• trace elements
• lake sediments
• geochemistry
• anthropic pressure
• multivariate statistics
• Pb isotopes
• Italy

In recent decades, there has been a growing interest in understanding how and to what degree pre-industrial activities have contributed to the dispersion of trace metals into the environment at local, regional, and supra-regional scales. The Mediterranean basin hosts some of the most important metallogenic provinces in the world and has a long history of civilization and anthropic modifications. Therefore, studying the history of trace metals pollution in this area can be interesting and useful for gaining insights into the temporal and spatial scope of human-environment interactions, evaluating the pollution legacy from human activities, and identifying natural background values for selected areas.
The aim of this project is thus to reconstruct the pollution history of Pb and other trace elements (such as Cr, Cu, Zn, Hg) for the Holocene in the central Mediterranean area, particularly in Italy, by studying lake sediments. For this purpose, four lacustrine cores were selected from different sites in Italy: Lake Ledro (Trento, Norhern Italy), Lake Accesa (Grosseto, Central Italy), Lake Lungo (Rieti, Central Italy), and Pantano Grande saltmarsh (Messina, Southern Italy), spanning the last 10, 8.5, 3, 4 ka years, respectively. These cores were collected in the framework of national and international collaborations for paleoenvironmental reconstructions, and depth-age models were already available. This project focused on the geochemical study of trace and rare earth (REE) elements, major elements, and carbonate contents. The analysis of these components was performed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and gasometric technique, respectively. For Lake Accesa, Pb isotope analyses were also performed using Thermal Ionisation Mass Spectrometry (TIMS).
The results of this study indicate that all lakes have been affected by anthropogenic activities starting from different historical periods in accordance with the local history of each lake. These sediments recorded mainly local disturbances and provided temporal data on local human-environment interactions and contaminant inputs.
Lake Accesa, which is the closest site to sulfide ore deposits among the studied lakes, recorded the highest values of potential toxic elements (e.g., Pb up to 1700 ppm), showing that the environment around mining sites can reach high levels of contamination. For this lake, Pb distribution pattern reflect increased input due to human activities during the Copper Age (~3300 and ~2350 BCE [Before Common Era]), the Bronze Age (~1550 BCE), and during a long phase started in the Middle Age (from ~700 CE [Common Era]). Lead isotopes indicated that Pb is mainly related to sulfide polymetallic deposits (Cu-Pb-Zn-(Ag)) surrounding the lake catchment. These findings are consistent with archeological evidence and corroborate the beginning of mining activities in southern Tuscany around 3300 BCE and the scope of metallurgical activity throughout the medieval period. Moreover, Lake Accesa exhibits higher Pb pollution during the medieval period (1000-1700 CE) compared to recent times, indicating that in this area pre-industrial activities have left a significant legacy of potential toxic elements in the environment, even greater of pollution due to recent activities. This points out that in some cases we need to go far back in time to identify the natural background values of an area, making the determination of these values more challenging.
Downcore variability of trace metals for Lake Ledro, Lungo and Pantano Grande seems mainly related to catchment erosion, at least from the bottom of the cores to about 1500 CE. However, for each lake, increased erosion of the catchment and/or changes in sediment supply can be related to changes in land use and basin management due to human activity. From 1500 CE, these lakes, and particularly Lake Lungo and Pantano Grande, show enrichments of trace metals (for Pb and Hg) compared to lithogenic elements (such as Al and Ti), likely due to population growth and the increase of industrial activities.
Since trace metal variability for each lake is primarily tied to local changes, there are no common Pb distribution patterns among these lakes, except for a general increasing trend towards the upper (and more recent) portions of the cores, indicating that, overall, population growth and increased human activities have enhanced the general dispersion of trace metals in the environment, as is well known.
This study also compared different methods for calculating natural background values in lake sediment cores (e.g., using statistical methods like percentiles and iterative methods, relative cumulative frequency curves, cluster analysis, and geochemical methods like biplots with a conservative element and normalization with reference values). These methods did not return unique values for each element, suggesting that is probably better to use more than one method to evaluate and interpret the different results and determine the most representative background values. Considering the pros and cons of the methods compared, a good compromise could be to employ both statistical and geochemical methods such as the relative cumulative frequency curve and the biplot with a conservative element. The values and thresholds of natural background in lake sediments mainly depend on the lithology of the basins. Accordingly, the highest values were returned from Lake Accesa, which is located near sulfide polymetallic ore deposits, at least for Pb (~70 ppm) and Zn (~540 ppm). For lakes Ledro, Lungo and Pantano Grande, Zn is mainly below the detection limit of the analysis suggesting that it is not very abundant in their catchments, while the calculated thresholds for the other elements are below 40 ppm for Pb and below 30 ppm for Cu, except for Pantano Grande which returned a value for Cu of around 65 ppm.
The results obtained in this study corroborate the usefulness and the need to study paleoenvironmental records to increase our understanding of environmental changes related to both natural and anthropogenic processes, to better manage and protect the environment in a scenario of increasing anthropogenic and natural pressures. Furthermore, the Lake Accesa case study pointed out that sedimentary archives located near mining areas can be invaluable for reconstructing the timing and scope of mining and metallurgical activities, combined with archaeological research. This study also provides the opportunity to increase the Mediterranean datasets for pollution reconstruction, against which other variables and future pollution reconstructions can be compared.