Geological context and sample quality

The Nama Group is a well-preserved terminal Neoproterozoic carbonate and siliciclastic sequence, ranging from upper shore-line/tidal flats to below-wave-base lower shoreface, deposited in a ramp system ∼550–541 Ma (refs 5, 16, 52, 53, 54, 55). Samples from nine shelf-to-basin sections within the Zaris and Witputs basins of the Nama Group encompass a range of palaeo-depths from outer- to inner-ramp settings (Supplementary Table 1). Stratigraphic correlations are well-established based on sequence boundaries and ash beds5,16. The age of the upper Nama Group is relatively well-constrained from U-Pb dating of three ash beds within the group, including one at 548.8±1 Ma in the Hoogland Member of the Kuibis Subgroup54, revised to 547.32±0.31 Ma (ref. 56). The base of the Nama Group is diachronous, but is between 553 and 548 Ma. The Proterozoic–Cambrian boundary is represented by a regionally extensive erosional unconformity near the top of the Schwarzrand Subgroup in the southern Basin53,54,57,58, which is overlain by incised-valley fill dated (U-Pb on an ashbed) at 539±1 Ma (ref. 54). Therefore, the Nama Group section spans at least 7 Myr and extends to within 2 Myr of the Ediacaran–Cambrian Boundary52.

Unweathered samples were selected and powdered or drilled avoiding alteration, veins or weathered edges. For Zebra River section, powders were drilled from thin section counterparts, targeting fine-grained cements. Carbonate rocks in the Nama Group are very pure, but they have all undergone pervasive recrystallization. Less than 15% of the samples in this study are dolomitized and there is no petrographic evidence for deep burial dolomitization in the Nama Group5,55.

Samples were logged for fossil occurrences and sampled within established sequence stratigraphic frameworks, using detailed sedimentology5,52,53 (see Supplementary Notes 1 and 2). The presence of different forms of skeletal biota, soft-bodied biota and trace fossils are reported for precise horizons where geochemical analyses have been performed5, indicated by grey lines in Figs 2 and 3. General local ecology, supported by additional information from the literature, is also marked, without associated grey lines. Our sampling focused on carbonates and hence skeletal fossils are over-represented compared with soft-bodied biota and trace fossils. We define ‘large’ skeletal animals as >10 mm in any dimension, which includes Cloudina hartmannae, some Namacalathus and Namapoikia.

Rare earth elements in carbonate rocks

Rare earth elements and yttrium (REY) have a predictable distribution pattern in seawater and non-biological carbonate rocks should preserve local water column REY at the sediment–water interface28. Ce anomalies develop progressively, but cutoff values are established to define negative and positive anomalies. We define a negative anomaly as , consistent with previous work59. A positive anomaly, using the same reference frame, would be defined as . However, as positive anomalies are not previously described from carbonate sediments, we cautiously use a higher cutoff, , to ensure any positive anomalies are environmentally significant with respect to positive anomalies recorded from some modern manganous waters (1.21–2.43) (see Supplementary Note 3 and Supplementary Fig 11 for discussion of cutoffs). Although positive or negative Ce anomalies in carbonate rocks probably represent seawater redox conditions, the absence of any Ce anomaly (0.9–1.3) is somewhat equivocal and could result from anoxic water column conditions or overprinting of any Ce anomaly during diagenesis or leaching31. Alternately, Ce anomaly formation may be disrupted in surface waters because of wind-blown dust or photo-reduction of Mn oxides60. Fe (oxyhydr)oxides may also be REY carriers, but do not contain the clear Ce enrichments observed in Mn (oxyhydr)oxides (see Supplementary Note 4 for discussion of the role of Fe (oxyhydr)oxides in REY cycling).

