Data from pore water (subterranean estuary) and seawater from Spiekeroog south (near ICBM time series station and campsite) and west beach ("Sturmeck"). South beach data were collected in August 2012, and west beach data were collected in November 2012. Pore water (event labels: 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 Stainless steel push-point lancets were insetred into the sediment, and pore water was withdrawn via vacuum (hand pumps) into nalgene polycarbonate bottles.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 (event label SW) was collected with polycarbonate bottles and from LDPE seepage meter bags (event label 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 dissolved organic matter (DOM) were done with FT-ICR-MS. The crosstables describe the molecular composition of DOM and associated Fe and Cu. They contain characteristic properties and classifications of molecular sum formulas as well as FT-ICR-MS signal intensities of each sum formula for each sample (see event table of https://doi.pangaea.de/10.1594/PANGAEA.902704). File name description of “Spiekeroog beach STE_ESI_x_Crosstab_x": pos & neg = positive and negative ionization mode of electrospray ionization (ESI). BSA & NWA = basic/strong acidic and neutral/weak acidic DOM fraction (solid phase extracted)