# Sediment Dynamics of the Neretva Channel (Croatia Coast) Inferred by Chemical and Physical Proxies
## 5 Discussions
### Organic Carbon  
Relationships between C/N ratios and the \(\delta^{13}\)C contents are commonly used as proxies to determine the source of organic matter in coastal areas [26; 39; 40]. In addition, the \(\delta^{13}\)C and \(\delta^{15}\)N isotopic compositions and the N/C ratio help to discriminate the marine versus terrestrial origins of the particles [41]. Since the photosynthetic processes and carbon sources are different between marine organisms and terrestrial plants, there is an inverse correlation between the \(\delta^{13}\)C and C/N ratio: A high \(\delta^{13}\)C values suggests a predominant influence of marine phytoplankton [42]. On the contrary, low \(\delta^{13}\)C values (lower than -27%) are indicative of vegetable sources (C3 plants) from inland; i.e., proximity to the shoreline and/or proximity to a source of organic matter of continental origin [43]. Here, we considered the N/C ratio instead of the C/N, because it behaves linearly in a mixing model [43; 44] and an estimate of the sedimentary organic carbon fraction is more reliable [45]. The observed N/C and \(\delta^{13}\)C distributions (Figure 2c) indicate that the sites close to the river mouth (01 BC and 17 BC) have a clear terrestrial signature due to the strong fluvial input. The marine input increases northward, and the transition between terrestrial and marine regimes may be located along the theoretical line joining sites 12G, 4 BC and 9 BC (Figure 2b). In addition, a strong negative correlation (\(-0.81\)) is observed between TOC and \(\delta^{13}\)C, a result in agreement with that of [17]. These data together with a lack of correlation with the clay fraction (Table 1), suggest riverine \({}^{13}\)C depleted terrestrial organic matter as the main source of sedimentary organic carbon [27]. These results allow us to hypothesize that the input material from the NR is quickly moved northward by marine currents and released not far from the river mouth, particularly in the area near and along the coast (north of the river mouth). The observed organic carbon distribution highlights a reduced fluvial influence moving seaward. However, surficial sediments may be affected by seasonal \begin{table}
\begin{tabular}{l l l l l l l l l l l l l} \hline  & **Pb** & **Cr** & **Ni** & **Porosity** & **TOC** & **TC** & **TIC** & **C/N** & \(\delta^{13}\)C** & **Fines** & **Sand** & **Silt** & **Clay** \\ \hline Pb & 1.00 & & & & & & & & & & & & \\ Cr & \(-\)0.04 & 1.00 & & & & & & & & & & & \\ Ni & 0.12 & **0.67** & 1.00 & & & & & & & & & & \\ Porosity & \(-\)**0.69** & **0.43** & 0.06 & 1.00 & & & & & & & & & \\ TOC & 0.10 & 0.09 & 0.30 & \(-\)0.10 & 1.00 & & & & & & & & \\ TC & \(-\)0.04 & \(-\)0.16 & \(-\)**0.72** & 0.09 & \(-\)**0.52** & 1.00 & & & & & & & \\ TIC & \(-\)0.06 & \(-\)0.16 & \(-\)**0.70** & 0.10 & \(-\)**0.64** & **0.99** & 1.00 & & & & & & \\ C/N & \(-\)0.03 & 0.04 & 0.07 & 0.12 & **0.59** & \(-\)0.29 & \(-\)**0.36** & 1.00 & & & & & \\ \(\delta^{13}\)C & \(-\)0.13 & 0.07 & \(-\)0.17 & 0.20 & \(-\)**0.82** & **0.40** & **0.50** & \(-\)**0.56** & 1.00 & & & & \\ fines & 0.09 & 0.19 & **0.68** & \(-\)0.11 & **0.70** & \(-\)**0.89** & \(-\)**0.92** & **0.45** & \(-\)**0.59** & 1.00 & & & \\ Sand & \(-\)0.10 & \(-\)0.21 & \(-\)**0.67** & 0.08 & \(-\)**0.73** & **0.87** & **0.91** & \(-\)**0.48** & **0.62** & \(-\)**1.00** & 1.00 & & \\ Slit & 0.12 & \(-\)0.17 & 0.31 & \(-\)**0.39** & 0.24 & \(-\)**0.60** & \(-\)**0.58** & 0.18 & \(-\)**0.45** & **0.67** & \(-\)**0.63** & 1.00 & \\ Clay & \(-\)0.05 & **0.42** & 0.24 & **0.42** & **0.40** & \(-\)0.07 & \(-\)0.13 & 0.23 & \(-\)0.02 & \(\mathbf{0.10}\) & \(-\)0.15 & \(\mathbf{-0.67}\) & 1.00 \\ \hline \end{tabular}
\end{table} 
Table 1: Linear correlation coefficients between Pb, Cr and Ni concentrations (\(\upmu\)g/g), porosity, organic carbon (TOC, %), total carbon (TC, %), inorganic carbon (TIC, %), C/N ratio, carbon stable isotope (\(\delta^{13}\)C, %o) and sediment compositions (fines, sand, clay, silt contents, %).