group of points represents the clear predominance of C3 land plants, while a second group reflects a more pronounced plank
