Large isotopic variability at the micron-scale in ‘Shuram’ excursion carbonates from South Australia

•First in-situ SIMS δ13C - δ18O values of the ‘Shuram’ excursion from South Australia.•We argue for a water column origin for very low δ13C values (down to -12 permil).•We also document very early diagenetic dolomite with δ13C up to +5 permil.•Large range (10-12 permil) in δ18O values indicate very low water/rock ratios.•Large δ13C heterogeneity implies large δ13C gradients in Ediacaran oceans.