# Sediment Dynamics of the Neretva Channel (Croatia Coast) Inferred by Chemical and Physical Proxies
## 5 Discussions
### Age Model and Sediment Accumulation Rates  
Activity-depth profiles of \({}^{137}\)Cs and \({}^{210}\)Pb were evaluated along the BCs to define an age model of the NC recent sedimentary sequence (Figure 3).  
Correlations between \({}^{210}\)Pb age determinations and depth are strongly model-dependent in cores with non-exponential \({}^{210}\)Pb profiles [46]. Assuming a constant \({}^{210}\)Pb flux below the surficial mixed layer, a \"constant flux-constant sedimentation\" model has been adopted to obtain an estimate of sedimentation accumulation rates (SARs) for the last hundred years. However, since the main assumptions of most common conceptual models in sediment chronologies refer to the inputs of particles and/or a radiotracer onto the sediment, in those samples where \({}^{210}\)Pb profiles were not suitable for model calculations (such as in samples 06 BC and 10 BC), rough estimates of sediment accumulation rates were obtained by assuming an age of 100 years at the depth where the \({}^{210}\)Pb background value of \(\approx\)18 Bq/kg is found (Figure 3d,e). Since physical mixing and bioturbation were neglected in the calculations, these apparent average rates have to be considered upper limits.  
Since the \({}^{210}\)Pb dating model requires validation by a second, independent stratigraphic tracer [47; 48], \({}^{137}\)Cs activity-depth profiles were considered in the calculations. Unfortunately, as mentioned above, the 1986 \({}^{137}\)Cs peak (Chernobyl accident) is not clear tracer [47; 48]rly identifiable in the investigated sedimentary intervals. This is probably due to the strong bioturbation and sediment mixing (see X-ray images in Figure 3f,g) as suggested before by [16]. Moreover, since the half-life of \({}^{137}\)Cs is 30.17 years, it could not be excluded that the concentration is below the experimental sensitivity. This effect may be enhanced in case of high contents of carbonate (such as in karstic areas) that are less reactive to radiotracers [49]. In order to overcome validation troubleshooting, the core depth where \({}^{137}\)Cs reached the instrumental detection limit was used as reference age, assuming it corresponded to the early 1960s, when a large number of nuclear bombs were exploded in the atmosphere.  
Estimated sediment accumulation rates (SARs; Figure 6) range from 1.9 to 8.5 mm/yr. These values differ partially from those previously estimated by other authors [16], ranging from 4 to 6 mm/yr and based only on the distribution of \({}^{137}\)Cs in core sediments. In particular, the lowest SAR value of 1.9 mm/yr was found at site 06 BC (Figure 6), located to the NW of the channel at the boundary with the Korcula Channel, a location comparable to that of sample K3 of [16] where a SAR of 4 mm/yr was estimated. In the sectors of the channel less influenced by the river outflow, SARs range from 1.9 to 2.3 mm/yr, while in the areas where the river influence is high, SARs range from 4.0 to 8.5 mm/yr. The highest accumulation rates are observed in front of the river mouth and along the northeastern coast with a peak of localized preferential accumulation at site 04 BC (8.5 mm/yr; Figure 6). Estimated accumulation rates are in agreement with the observed water circulation, where a hygpycnal river plume formed at the river mouth distributes particles toward the open sea.  
Apparent accumulation rates (Figures 3 and 6) suggest that most surficial sediments in the NC result from the present day seasonally river-dominated coastal sedimentary regime, with few relict deposits in its southernmost sector, south of the river mouth. According to the calculated SARs, our BCs record a time interval ranging from 18 (04 BC) to 100 years (06 BC) ago.