Trace element partitioning in biogenic high-Mg calcite (Spirorbis) under in-situ near-natural conditions

High-Mg calcite biominerals are valuable archives of environmental change, yet their geochemical behavior under variable climate conditions remains underexplored. We cultured the tube-dwelling serpulid polychaete Spirorbis spirorbis in outdoor flow-through benthocosm systems simulating near-natural shallow-water habitats in the brackish Kiel Fjord (salinity: 12–18). Across three seasonal experiments with different controlled combinations of ambient and elevated pCO 2 (pH NBS 7.7–8.6) and temperature (10–20°C), we assessed the potential of the calcite shell of S. spirorbis as a geochemical archive. S. spirorbis produces high-Mg calcite (∼10 mol% MgCO 3 ). Trace and minor element compositions (Mg/Ca, Li/Ca, Li/Mg and Ba/Ca ratios) respond significantly to temperature and salinity. The Li/Mg partition coefficient shows a robust inverse relationship with temperature: D Li/Mg = − 0.0012 × T(°C) + 0.92 (R 2 = 0.95). Shell Sr/Ca, Li/Ca, and Ba/Ca ratios increase with pH, reflecting biologically mediated or precipitation rate-related kinetic effects on element incorporation. Ba/Ca ratios strongly correlate with seawater Ba/Ca (R 2 = 0.94, D Ba = 0.92), making it a promising proxy for coastal salinity in estuarine systems: salinity = -0.37 × Ba/Ca (µmol/mol) + 27.14 (R 2 = 0.97). Elemental mapping reveals chevron-aligned heterogeneities in Mg, Sr, Ca, and S with Mg-depleted outer layers suggesting mineral stabilization. S. spirorbis calcite displays strong environmental sensitivity and minimal vital effects, indicating a high potential as a multiproxy archive for reconstructing seawater temperature, salinity, and geochemistry in coastal ecosystems.