Abstract
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 δ13C and δ18O. Low δ13C values suggest that aragonite grew as a result of anaerobic oxidation of methane and observed δ18O values indicate that the temperature of aragonite crystallization was 1.7–1.9 °C.
Highlights
Authigenic carbonates, which are a significant ocean carbon sink, occur widely as carbonate crusts near cold methane seeps at the seafloor [1,2]
We compared our data with published values for elemental concentration in NIST-610, NIST-612, and MACS-3 (Tables S1–S3)
100% was observed between certified data for NIST glass standards and when NIST glasses were treated as unknowns and reduced with MACS-3 carbonate standard for Na, Zr, and U; and a difference greater than 10% was observed for Mg, S, Mn, Fe, and Ba (Tables S1 and S2)
Summary
Authigenic carbonates, which are a significant ocean carbon sink, occur widely as carbonate crusts near cold methane seeps at the seafloor [1,2]. The commonly accepted mechanism of carbonate precipitation by AOM is expressed as [3,4]: CH4 + SO4 2− → HCO3 − + HS− + H2 O (1). HCO3 − + Ca2+ → CaCO3solid + H+ (2) This mechanism, in which HCO3 − is released, increases alkalinity and promotes carbonate crystallization within seafloor sediment pore space as well as the production of authigenic carbonate rock. McVeigh et al [5] reported that AOM was directly observed within overlying authigenic carbonate rocks collected at the seep site studied in this work. The reconstruction of methane seepage in the past through analysis of seep associated authigenic carbonate rock could lead to an improved understanding of the evolution of ocean and atmospheric carbon cycles, as well as past climate change events. 54,56,58 Fe, 55 Mn, 88 Sr, 90,91 Zr, 137,138 Ba, 235,238 U 60 s 20 s
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