Biogeochemical variables along the soil profile of three saltmarsh cores sampled in an estuary of the Gulf of Biscay

This database contains data on different biogeochemical variables measured in three soil cores (32-39 cm long) sampled in a saltmarsh community of the Bay of Santander (Gulf of Biscay). Soil cores were extracted in June 2019 within an area of 25 m2 in a high marsh community located at the mouth of the Miera estuary, within the Bay of Santander (43.452136°/ -3.748134°) by manually hammering a PVC tube (60 cm L * 7 cm Ø). Compression was measured during sampling of each of the cores. The cores were preserved frozen until processing. The longest core (BS2A1, 39 cm) was sliced every 1 cm, whereas the other two cores (BSA2, BS2A3) were sliced every 2 cm for the top 20 cm and every 5 cm for the deeper layers. Each sediment slice was measured for wet volume and dried at 60 ºC for a minimum of 72 h. The dry weight of each slice was measured and used together with wet volume to estimate sediment dry bulk density (DBD in g·cm-3). Soil organic carbon content (Corg % DW) was measured every two or three slices along the sediment depth profile of each core. Corg was analyzed in the IHLab Bio laboratory of the IHCantabria using a TC analyzer (Shimadzu TOC-L + SSM-5000A). Grain size analysis was performed every other sample at the Universitat de Barcelona with a Beckman Coulter LS GB500. Organic Corg isotopic signature (δ13Corg) (in pre-acidified subsamples) was measured using an Elemental Analyzer Flash IRMS coupled with an Isotope Ratio Mass Spectrometry (DeltaV A) at the Universidad de la Coruña. The years of sediment accumulation were estimated from concentration profiles of 210Pb, determined by alpha spectrometry through the measurement of its granddaughter 210Po, assuming radioactive equilibrium between both radionuclides. About 100–200 mg aliquots of each sample were spiked with 209Po and microwave digested with a mixture of concentrated HNO3 and HF. Boric acid was then added to complex fluorides. The resulting solutions were evaporated and diluted to 100 mL 1M HCl and Po isotopes were auto plated onto pure silver disks. Polonium emissions were measured by alpha spectrometry using PIPS detectors (CANBERRA, Mod.PD-450.18 A.M). Reagent blanks were comparable to the detector backgrounds. Analyses of replicate samples and reference materials were carried out systematically to ensure the accuracy and the precision of the results. The supported 210Pb was estimated as the average 210Pb concentration of the deepest layers once 210Pb reached constant values. Then, excess 210Pb (210Pbxs) concentrations were obtained by subtracting the supported 210Pb from the total 210Pb. Age model of the sediment depth profile records was obtained by modeling the 210Pbxs concentration profiles along the accumulated mass at each site. The model age of the sediment record was estimated using the Constant Flux: Constant Sedimentation model (CF:CS, 76). In order to assess the impact of the bridge construction on the biogeochemical properties of the saltmarsh soil, we compare all biogeochemical properties across two sections of the core, divided based on the results of 210Pb dating: sediments accumulated before and after the building of the bridge (i.e., before vs. after 1978).concentration profiles of 210Pb and applying the Constant Flux: Constant Sedimentation model (CF:CS, Krishnaswamy et al. 1971).