Characterization of carbonate crust from a recently discovered methane seep on the north Atlantic continental margin of the USA

This study is focused on mineralogical and chemical characterization of an authigenic carbonate rock (crust) collected at a recently discovered cold seep on the US North Atlantic continental margin. X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the carbonate rock is composed of microcrystalline aragonite cement, white acicular aragonite crystals (AcAr), equant quartz crystals, small microcrystalline aluminosilicates, and trace amounts of iron sulfide microcrystals. Element/calcium ratios were measured with laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) using a calcite standard, which was prepared by annealing USGS certified carbonate powder (MACS-3). The occurrence of microscopic, non-carbonate inclusions precluded evaluation of trace elements in the aragonite cement, but allowed for in situ analysis of AcAr crystals. Carbon and oxygen isotopes were analyzed via isotope ratio mass spectrometry (IRMS) and expressed as δ ¹³ C and δ ¹⁸ O. Low δ ¹³ C values suggest that aragonite grew as a result of anaerobic oxidation of methane and observed δ ¹⁸ O values indicate that the temperature of aragonite crystallization was 1.7–1.9 ^◦ C.