Granse, Dirk; Muhr, Jan; Niessner, Andreas; Jensen, Kai: Radiometric dating reveals sediment dynamics of restored polders and a reference salt marsh: implications for accretion and blue carbon in the Wadden Sea [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.997869 (dataset in review)

Salt marshes are valued for their ecosystem services, including blue carbon storage, biodiversity support, and wave attenuation. Reconstructing long-term sedimentation dynamics is essential for assessing accretion rates and blue carbon stocks, and for projecting the future development of salt marsh ecosystems under global change. Radiometric dating, based on lead (²¹⁰Pb) and cesium (¹³⁷Cs) profiles, is a suitable method for this purpose. We applied it to soil cores from restored polders and a reference salt marsh in the souther part of the German Wadden Sea. A total of three soil cores (topsoil down to a maximum depth of 90 cm) were collected from two former polders, restored 15 years ago (recently grazed) and 30 years ago (managed, ungrazed), and from a reference salt marsh with similar geomorphological settings. Cores were taken by driving a tube (10 cm in diameter) into the soil, and sampling-related compaction was corrected. Cores were cut into 2 cm slices, dried to constant weight at 105 °C, and ground to <630 µm. Homogenized, sealed samples were analyzed at Laboratory of Radioisotopes, University of Göttingen after an equilibration period of 28 days, using a high-purity germanium detector (ORTEC C-Series PROFILE GEM Coaxial P-type, Ametek GmbH, Meerbusch, Germany; relative efficiency 30%, resolution 1.9 keV at 1.33 MeV) with a counting time of 69 h. Energy and efficiency calibration was performed with a commercially available multi-gamma standard solution (No. 7603, Eckert & Ziegler Isotope Products, Valencia, CA, USA). Activities of ²¹⁰Pb, ²¹⁴Pb, ²¹⁴Bi, ¹³⁷Cs, and ²⁴¹Am were determined from the peaks of discrete gamma energy lines using LabVision (v. 3.2.7.0, AMETEK/ORTEC). The measurements revealed distinct ¹³⁷Cs peaks, most likely corresponding to the 1986 Chernobyl accident and the nuclear weapons tests of the 1950s, while ²¹⁰Pb activity decreased with soil depth in all cores. Radiometric dating of soil cores from restored polders and the reference salt marsh thus provides valuable data for comparing the sediment dynamics of restored polders with those of a natural salt marsh.