See Estes el al. (2019) for complete methods.

Sediment slurries were generated by subsampling ~0.1 g of wet sediment from samples stored at 4 °C into a sterile microcentrifuge tube. Then, 0.2–0.5 ml of 0.2 μm filtered 18 MΩ water was added
and the sample shaken. Sediment slurry (1–10 μl) was pipetted onto silicon wafers and air-dried. The beamline was operated with a 500 l mm–1 spherical grating monochromator and entrance and exit slits set to 40 μm, which yielded an absolute energy resolution of less than 0.3 eV. The samples were attached to an aluminium sample stick in a single load and analysed under ultrahigh vacuum conditions (pressure ~10−9 mbar). The measurements were made in the total electron yield (TEY) mode on a spot size of less than 1 mm2 using a grazing incidence angle
of 45°, where previous trials determined that the incidence angle did not yield a
difference in results. The TEY mode was selected instead of fluorescence as we observed dampening of the fluorescence signal, probably due to matrix-induced absorption.

Spectra were collected around the C 1s edge, from 260 to 340 eV, with a dwell time of 0.2 s. To avoid beam damage and variation of the background due to charging, scans were taken at different positions on the sample. The spectra analysed were the average of 2–3 scans taken at different positions on the sample. The dark current was measured prior to the collection of each spectrum and subtracted from the raw data. Spectra were then normalized to no load current measured by a mesh upstream of the chamber with freshly evaporated gold.

Instruments:

Bulk carbon NEXAFS spectroscopy was conducted on beamlines 8-2 and 10-1 at the Stanford Synchrotron Radiation Lightsource.

The absolute energy calibration of the carbon spectra was achieved by shifting the energy such that the first dip in the incoming intensity due to carbon contamination on the beamline optics (carbon dip) occurred at 284.7 eV.