Marine redox fluctuation as a potential trigger for the Cambrian explosion

735 ERRATUM: Marine redox fluctuation as a potential trigger for the Cambrian explosion Guang-Yi Wei, Noah J. Planavsky, Lidya G. Tarhan, Xi Chen, Wei Wei, Da Li, and Hong-Fei Ling ORIGINAL ARTICLE: 2018, v. 46, no. 7, p. 587–590, https://doi.org/10.1130/G40150.1, First published 30 May 2018 ERRATUM PUBLICATION: 2018, v. 46, no. 8, p. 735, https://doi.org/10.1130/G40150E.1, First published 3 July 2018 In Figure. 2B, seawater δ238U values are calculated as a function of relative proportions of suboxic (ASOx) and anoxic (AAOx) seafloor areas. The correct Figure 2B is provided here.

U isotopes were measured with Thermo Neptune plus MC-ICP-MC instruments with a ESI Apex IR desolvating system.U isotope analyses were under low-resolution (LR) and beam intensities for 232 Th, 233 U, 234 U, 235 U, 236 U, 238 U were measured in Faraday cups of L3, L2, L1, C, H1, H3, respectively. 232Th was analysed in order to monitor the interference of 234 Th in the sample solution.All of the U isotope ion beam signals were measured with the 10 11 Ω resistance.

DR2. Ce anomaly calculation
Ce anomaly is calculated using equation Ce/Ce* = Ce N / (Pr N 2 / Nd N ) of Lawrence et al. (2006) to avoid the effect of La overabundance that may disguise the genuine Ce anomaly (Ling et al., 2013), where lower subscript N denotes normalization of concentrations against the post-Archean Australian shale (PAAS; McLennan, 1989).

Yangtze Gorges region
The Ediacaran and the early Cambrian successions in this region consist of the Doushantuo Formation, Dengying Formation (the Gaojiaxi section in this study) and Yanjiahe Formation (the Yanjiahe section in this study), which records sedimentary deposition in shallow water below or near wave base in a continental inner shelf.According to lithostratigraphic criteria, the Doushantuo Formation in the Yangtze Gorges area can be subdivided into four members (e.g., Jiang et al., 2011).Doushantuo Member I comprises a ca.5-m-thick cap dolostone overlying the Marinoan glacial diamictite of the Nantuo Formation.The bottom part of the cap dolostone contains a volcanic ash layer that has been dated at 635.2 ± 0.6 Ma with zircon U-Pb methods (Condon et al., 2005).Doushantuo Member II is ca.70 m thick and comprises interbedded organic-rich shale and dolostone beds with abundant pea-sized cherty nodules which contain complex microfossils including acanthomorphic acritarchs, probable animal eggs, embryos, multicellular algae, and cyanobacteria (e.g., Yin et al., 2007;McFadden et al., 2008).
Doushantuo Member III is a ca.50-m-thick interval comprised of dolostones intercalated with cherty layers in the lower part and interbedded with limestone beds in the upper part.
Doushantuo Member IV is characterized by a 10-m-thick organic-rich black shale interval used as marker beds for stratigraphic correlation (e.g., Jiang et al., 2011).
The  (Jiang et al., 2012).The Ediacaran-Cambrian boundary is suggested to be at the bottom of the lower dolostone layer where the small shelly fossil assemblage zone I first occurs (Chen, 1984).
The Yanjiahe Formation is overlain by Shuijingtuo Formation with stratigraphic discontinuity between the two formations.The Shuijingtuo Formation can be correlated with the Niutitang Formation or Jiumenchong Formation in slope and basin area, South China, which is considered the base of Cambrian Stage 3 (Jiang et al., 2012).

