Geochemical and Sr-Nd isotopic characterization of Middle Pleistocene sediments from the paleontological site of La Polledrara di Cecanibbio (Sabatini Volcanic District, central Italy)

Six stratigraphically-related samples of freshwater sediments from the Middle Pleistocene paleontological site of La Polledrara di Cecanibbio to NWof Rome in the Sabatini Volcanic District (SVD) have been characterized by their mineralogical, geochemical, and Sr-Nd isotopic compositions. These volcaniclastic sediments filled a valley incised by a former watercourse flowing through volcanic rocks. The samples are composed of abundant analcite after leucite, subordinate augite, sanidine, fluorite (except for the stratigraphically-deepest and most SiO2-poor sample), and minor anorthite. Quartz is present only in the stratigraphically-uppermost samples. The mineralogical composition is consistent with the origin of the sediments from different source rocks of the SVD. Major and trace elements allow for distinguishing among the samples. In particular, the four stratigraphically- uppermost samples share close SiO2, intermediate between the contents of the other two samples. Although several chemical characteristics of the Polledrara samples can be explained by impact of supergene processes on the sediments, the overall composition supports an origin from different source rocks of the SVD, in particular the “Tufi Stratificati Varicolori di La Storta” (TFVLS) and older pyroclastic rocks. The 87Sr/86Sr ratios of the samples (0.710182-0.710433) support the distinction of the sediments based on chemistry and mineralogy, and their Sabatinian origin. The isotopic range overlaps that in the lavas of Phases III-IV of the SVD and is slightly lower than the range in the TFVLS, likely because of advanced analcitization of the sediments. The ammonium acetate extracts from the samples show a narrow isotopic range (0.709613-0.709735), which is lower than the range of the bulk sediments but close to the isotopic composition of fossil elephant remains in the Polledrara sediments. Lastly, except for the most SiO2-rich sample exhibiting a higher value (0.512227), the 143Nd/144Nd ratios of the other samples show a narrow range (0.512147e0.512171) and cannot be distinguished from each other. The isotopic range is similar to that of the TFVLS, but higher than that of Sabatinian lavas, supporting the origin of the sediments from the former rocks.