Fig. 1: Fe-OC spatial distribution within goethite ooids.
From: The geologic history of marine dissolved organic carbon from iron oxides
a, Raman spectra of synthetic goethite synthesized without organic matter (black line) and of a natural goethite ooid-containing organic matter (red line). The synthetic goethite was synthesized at 70 °C using FeCl3 as the Fe(III) source at pH 7 and aged for 60 days. The red spectrum is from a goethite ooid sample from the Aseri Formation (Ordovician, Estonia), the same as in d. A diagnostic peak at approximately 1,590 cm−1 (G-band) indicates the presence of organic carbon in goethite (Methods). b,c, An optical microscopy image and corresponding 2D Raman map of a modern ooid from Panarea Island, Italy, showing the presence (purple) and absence (turquoise) of Fe-OC at each pixel. d,e, Example line scans of goethite ooids from the Aseri Formation and the Sillaoru Formation (both Ordovician, Estonia), respectively, showing the spatial distribution of Fe-OC. The presence of OM was identified using the ratio I1,450–1,700/I1,200–1,450 (where I represents the integrated intensity within the defined spectral windows), and a threshold value determined from the raw data was used to convert calculated values to a binary format indicating the presence or absence of organic matter (Methods). Scale bar, 100 μm (b).
