Pialassa Baiona is a temperate coastal lagoon connected with the Northwestern Adriatic Sea (44° 280 N and 44°310 E). Sediment cores and surface sediments were collected in two habitats within Pialassa Baiona: a) a salt marsh habitat characterized by the presence of saltmarsh vegetation, and b) a human impacted habitat close to anthropogenic source inputs. Cores were sampled by inserting one cylindrical Plexiglas hand corer (5-cm diameter, 50 cm long) into the sediment to a depth of 20-25 cm, and surface sediment samples (0–5 cm) were collected with a stainless-steel grab sampler. The cores were extruded in the field, sectioned into 1‐2 cm intervals, and analyzed for total organic carbon (OC), total nitrogen (TN), carbon isotope ratio (δ13C) and dry bulk density (upper 20 – 25 cm) and analyzed for organic carbon (OC), total nitrogen (TN), carbon isotope ratio (δ13C) and dry bulk density. Organic carbon (OC), total nitrogen (TN), and carbon isotopes (δ13C = [(13C/12C)sample/(13C/12C)standard − 1] × 1,000) were measured in sediment cores and surface sediments using a using a FINNIGAN Delta Plus XP mass spectrometer directly coupled to Thermo Fisher FLASH 2000 CHNS Elemental Analyzer. OC and TN content was expressed as the weight percentage of dried sediment, and carbon isotope results were reported in the standard delta notation with respect to the Vienna Pee Dee Belemnite. A ‘three-end member’ mixing model was used to estimate the relative contribution of the different organic matter (OM) sources: marine phytoplankton (FM), riverine-estuarine phytoplankton (FRE), and C3 saltmarsh plant material (FSM): δ13Csample = δ13CM x FM + δ13CFE x FFE + δ13CSM x FSM C/Nsample = C/NM x FM + C/NFE x FFE + C/NSM x FSM 1 = FM + FRE + FSM where FM, FRE and FSM are the relative contributions of marine phytoplankton, riverine-estuarine phytoplankton, and saltmarsh plant material, respectively. δ13CM (-18.97‰) and C/NM (4.95), and δ13CRE (-30.70‰,) and C/NRE (6.63) represent the end member POC signature recorded during distinct marine and estuarine phytoplankton blooms in the coastal lagoon (Guerra et al., 2013).The C3 saltmarsh plant end-member was selected on the basis of typical δ13CSM (-20.2‰) and C/NSM (15.35) values for Juncus spp. (Gebrehiwet et al., 2008; Hughes and Sherr, 1983; J. I. Kelleway et al., 2017; Kemp et al., 2012, 2010; Lamb et al., 2006). References Gebrehiwet, T., Koretsky, C.M., Krishnamurthy, R. V., 2008. Influence of Spartina and Juncus on saltmarsh sediments. III. Organic geochemistry. Chem. Geol. 255, 114–119. https://doi.org/10.1016/j.chemgeo.2008.06.015 Guerra, R., Pistocchi, R., Vanucci, S., 2013. Dynamics and sources of organic carbon in suspended particulate matter and sediments in Pialassa Baiona lagoon (NW Adriatic Sea, Italy). Estuar. Coast. Shelf Sci. 135, 24–32. https://doi.org/10.1016/j.ecss.2013.06.022 Kelleway, J.I., Saintilan, N., Macreadie, P.I., Baldock, J.A., Ralph, P.J., 2017. Sediment and carbon deposition vary among vegetation assemblages in a coastal salt marsh. Biogeosciences 14, 3763–3779. https://doi.org/10.5194/bg-14-3763-2017 Kemp, A.C., Vane, C.H., Horton, B.P., Culver, S.J., 2010. Stable carbon isotopes as potential sea-level indicators in salt marshes, North Carolina, USA. Holocene 20, 623–636. https://doi.org/10.1177/0959683609354302 Kemp, A.C., Vane, C.H., Horton, B.P., Engelhart, S.E., Nikitina, D., 2012. Application of stable carbon isotopes for reconstructing salt-marsh floral zones and relative sea level, New Jersey, USA. J. Quat. Sci. 27, 404–414. https://doi.org/10.1002/jqs.1561
