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• Seawater chemistry changed from stratified to homogeneous during the early Cambrian. • Ocean homogenization was driven by vertical mixing of the seawater after ~535 Ma. • Upwelled nutrient-rich water and down-welled O 2 -rich seawater likely promoted the early Cambrian biodiversity. Phosphorite successions in the nearshore Meishucun section and the offshore Gezhongwu section of the Nanhua Basin, South China, were investigated to decipher the circulation pattern of the early Cambrian ocean. The 539–535 Ma phosphorites from the Meishucun section contain authigenic phosphorus-rich mineral aggregates (collophanite grains) with much lower bio-limiting elements, Zn, Cd and Ni, than those from the Gezhongwu section. In contrast, collophanite grains in the 535–526 Ma phosphorites from both sections have similar concentrations of these elements. This change indicates that the ocean likely transformed from a chemically stratified state to a homogeneous state at ~535 Ma. Phosphorites from both sections have constant 87Sr/86Sr ratios and εNd(t) values, supporting the interpretation of a relatively constant terrestrial input into the Nanhua Basin between 539 Ma and 526 Ma. Instead, negative δ13C excursions and similar 208Pb/204Pb ratios of the 535–526 Ma phosphorites are indicative of an extensive upwelling of the deep oceanic water. In combination with previous studies of deep ocean oxygenation, our work provides evidence for the vertical mixing of the ocean between 535 Ma and 526 Ma, which introduced O 2 -rich water at the ocean bottom to promote early Cambrian biodiversity. [ABSTRACT FROM AUTHOR]

• The Doushantuo glaucony have consistent and high K 2 O with low but variable TFe 2 O 3 , consistent with Al–Fe substitution during glauconitization. • The wide occurrence of glaucony documents a generally suboxic early diagenetic environment on the seafloor. • The lack of a negative Ce anomaly in phosphorites confirms deposition and diagenesis in suboxic environments. • Apatite-glaucony association in lower Doushantuo Formation resulted from Fe-redox driven diagenesis. Major phosphogenic events took place in the late Paleoproterozoic and the Ediacaran, both of which followed an episode of atmospheric oxygenation. These apparent coincidences have led to suggestions that the Precambrian phosphorus cycle was partly regulated by oceanic redox conditions, although direct evidence is lacking. In this paper, we report the discovery of glaucony in drill cores from the phosphorite-rich Ediacaran Doushantuo Formation at Weng'an, South China, which provides new insights into redox conditions and geochemical cycles during the deposition and early diagenesis of the lower Doushantuo Formation. The presence of flaser and lenticular bedding in the sandstones suggests deposition in a tidally-influenced shallow water environment. Apatite is present as millimeter-scale flasers in sandstones, but more commonly as intraclastic grains. Glaucony typically occurs as interstitial aggregates cementing and replacing detrital minerals and phosphatic grains, with an upsection decrease in abundance. The Doushantuo glauconies have consistently high K 2 O (7.5–9.6 wt%). In particular, phosphorite-hosted glaucony has generally higher total iron (TFe 2 O 3) than sandstone-hosted glaucony, consistent with Al3+–Fe3+ substitution in the octahedral site, as supported by Raman peaks slightly shifted towards lower wavenumbers for higher Fe content. A decrease in detrital input probably resulted in limited Al availability during early diagenesis, which in turn led to the waning of glaucony upsection. The phosphate grains do not show any negative Ce anomalies, consistent with precipitation of authigenic apatite in suboxic environments with insignificant preferential remobilisation of Ce during deposition and early diagenesis. REE + Y features show variable diagenetic REE enrichment in phosphorites, but identical concave-down patterns. The wide occurrence of glaucony cement further documents the presence of a generally suboxic early diagenetic environment on the seafloor, which was in active exchange with bottom seawater. Slight fluctuations of redox states in the porewater and bottom water would have promoted the cycling of Fe, enrichment of phosphate and REE, and extensive glaucony authigenesis. Therefore, the glaucony-bearing phosphorites in the Doushantuo Formation are a direct result of Fe-redox driven diagenesis. Apatite-glaucony association likely forms in suboxic environments and is considered as a useful mineralogical proxy for paleoredox conditions on the Precambrian ocean floors. [ABSTRACT FROM AUTHOR]

• 3.3–3.4 Ga may be the maximum deposition age of the Caozhuang Group. • Caozhuang zircons may derive from protracted reworking of a long-lived mafic source. • Anshan complex may contribute most of Caozhuang detrital zircons. Caozhuang Group (or Caozhuang supracrustal sequence) in Eastern Hebei, North China Craton (NCC) may represent the oldest supracrustal unit of China. Abundant Eoarchean and Paleoarchean detrital zircons (up to 3.88 Ga) in paragneisses of Caozhuang Group provide valuable memories of the earliest crustal evolution of China. There still remains confusion about the deposition time of the Caozhuang group, the origin and the igneous host rocks of their detrital zircons, and the earliest crustal evolution of NCC. In this study, the combined U–Pb and Lu–Hf datasets provide evidence that ~3.3–3.4 Ga is a probable estimate for the maximum deposition age of the Caozhuang Group. In addition, the increasingly subchondritic Hf isotopic compositions between 3.9 and 3.4 Ga reflected by the broadly linear Hf isotope evolution defined by Caozhuang detrital zircons may support the protracted intra-crustal reworking of a long-lived mafic source in composition with 176Lu/177Hf of 0.027. The Lu-Hf composition of the mafic source is similar to the average value of ocean floor basalts (176Lu/177Hf = 0.028) and Eoarchean-Paleoarchean Caozhuang mafic rocks (176Lu/177Hf = 0.026) in the same area. Based on the similarities between Caozhuang detrital zircons and TTG-hosted zircons of Anshan complex, as well as the significant differences with other Archean ancient complexes, the Anshan complex, where the oldest rock unit of China is preserved, may be the suitable host rocks for Caozhuang detrital zircons. [ABSTRACT FROM AUTHOR]