Analyses
X-ray diffraction (XRD) was used to determine the mineralogy of Lake Fryxell carbonates. Four pieces of freeze-dried carbonate, two each from depths of 9.0 and 9.7 m (Table S1), were crushed into powders using a ceramic mortar and pestle, and 230-340 μg of each powder was weighed out for analysis. XRD analysis was performed using a Rigaku SmartLab diffractometer (Tokyo, Japan) with Cu Kα radiation (wavelength = 1.54059 Å) and a DeteX Ultra detector. Spectra were collected in the 3-150° 2θ range with a step size of 0.02° and a count time of 6°/minute.
Transmitted- and reflected-light microscopy were performed on thin sections and billets using Nikon SMZ25 stereo and Optiphot-Pol petrographic microscopes (Tokyo, Japan) to assess petrographic characteristics of these carbonates. Cathodoluminescence (CL) microscopy was conducted using a Nuclide Corporation Luminoscope ELM-2A cold-cathode microscope. Polished thin sections and billets were placed in the Luminoscope chamber under low vacuum (30-50 mtorr) and CL images were captured using a voltage of 5.5-6.0 kV to obtain a beam current of 0.7-0.8 mA.
Samples for δ13C/δ18O analysis were extracted from billets using a hand-held Proxxon Micromot 60/E drill (Hickory, NC, USA) with Kodiak Cutting Tools carbide end mill bits (Syracuse, NY, USA) of diameters 1/64” and 1/32”. Carbonate powders were washed in 30% hydrogen peroxide overnight to remove organic carbon, then dried at 50-70 °C for several hours. Carbon and oxygen isotope ratios were measured at the Center for Stable Isotopes, University of New Mexico using the method described by Spotl and Vennemann (2003). Samples were loaded in 12 mL borosilicate exetainers, then the exetainers were flushed with He and the samples were reacted with H3PO4 for 12 hours at 50º C. The evolved CO2 gas was measured by continuous flow isotope ratio mass spectrometry using a Thermo Fisher Scientific Gasbench coupled to a Thermo Fisher Scientific Delta V Plus Isotope Ratio Mass Spectrometer (Waltham, MA, USA). Isotope ratios are reported in delta notation on the VPDB scale.
Elemental composition data were acquired by laser ablation and inductively coupled plasma optical emission spectrometry (LA-ICP-OES). Elemental analysis focused on concentrations of Mn and Fe ([Mn] and [Fe], respectively). In-house standards were prepared by diluting BCS-CRM 393 limestone and NBS 2710 Montana soil with BCS-CRM 313 high-purity silica to create a set of references encompassing a wide range of [Mn] and [Fe]. LA-ICP-OES was performed using a New Wave Research UP-213 laser ablation system (Fremont, CA, USA) connected to a PerkinElmer Optima 4300 DV optical emission spectrometer (Waltham, MA, USA) with Ar plasma and Ar as a carrier gas at a flow rate of 600 mL/min. [Mn] and [Fe] were measured at selected locations on four billets corresponding to thin sections of Lake Fryxell carbonates (see Table S1). Ablation was conducted with the laser set to 80% power and 10 Hz firing speed. Linear transects were scanned with a spot size of 55 or 80 μm and a scanning speed of 4-8 μm/s; single-spot analyses were performed with a spot size of 55-100 μm and a dwell time of 45-75 seconds.