ISOTOPIC AND MINERALOGIC BIAS INTRODUCED BY PULVERIZATION OF ARAGONITE

Stable oxygen and carbon isotope data of biogenic and abiogenic aragonite are of great relevance in paleoclimate research. Wet-chemical analysis of such materials typically requires fine-grained powder. We studied the potential risk of unintentional calcite formation and isotope shift caused by different grinding and milling methods. Shells of the bivalve mollusk, Arctica islandica, and aragonite sputnik crystals were pulverized using a set of commonly used methods, including a hand-held drill, a vibromill operated at various settings (with and without liquid nitrogen as a coolant; changes in milling ball diameters, frequencies and processing durations) and an agate mortar and pestle. Identification of mineral phases was completed by powder X-ray diffraction (PXRD), Raman spectroscopy and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. The calcite content was quantified by PXRD Rietveld refinement. Increasingly vigorous grinding resulted in larger fractions of calcite (0.5 – 4.2 wt%) and a concomitant δ¹⁸O and δ¹³C decrease, specifically in bivalve shell material. The only method that produced pure aragonite powder was manual pounding of the aragonite sputniks with an agate mortar and pestle. None of the studied, commonly used machine-based pulverization methods generated pure aragonite powder from samples consisting originally of aragonite. These findings have significant implications for light stable isotope-based paleoclimate reconstructions. Except for abiogenic aragonite powder prepared by manual pounding, paleotemperatures would be overestimated by up to 2.7 °C with the settings tested here.