Xiaotan section
The Xiaotan section is located on the southern bank of the Jinsha River, NE Yunnan Province, which represents an inter shelf sedimentary environment (Li et al., 2013).The Xiaotan section comprises, from the oldest to youngest, late Ediacaran Dengying Formation (upper Donglongtan Member, Jiucheng Member and Baiyanshao Member), early Cambrian Zhujiaqing Formation (the Daibu Member, Zhongyicun Member, Dahai Member), Shiyantou Formation, Yu'anshan Formation, Hongjingshao Formation, and middle Cambrian Wulongqing Formation.
In the Dengying Formation, the Donglongtan Member consists of laminated dolostone; and the Jiucheng Member consists of shale that has been eroded away by the river; and the Baiyanshao Member contains thickly bedded to massive dolostone with colors of grey to dark grey.In the Cambrian Zhujiaqing Formation, the Daibu Member consists of interbedded thin-intermediate, dark, dolomitic cherts and pale yellowish siliceous dolosutone, representing a transgressive system tract.The Zhongyicun Member, overlying the Daibu Member, is a phosphatic unit with grey, thick bed of laminated phosphorite.Small shelly fossil assemblage (SS1 and SS2) have been found in the Zhongyicun Member.At the correlative Meishuncun section, a SIMS U-Pb zircon age of 535.2 ± 1.7 Ma has been reported from the middle of the Zhongyicun Member (Zhu et al., 2009).The Dahai Member consists of pale grey, thickly bedded limestone and contains small shelly fossil assemblage (SS3).The Shiyantou Formation comprises grey to dark grey, bedded quartz siltstone in its lower part, representing a condensed deposition and dark grey to black shale with small shelly assemblage (SS4) in the upper part (Li and Xiao, 2014).A SHRIMP U-Pb zircon age of 526 ± 1.1 Ma was reported for the basal Shiyantou Formation in the correlative Meishucun section (Compston et al., 2008).The Yu'anshan Formation, which is characterized by the first appearance datum (FAD) of trilobites in the basal formation, consists of black shale in the lower part and carbonate-rich siltstone in the upper part.The Hongjingshao Formation comprises dark reddish thickly bedded sandstone.The Wulongqing Formation in the Xiaotan section comprises grey thickly bedded limestone, interbedded yellow silty mudstone, which is suggested to be deposited in the lower Cambrian Stage 4 (Zhu et al., 2010).

DR4. Uranium mass balance model
Under In modern suboxic settings, the U isotope fractionation associated with suboxic sinks is less well constrained.The +0.1‰ fractionation for suboxic sinks used in this study is based on a few measurements of Peru margin sediments (Weyer et al., 2008).In modern anoxic settings, the U isotope fractionation for anoxic sinks are relatively large and range from +0.4‰ to +0.85‰ (Weyer et al., 2008;Andersen et al., 2014;Holmden et al., 2015;Stirling et al., 2015;Tissot and Dauphas, 2015;Rolison et al., 2017).+0.7‰ is chosen as the average U isotope fractionation between the anoxic sediment and seawater for our baseline model.For modern oxic settings, oxic U sinks include marine carbonates, metallic deposits (Fe-Mn crust), oceanic crust alteration, pelagic clay, and coastal retention.The average U isotope fractionation from modern oxic sediments is -0.043‰, modified after Wang et al. (2016), where U isotope fractionation between seawater and marine carbonate is considered to be 0‰ (Anderson et al., 2017).
Baseline parameters used in the mass balance model are shown in Table DR2.Sensitivity analyses of changing U isotope fractionation between seawater and anoxic/suboxic sediments are shown in Fig. DR4.Larger U fractionation factors between seawater and anoxic/subxoic sediments result in even smaller areas of anoxic seafloor and greater predominance of oxic bottom waters for modern-like δ 238 U values.(Baimatuo Mb.), Gaojiaxi-Yanjiahe section.
the steady-state of global uranium mass balance, δ 238 U values of global seawater is determined by the fractions of uranium fluxes to sediments in the three redox settings: anoxic (f AOx ), suboxic (f SOx ), and oxic (f Ox ).If riverine uranium is considered as the main input of oceanic uranium reservoir, we have ; 1 Isotope mass balance is described by: • • • where each δ 238 U sink equals δ 238 U SW + Δ 238 U sink-seawater .The modelling results of seawater δ 238 U values as a function of F AOx / F Total and F Ox / F Total are shown in Fig. 2A in the main text.The uranium output rates (F output , mol m -2 yr -1 ) in various redox settings are assumed to be controlled by first-order kinetics with respect to the coeval uranium reservoir in the global ocean (R), that is, each F output = F output0 ×R/R 0 (subscript 0 represents the modern values).Replacing each f output (= F output /F Total ) in the isotope mass balance equation with: We can get the seawater δ 238 U values as a function of areal proportions of anoxic condition and suboxic condition (A AOx /A Total and A SOx /A Total , respectively).The modelling results of seawater δ 238 U values as a function of A AOx /A Total and A SOx /A Total are shown in Fig. 2B in the main text.

Fig DR1 .
Fig DR1.Simplified geological map of the Yangtze block (after Jiang et al., 2012) and stratigraphic columns of the Yangtze Gorges area and Xiaotan section.The ages of every formation in the studied sections are modified from Ling et al. (2013); Li et al. (2013) and references therein.

Fig DR3 .
Fig DR3.Analyses of δ 238 U vs. U/Ca ratio (A) and Mg/Ca ratio (B) as the tracers for effects of authigenic U accumulation and dolomitization on U isotopic composition in the carbonates.

Table DR1 .
Geochemical data of the Gaojiaxi-Yanjiahe section and the Xiaotan section

Table DR2 .
Parameters of the uranium isotope mass balance model