# Strategic Elements in Holoceneene Sediments of the Tinto River Estuary (SW Spain)
## 3 Results and Discussion
### Strategic Elements and Paleoenvironmental Evolution  
The base of core A is composed of sandy alluvial sediments deposited between 8.2 kyr BP and 6.5 kyr BP (Figure 1D: samples A-25 to A-23). They show low-to-very-low average contents of Al (2.93-3.29%; Mean-M-: 3.05%), Be (0.9-1.07 mg kg\({}^{-1}\); M: 0.97 mg kg\({}^{-1}\)), Li (15.2-18.1 mg kg\({}^{-1}\); M: 16.3 mg kg\({}^{-1}\)), Mo (0.38-0.63 mg kg\({}^{-1}\); M: 0.48 mg kg\({}^{-1}\)), Sb (0.5 mg kg\({}^{-1}\)), Sc (3.6-4.3 mg kg\({}^{-1}\); M: 3.9 mg kg\({}^{-1}\)), and Ti (0.29-0.31%; M: 0.3%) in relation with the rest of the core (Figure 2 and Table 1). These concentrations are similar to those of the upper anthropic filling of core A and other cores obtained in this estuary (Figure 1C), indicating that this filling came from the alluvial materials surrounding La Fontanilla Cove [32].  
This cove was flooded during the maximum of the MIS-1 transgression between 6.5 kyr BP and 5.3 kyr BP [36]. In these 1.2 kyr, almost 3 m of bioclastic sands (samples A-22 to A-19) were deposited, with a two-fold increase in the strategic elements around 6.2 kyr BP (Figure 2: sample A-21). This increase came from natural processes such as acid rock drainage, which can be inferred from the chemical concentrations of Pleistocene terraces in the Tinto River [37]. The final phase of this transgression saw a decrease in these concentrations, which reached values similar to those of the underlying alluvial sediments for most of the elements analyzed.  
The second peak of strategic elements was marked by the beginning of mining activities in the Iberian Pyritic Belt around 4.4 kyr BP (Figure 2: sample A-17) and by the transition from ebb-tide channels to marshes (Figure 1D). This conjunction led to a remarkable increase in the concentrations of almost all strategic elements (e.g., Al: up to 6.64%; Li: up to 55.2 mg kg\({}^{-1}\); Sc: up to 10.9 mg kg\({}^{-1}\)). This increase coincided with a peak of other elements in the sediment cores of this estuary, such as Cu, Pb, or As [38].  
Figure 2: Age model and vertical distribution of strategic elements in core A.  
The peak of the Roman exploitation (2-1.9 kyr BP) led to a further increase in the concentrations of all these elements (Figure 2). This increase was especially significant for Mo (up to 2.03 mg kg\({}^{-1}\)) and Sb (up to 1.3 mg kg\({}^{-1}\)), which doubled their concentrations in relation to the underlying sediments. During this period, it is estimated that 25 million tons of pyrite were extracted from the Iberian Pyrite Belt [39].  
The last mining activities (1850-1990) and recent industrial wastes (1960-1990) witnessed a fourth peak in strategic elements (samples A-8 to A-6) with the highest concentrations of core A (Figure 2; Al: up to 8.98%; Be: up to 2.78 mg kg\({}^{-1}\); Li: up to 76 mg kg\({}^{-1}\); Mo: up to 6.57 mg kg\({}^{-1}\); Sb: up to 6.1 mg kg\({}^{-1}\); Sc: up to 17.9 mg kg\({}^{-1}\); Ti: up to 0.48%). Numerous previous studies of sediment cores extracted from the estuary of the Tinto River also show that this pollution episode was the most important in the paleoenvironmental evolution of its basin [40,41].  
The progressive siltation of La Fontanilla Cove caused the loss of tidal connection in this inner zone, which explains the low values of strategic minerals in the marshes and alluvial sediments of the upper part of core A (samples A-5 to A-2). Consequently, the distribution of these strategic metals also allows us to detect environmental changes.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**SAMPLE**} & **Al\({}_{2}\)O\({}_{3}\)** & **Be** & **Li** & **Mo** & **Sb** & **Sc** & **TiO\({}_{2}\)** \\  & **\%** & **mg kg\({}^{-1}\)** & **mg kg\({}^{-1}\)** & **mg kg\({}^{-1}\)** & **mg kg\({}^{-1}\)** & **mg kg\({}^{-1}\)** & **\%** \\ \hline  
\end{tabular}
\end{table}
Table 1: Concentrations of strategic elements in core A.