Stable isotopes as tracers of provenance in Holocene sediments from the Venetian-Paduan area (NE Italy)

This work reports an ab initio study on the carbon (C), nitrogen (N), and sulphur (S) elemental and isotope compositions of the Padanian Plain sediments collected in the Venetian-Paduan area (North Eastern Italy). The investigated sediments were already characterized by previous research as Western- and Eastern-Alpine provenance, and were conveyed to the plain by Po and Adige River, respectively (Natali & Bianchini, 2017). The sample group characterised by high Ni/Zn and Cr/Pb values conforms to modern Po River sediments, whereas a second group showing low Ni/Zn and Cr/Pb values conforms to the geochemical signature of modern Adige River sediments. However, this work challenges these assertions observing that 13C/12C, 15N/14N, 34S/32S are significantly different in Po and Adige River sediments. Our hypothesis is that the CNS geochemical signal is 1) mainly regulated by the inorganic and organic fractions included in the alluvial sediments, and 2) these organic fractions have in turn a specific composition in the distinct source catchments. For carbon, the TC isotopic fingerprint depends on the OC and IC contents and their relative isotopic ratio. The difference cannot be related to a distinct fertilization history as proposed for other study-cases (Kanstrup et al., 2011) and must be interpreted as a distinctive natural character of the sediment source area, which is peculiar for every hydrological basin (Li et al., 2020). In general, the δ13COC is controlled by the distribution of C3 and C4 plants, as according to their photosynthetic pathways ranges from −21‰ and −35‰ for C3 plants and from −9‰ to −20‰ for C4 plants. Moreover, in aquatic ecosystems the isotopic composition of the transported organic matter is also influenced by the autochthon growth of biomass constituted by algae and plankton. On the other hand, the δ13CIC is controlled by the presence of lithogenic (i.e., primary) or pedogenic (i.e., secondary) carbonates, which have values close to 0‰ or negative, respectively. We found out that the different CNS isotope fingerprint of Po and Reno River sediments is natural and not induced by anthropogenic contributions, but doesn’t necessarily reflect a lithogenic signature, i.e., it is not solely related to different parent rock types in the Po and Adige River catchments. In fact, we infer that bio-geochemical processes, characterized by distinct ecological conditions in the Po and Adige River catchments, are recorded in the CNS isotopic signatures. Po River sediments are generally few hundreds of years older and pertain to a basin having a path of nearly seven hundred kilometres, much longer that of Adige River. Consequently, soils developed on Po River sediments are comparatively more mature and record a more complete spectrum of biogeochemical processes that were more intense and affected nitrogen/sulphur compounds generating the distinctive isotope signatures. More in general, the presented data increase the knowledge on the local elemental and isotopic backgrounds. This is important because many pollutants contain significant CNS concentration and specific isotope composition. Therefore, they serve as natural baseline and will provide a new tool to recognize possible anthropogenic anomalies in the studied area