Abstract: This database contains biogeochemical data on different biogeochemical variables measured in three soil cores sampled in a saltmarsh community of the Bay of Santander (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 (?13Corg) (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 is

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