Diagenetic phosphates, Fe and Mn (oxyhydr)oxides, organic matter and clays can potentially affect the REY signatures of authigenic sedimentary rocks if they are partially dissolved during the leaching process61,62,63. Care has been taken to partially leach samples, to isolate the carbonate phase without leaving excess acid, which may leach contaminant phases (see ref. 31 for detailed discussion of methodology). Powdered calcite samples were cleaned in Milli-Q water and pre-leached in 2% nitric acid, to remove adsorbed and easily exchangeable ions associated with clay minerals. The remaining sample was partially leached, also in 2% (w/v) nitric acid, to avoid contributions from contaminant phases such as oxides and clays31. The supernatant was removed from contact with the remaining residue, diluted with 2% nitric acid and analysed via inductively coupled plasma mass spectrometry in the Cross-Faculty Elemental Analysis Facility, University College London. This leaching method has been designed to extract the carbonate-bound REY pool without contributions from (oxyhydr)oxides or clays31. These same leachates were also analysed for major element concentrations (Mg, Fe, Mn, Al and Sr) via inductively coupled plasma optical emission spectrometry. Oxide interference was monitored using the formation rate of Ce oxide and the formation of 2+ ions was monitored using Ba2+. All REY concentrations were normalized to post-Archean Australian Shale.

Standard solutions analysed after every ten samples were within 5% of known concentrations. Replicate analyses on the inductively coupled plasma mass spectrometry give a relative s.d. <5% for most trace elements, with a larger s.d. for the heavy REE that sometimes have non-normalized concentrations <0.5 p.p.b. Carbonate standard material CRM 1c was prepared using the same leaching procedure as the samples and repeat analyses give a relative s.d. <5% for individual REY concentrations, and calculated Ce anomalies (average=0.80) give a relative s.d. <3%.

Mn/Sr ratios are <1 for the majority (97%) of samples and δ18Ocarb is >−10‰, indicating minimal open-system elemental and isotopic exchange during diagenesis, and excluding deep burial dolomitization (Supplementary Fig. 12). Ce anomaly data are only presented for carbonates that preserve seawater REY features (smooth patterns with molar Y/Ho>67)28,31, indicating they originate from the carbonate portion of the whole rock, without contributions from detrital or oxide phases. For samples with Y/Ho>67, 85% also have ∑REE <2 p.p.m. and all have ∑REE <10 p.p.m. La anomalies, and small positive Eu and Gd enrichments are prevalent in samples with Y/Ho>67 (Supplementary Fig. 11 and Supplementary Note 5 for discussion of Y anomaly thresholds). Positive Ce anomalies are associated with low Mn/Sr ratios (<1) and low Al, Zr, Ti, Fe and Mn contents in the leachate (<0.2 wt% for Fe and <500 p.p.m. for Mn), indicating minimal contamination due to diagenetic exchange, leaching of clays or Fe–Mn (oxyhydr)oxide phases (Supplementary Fig. 12).

Rare earth elements in shales

Shales from throughout the Zebra River section, including inter-reef deposits and lateral subordinate shales between grainstone horizons, were fully digested using HNO 3 -HF-B(OH) 3 -HClO 4 at the University of Leeds. These full digestions include the dominant siliciclastic component, but would also encompass any subordinate (oxyhydr)oxide phases, organic matter or carbonate components. The full digestions were dried down, washed twice in 50% nitric acid and resuspended in 2% nitric acid for analysis on an inductively coupled plasma mass spectrometry in the Cross-Faculty Elemental Analysis Facility, University College London.

Relative to standardized shale composition, post-Archean Australian Shale64, the Zebra River shales show consistent patterns (Supplementary Fig. 13), with middle-REY enrichment (bell-shaped index=1.25) and negative Y anomalies (shale-normalized Y/Ho=0.88), but no anomalous Ce behaviour. These patterns resemble those reported for Fe (oxyhydr)oxides65,66 and may well derive in part from the high Fe ox contents of these shales (up to 1.2%). Shales carry a ‘continental-type’ REY pattern and represent a baseline from which surface-solution fractionation of REY begins, and thus they are commonly used to normalize seawater REY patterns.

