Extensive primary production promoted the recovery of the Ediacaran Shuram excursion

Member IV of the Ediacaran Doushantuo Formation records the recovery from the most negative carbon isotope excursion in Earth history. However, the main biogeochemical controls that ultimately drove this recovery have yet to be elucidated. Here, we report new carbon and nitrogen isotope and concentration data from the Nanhua Basin (South China), where δ13C values of carbonates (δ13Ccarb) rise from − 7‰ to −1‰ and δ15N values decrease from +5.4‰ to +2.3‰. These trends are proposed to arise from a new equilibrium in the C and N cycles where primary production overcomes secondary production as the main source of organic matter in sediments. The enhanced primary production is supported by the coexisting Raman spectral data, which reveal a systematic difference in kerogen structure between depositional environments. Our new observations point to the variable dominance of distinct microbial communities in the late Ediacaran ecosystems, and suggest that blooms of oxygenic phototrophs modulated the recovery from the most negative δ13Ccarb excursion in Earth history.

were obtained simultaneously using a calibration against Acetanilide as a procedural standard.
Nitrogen isotope analyses were performed by combustion in a Thermo Finnigan Flash EA (1112 series) on-line to a Delta Plus XL mass spectrometer. δ 15 N was calculated with respect to the δ 15 N value of air using the BROC2 standard as a within-run laboratory standard calibrated against USGS40 and USGS41. Overall analytical accuracy for these samples is, on average, better than 0.2‰. Replicate analysis of some samples (n=8) indicated a reproducibility of <0.2‰ (1 SD).

d) Optical microscopy and Raman spectroscopy
Raman spectra and hyperspectral images of organic matter were obtained in the Geological Spectroscopy Laboratory at UCL with a WITec Confocal Raman Imaging system using a 532 nm laser at up to 1000x magnification. Carbonaceous targets were detected in standard petrological thin sections (30 μm in thickness) using an optical microscope (Olympus BX51) equipped with 4x,10x,20x,50x and 100x objectives. Cosmic ray reduction was applied to all spectra, and their backgrounds fitted to a polynomial function and subtracted.

Isotopic signatures: Post-depositional analysis
Post-depositional alteration assessment is critical to interpreting the chemostratigraphies of C and N in their depositional environments. This is because metamorphic processes alter

a) Geochemical cross-plots
The extent of diagenetic and metamorphic alteration on the studied samples was evaluated using linear relationships of elemental abundances in four different plots: δ 13 Corg-TOC, TOC-TN, TN-δ 15 N and Mn/Sr-δ 13 Ccarb. The δ 13 Corg -TOC plot is generally used to assess whether the degradation of organic matter during diagenesis affects the δ 13 Corg data 6 . This is because degradation of organic matter during diagenesis would have caused 13 C-depleted loss and residues to be enriched in 13 C 2,4 . However, the data from the newly studied samples, shown in Figure S3-a, present no negative correlation between δ 13 Corg and TOC values, indicating insignificant post-depositional effects δ 13 Corg values. The TOC-TN plot is usually used to assess the co-variations of nitrogen content with TOC and whether N is derived from organic matter or sourced from inorganic clay-bound nitrogen. In Figure S3-b, it is observed that total nitrogen positively correlates with total organic carbon in the six sections showing a linear relationship with high correlation coefficients (R >90) in most sections, except in Taoying and Fengtan. The linear relationship intercepts on the N axis in these latter sections, meaning that clay-bound nitrogen contributes to the measured N data only in these sections.
Thus, it can be inferred that, in general, N mainly occurs in organic matter, and an insignificant isotopic fractionation by diagenesis or metamorphism is expected. The TN-δ 15 N plot is generally used to assess whether the loss of N may have substantially altered the δ 15 N values. This is because δ 15 Nbulk values in sediments typically increase with decreasing TN content due to the preferential loss of 14 N during thermal alteration 7 . However, such a negative correlation is not found in the new data, indicating an insignificant alteration of δ 15 N values and minimal N loss (Fig. S3-c). Finally, the Mn/Sr ratio is used to evaluate the possible impact of meteoric diagenesis on carbonate. The interaction of carbonates with meteoric fluids during diagenesis, metamorphism, or dolomitization increases the Mn/Sr ratio and decreases the δ 13 Ccarb 8 . This is because Sr leaches from carbonate during meteoritic diagenetic processes, and Mn is incorporated in diagenetic cement. It was suggested that carbonates with Mn/Sr <10 usually retain near primary δ 13 Ccarb abundances 9 . The Mn/Sr-δ 13 Ccarb plot in Figure S3-d shows that most of the measured Mn/Sr ratios in this study are <10, and no considerable decrease in δ 13 Ccarb is seen. Thus, meteoric diagenesis on carbonate samples is negligible.  Figure   S4 shows examples of the fitted curves after spectra deconvolution in proximal and distal sections. Two different area ratios, RA1 and RA2, are calculated with a result of 0.60-0.62 and 1.50-1.65, respectively. Results show peak metamorphic temperature (T1) estimates between 280 o C and 309 o C (Table S2), approximately at the sub-greenschist metamorphic facies and point to insignificant isotopic fractionation. These temperatures are consistent with previous temperatures reported for the Doushantuo Fm. 10,11 . In addition, these results also show a thermal basin distribution with temperatures 280-289 o C in the proximal sections 300-309 o C in the distal sections.

Member IV stratigraphic correlation
One of the principal uncertainties in the Nanhua basin is the stratigraphic correlation of In our stratigraphic correlation, we use stratigraphic, paleontological and geochemical arguments to support the "Z" correlation for Member IV. The segmented geometry on the Yangtze platform during deposition of the Doushantuo Fm. likely favoured abrupt lateral variation of facies by which the UD interval would have thinned down and pinched out, to a point where it eventually vanished even over a short distance (< 50 km) 12 . Also, taxonomic differences, principally derived from morphological traits, have been reported between the Miaohe Member and Shibantan Member (Dengying Fm.) biotas 21,22 . Moreover, the Eoandromeda taxa, present in the Miaohe biota, has also been found in Australia, suggesting that the Miaohe biota forms part of the 'White Sea assemblage' 23 whereas the assemblage of fossils in the Shibantan biota is typical of the younger Nama assemblage 22 . In terms of geochemical differences, it has been widely reported that Member IV and the Dengying Fm.
have distinct 13 C-depletions in organic matter 13,24,25 Figure S2 shows the Nanhua basin stratigraphic correlation where the black shales described in this study correlate with the Member IV of the Doushantuo Fm. and are interpreted to be equivalent to the Shuram excursion. Fig. S1 Figure S1b. The correlation of Member IV in the shelf lagoon is the same as the "Z" correlation 3 , where the Miaohe Member in the western region (Qinglinkou) correlates with the single black shale unit in the central region (Jiulongwan). δ 13 Ccarb and δ 13 Corg data from this study, Jiulongwan 4 , Siduping 5 and Fengtan This study, 6,7 .    Table   Table S2 Parameters