# Natural Versus Anthropic Influence on North Adriatic Coast Detected by Geochemical Analyses
## 4 Discussion
### Geochemical Signature of Marine Sediments  
As reported in several studies [17; 48; 49; 50], Cr, Ni and Mg concentrations can be used to discriminate the source areas of sediments. We compared Cr, Ni and Mg contents in the sediments from the S1 and E1 sites with those from the literature. In detail, we considered the geochemistry of deposits from the Po and Apennines rivers [30] and from boreholes in the inland areas [27]. The comparison shows that the concentrations of Cr and Ni in sediments from the Po River delta (S1 site) are very similar to those detected in sediments recovered along the Po River [30], suggesting for this area a prevailing to exclusive sediments supply from the Po River (Figure 6a). On the contrary, Cr and Ni concentrations measured in sediments from the E1 site show a wider dispersion with values comparable to those of the Northern Apennines rivers (Reno, Lamone, Fiumi Uniti, Bevano, Savio Rubicone, Uso and Marecchia), indicating an Apenninic sedimentary source for this area. These different sources of sedimentary material are in agreement with previous investigations [17; 27]. It is noteworthy that the Cr and Ni concentrations of S1 site exceed the maximum permissible concentrations, according to the Italian environmental legislation, for unpolluted sites [51] (Table 4). These have a natural origin, derived from the outcrops of ultramafic rocks in the Western Alps and in the Northern Apennines, as documented by previous studies on both river and marine sediments [8; 30; 49]. Figure 6: (a) Binary plots of Ni vs. Cr and (b) Ni/Al2O3 vs. MgO/Al2O3 ratios. Our samples are indicated as green and red dots. Reference data for rivers (light green, Apennines; light red, Po River) derive from tables presented in [30]. Reference data for boreholes (empty big circles) represent the average values presented in [27]. Provenances fields (green and red circles) are drawn in accordance with literature data [17; 26; 28; 50; 51]. In addition, Ni and Mg concentrations also highlight different input sources for our samples (Figure 6b). The Po River is the main source of Cr and Ni for the Adriatic Sea, accounting for \(\sim\)50% of total Cr and Ni fluvial inputs [8]. According to a recent study [17], Ni/Al\({}_{2}\)O\({}_{3}\) and MgO/Al\({}_{2}\)O\({}_{3}\) ratios close to 6 and 0.3 mg kg\({}^{-1}\), respectively, reflect an Apennine signature of the E1 samples, whereas values of 10 and 0.32 mg kg\({}^{-1}\) document the Po River provenance for the S1 samples (Figure 6b). Samples collected in the 1980s [53] were characterized by a southward decrease in magnesium rates in the northern-central Adriatic Sea, which is probably due to a higher content of dolomite (rich in Mg) brought by the Po and Alpine rivers. Based on the observations reported by [PERSON] et al. [28], a value of Cr/Al\({}_{2}\)O\({}_{3}\) ratio \(>\) 11.5 can be considered a clear signal of the sediment provenance dominated by the Po River supply. These data are consistent with the relatively high Cr concentrations detected in S1 sediments (Table 3 and Figures 5 and 7). The upward increasing trend of Cr/Al\({}_{2}\)O\({}_{3}\) in the E1 depth profile (Figure 7) may be interpreted as a gradual increase of the Po River contribution in the E1 site over the years, in particular after the 1940s. This trend seems in contrast with the decrease of sediment supply by the Po River recorded after 1945 (with a generalized phase of degradation and partial retreat of the entire Po River delta) caused by the construction of a dam [21; 54], riverbed mining activities and the channelization of watercourses [55]. The most significant retreat phase, with rates in the order of tens of meters per year, reached its peak between 1954 and 1978 [56] when the E1 core recorded the highest Cr/Al\({}_{2}\)O\({}_{3}\) ratio (Figure 7). The reduction in sediment supply, combined with the recent increase in storm events produced an increase of coastal erosion in the area [18; 19; 20; 57].