Fe speciation in carbonates and siliciclastics

The Fe speciation method quantifies Fe that is (bio)geochemically available in surficial environments (termed Fe HR ) relative to Fe T . Mobilization and subsequent precipitation of Fe in anoxic water column settings results in Fe HR enrichments in the underlying sediment. The nature of anoxia (that is, sulfide-rich or Fe-containing) is determined by the extent of sulfidation of the Fe HR pool1. Fe speciation data for carbonate rock samples discussed here and accompanying interbedded siliciclastic rocks come from previously published data5. The Fe-speciation technique was performed using well-established sequential extraction schemes1,67. The method targets operationally defined Fe pools, including carbonate-associated-Fe (Fe Carb ), ferric oxides (Fe Ox ), magnetite (Fe Mag ), pyrite Fe (Fe Py ) and Fe T . Fe HR is defined as the sum of Fe carb (extracted with Na-acetate at pH 4.5 and 50 °C for 48 h), Fe ox (extracted via Na-dithionite at pH 4.8 for 2 h), Fe mag (extracted with ammonium oxalate for 6 h) and Fe py (calculated from the mass of sulfide extracted during CrCl 2 distillation). Fe T extractions were performed on ashed samples (8 h at 550 °C) using HNO 3 -HF-H 3 BO 3 -HClO 4 . All Fe concentrations were measured via atomic absorption spectrometry and replicate extractions gave a relative s.d. of <4% for all steps, leading to <8% for calculated Fe HR . Fe Py was calculated from the wt% of sulfide extracted as Ag 2 S using hot Cr(II)Cl 2 distillation68. A boiling HCl distillation before the Cr(II)Cl 2 distillation ruled out the potential presence of acid volatile sulfides in our samples. Pyrite extractions give reproducibility for Fe py of 0.005 wt%, confirming high precision for this method. Analysis of a certified reference material (PACS-2, Fe T =4.09±0.07 wt%, n=4; certified value=4.09±0.06 wt%) confirms that our method is accurate. Replicate analyses (n=6) gave a precision of ±0.06 wt% for Fe T and a relative s.d. of <5% for the Fe HR /Fe T ratio.

Calibration in modern and ancient marine environments suggests that Fe HR /Fe T <0.22 indicates deposition under oxic water column conditions, whereas Fe HR /Fe T >0.38 indicates anoxic conditions1. Ratios between 0.22–0.38 are considered equivocal and may represent either oxic or anoxic depositional conditions. For sediments identified as anoxic, Fe py /Fe HR >0.8 is diagnostic for euxinic conditions and Fe py /Fe HR <0.7 defines ferruginous deposition1. Although originally calibrated for siliciclastics1,32, enrichments in Fe HR /Fe T can also be identified in carbonates deposited under anoxic water column conditions69. These Fe HR enrichments can far exceed Fe HR contents expected under normal oxic deposition, where trace amounts (∼0.1 wt%) of Fe may be incorporated into carbonates, or precipitate as Fe–Mn coatings69. However, although early dolomitization in shallow burial environments does not generally cause a significant increase in Fe HR , late-stage deep-burial dolomitization may significantly increase Fe HR 69, but there is no petrographic evidence for deep-burial dolomitization in our samples5,16. Consistent with a recent calibration69, we have limited the application of Fe speciation to carbonate samples with >0.5 wt% Fe T , which buffers against the impact of non-depositional enrichments in Fe HR 69. Where Fe T is very low (<0.5wt%), this may indicate deposition under oxic conditions69. In addition, however, we stress that all of our redox interpretations based on Fe speciation in carbonates are entirely consistent with data from siliciclastic horizons interbedded with and/or associated with carbonate rocks contained within the same m- to dm-scale depositional cycle5.

Equivocal Fe HR /Fe T ratios could be a consequence of dilution of a high water column Fe HR flux through rapid sedimentation32 (for example, in turbidite settings) or post-depositional transformation of unsulfidized Fe HR minerals to less reactive sheet silicate minerals8,67. Further, local Fe HR enrichments can occur due to preferential trapping of Fe HR in inner shore or shallow marine environments (for example, flood plains, salt marshes, deltas and lagoons). However, none of the presented Fe HR data here are from rocks that show evidence of turbiditic deposition and are from dominantly open marine settings. Oxidative weathering may result in mineralogical transformation of Fe minerals. Oxidation of siderite would transfer Fe carb to the Fe ox pool and hence any interpretation of ferruginous or euxinic signals would remain robust. The weathering of pyrite to Fe ox would not affect interpretation of anoxic signals (Fe HR /Fe T >0.38), but may reduce the Fe py /Fe HR ratio, giving a false ferruginous signal in a euxinic sample. In the extreme and highly unlikely scenario that all Fe ox in our samples is a product of pyrite weathering, ∼10% of the anoxic samples would give a euxinic signal. However, significant Fe carb (>20% of the Fe HR fraction) occurs in ∼57% of anoxic samples, indicating that the rocks have not been completely weathered and hence this extreme scenario is unlikely.

Data availability

All relevant data are available to download in the data repository associated with this manuscript and further details on the Fe-speciation data are available in ref. 5.