Data from pore water (subterranean estuary) and seawater from Spiekeroog south (near ICBM time series station and campsite, 53°45'13.5"N 7°40'22.5"E) and west beach ("Sturmeck", 53°46'10.0"N 7°40'26.2"E). South beach data were collected in August 2012, and west beach data were collected in November 2012. Sample abbreviations: SB=South Beach, WB=West Beach. DUNE=most landward station near dunes, MIX=mid-way station between dune base and low water line, LTWL=low tide water line. Pore water (DUNE, MIX, LTWL) sample collection was conducted at different sediment depths (50, 100, 150 cm below sediment surface). Stainless steel push-point lancets were insetred into the sediment, and pore water was withdrawn via vacuum (hand pumps) into nalgene polycarbonate bottles. Filtration was done using inline PES cartridge filters. The vacuum bottles were argon gas-flushed to avoid oxygen contamination. Sea water (SW) was collected with polycarbonate bottles and from LDPE seepage meter bags (SP) and filtered upon return to the laboratory (same day, PES filter cartridges). Sample collection was trace organic and metal clean (soaking and rinsing of bottles, tubing, and filters with diluted HNO3 and HCl suprapur), with sample materials consisting of polyethylene, polypropylene, and polycarbonate. Solid-phase extraction was done with BOND Elut PPL cartridges and elution with Methanol Optima grade. Measurements were done with VA Computrace 757 (Cu ligand concentrations and stability constants), HR-ICP-MS (Cu, Fe, and Mn concentrations), FT-ICR-MS (DOM), spectrophotometry (nutrients), and TOC analyzer (DOC and TDN). Trace metal concentrations (Fe, Cu, Mn) and speciation (oxidation state, size fractions, and organic association), as well as nitrogen species and concentrations: "Spiekeroog beach subterranean estuary environmental data". Concentrations (µM or nM) are denoted in the headers. CuL1=Cu-binding ligands concentrations (nM), logK1=Cu-binding ligand stability constant. Molecular composition of dissolved organic matter and associated Fe and Cu: "Spiekeroog beach STE_ESI_x_Crosstab_x". pos & neg = positive and negative ionization mode. BSA & NWA = basic/strong acidic and neutral/weak acidic, refer to the extracted DOM fraction. Processed figures and tables based on the original data are published here: https://doi.org/10.1016/j.gca.2019.06.004

Supplement to: Waska, Hannelore; Brumsack, Hans-Jürgen; Massmann, Gudrun; Koschinsky, Andrea; Schnetger, Bernhard; Simon, Heike; Dittmar, Thorsten (2019): Inorganic and organic iron and copper species of the subterranean estuary: Origins and fate. Geochimica et Cosmochimica Acta, 259, 211-232